Rebuilt 454 still can't rev past ~3000 RPM Solved!

Adam- just throwing out some more quick thoughts.

I expect you have done this but I don't think I noticed it in the videos. While running WOT underway did you also try manually gradually opening the secondaries fully to see if there was any RPM gain (or stumble)?

Also , have you tried watching the timing light strobe flash when placing the inductive pickup on each successive plug cable to see if there is any noticeable difference in brightness or consistency of the flash, or of the steadiness of the timing reading at higher rpm?

You mentioned the carb had been replaced I believe and fuel pump changed from mechanical to electric/6 psi. . Just asking, are you sure the sight tube vac inlet port at the carb ( from the old pump) is well-sealed now? Apologies if that has been previously covered .

*** New thought to consider:

Here's the timing curve graph for your description of '92-96 Mercruiser 7.4L Bravo, Gen V ( T-bolt V ignition, yes?):

http://www.boatfix.com/merc/bullet/95/95_11.PDF ( p. 19 . Note the graph is for advance only & initial timing component has been subtracted.)

it looks to me like the advance-only should normally be around 26* at 2000 RPM, which with your 8* initial timing should show about 34* total timing at 2000 and above , or 0* on the engine timing scale using an advance timing light set for 34* BTDC.

Your engine may may have timing limited in knock partial-retard mode.

I think most of us may have been thinking Knock Retard would not be applicable to your carbed engine, but this diagram of the harness and sensors shows diagram # 34 which is part # 805544T , a knock sensor.
http://www.marinepartsplus.com/cata..._V-8_1992-1996/821723+92/1621-90#805544T_jump

Here's the part:
http://www.marineengineparts.com/sierra-marine-18-7703-knock-sensor.html

If indeed your engine does employ that part with the T-Bolt V ignition module, I would 1st try plumbing in a clean portable O/B tank of fresh new 91-93 octane-rated gas or the closest you have to that rating locallly, and go for a run. If that doesn't help, at this point it might be worth swapping in a new sensor. Cheaper than a Thunderbolt module if swapping parts , as I don't know how to test a knock sensor on a non-injected engine. (if there really is one.)

If I have the wrong diagram for your described engine & ser# then this would not apply, but I'm a little optimistic.

By the way, if you subscribe ($20) to this forum to "anchor up" you can have full access to the extensive GVP "Mechanical & How-to Articles Forum " here with LOTS of valuable diagnostic/service info including many Merc manuals and so much more (like the info above).
 
Hmm... Our new "357" engine is carbed but has knock sensor. Never thought a 1995 carbed MerC would have one. Our 1996 5.7 EFI did, but it had ECM and all that stuff.

OK "WHAT IF" this knock sensor is actually "hearing" knock, or there isn't knock and the sensor "thinks" there is...?

I scanned through, not sure if I found whether the timing advances to spec then backs off (retards to less advance) when the RPM drops. Have you been able to monitor the advance as it comes up, reaches 4600, then drops to 3200?
 
J-D;

Good points, thanks! I have never been fortunate enough to have the timing light hooked up on a run where we've been able to rev it up beyond 3000 RPM, so I can't say what the timing advance does in that case. However, I have watched the timing on runs to 3000 RPM multiple times, and the timing advance follows the curve listed in the manual - more or less. I'm only a few degrees off, which I have attributed to the fact that I'm using a $19 timing light I bought at Harbor Freight! :) But yeah, the timing advance goes up and down with RPM like you'd expect.

Regarding the knock sensor, that's a very good point, and one I address below:

Sandy;

Thanks for the reply! I have indeed manually pushed the secondaries open just to be sure there is no restriction. And at full throttle, they open all by themselves, but the fuel spray pattern isn't full like it should be. (You can see this in the video too.)

I have not connected the timing light to any of the other plug wires. Just to clarify, this is to check to ensure all plugs have more or less the same magnitude of spark? (As in, enough to consistently trigger the flash in the timing light?) I can do that the next time we have it in the water easily enough.

Your other comments about ignition timing advance are spot-on and mirror my thoughts over the past month. I have a copy of the Merc manual for this engine (#16), and I've looked at the exact timing advance graph that you posted. If I read it correctly, the *total* advance is 34 degrees at 2000 RPM and higher. That's 26 degrees from the Thunderbolt V ignition module plus 8 degrees base timing. And I measured 30 degrees, which is really close... Would 4 degrees be enough to cause all this?

That being said, the primary difference between Thunderbolt IV and Thunderbolt V is the addition of a knock sensor on the STBD side of the engine. (My old Bayliner had a Merc 4.3 with Thunderbolt IV, and after speaking with GVP about that rebuild he schooled me on the differences in the Thunderbolt series.)

I don't know how to test a knock sensor, but I did verify it was hooked up correctly. (There's only 1 wire to it, so presumably it grounds out when it detects a knock?) I have considered pulling the wire off just to see what happens, but given that the ignition is already advancing really close to max, I don't think this is the problem. My understanding is that when the knock protection kicks in, the timing is retarded all the way back to base timing, and we're definitely not seeing that.

Incidentally, the mount for the knock sensor on his engine looks different from both the pictures in the link you posted and the pictures in the Merc manual. (The manual matches your link, however.) His mount is squared off and angles towards the stern at about a 45 degree angle. Honestly though for no more than it costs, it might be worth just replacing the thing to see what that does...

One other thing you mentioned has got me thinking: You talked about the vacuum port on the carb... This would be on the stbd side of the carb, just forward of the fuel line connection, and it angles slightly aft, yes? If that's the port we're talking about, there is a piece of clear tubing connected from there to the top of the old (and now disconnected) mechanical fuel pump. But yeah, thinking about this, there's no reason for *anything* to be connected to that port now. (It was initially designed to catch fuel leaking from a failed fuel pump diaphragm, but that isn't an issue now.) Moreover, if there was air leaking into that tubing, I can see how that could change the performance of the carb...

I think the first order of business is to plug off that port with a tight-fitting rubber cap, and maybe a clamp over the top just for good measure... It couldn't be sucking too much air, but maybe just enough to screw with the jetting on the secondaries.?. Then I guess I should hook up the timing light and see if disconnecting the knock sensor changes the ignition advance any...

OK, I've got a game plan for the next time my son puts the boat in the water! (And I'm really, *really* getting too old for all this mechanical stuff. There's a reason I sold my boat 8 years ago...)

Adam

PS: Regarding the Anchor list, I was a member here years ago, but I sort of lost touch after I sold my boat. Was honestly shocked to not see George around when I returned last year... Still, it's a worthy cause, so I re-upped! :)
 
Adam- I don't think knock sense necessarily causes timing to retard to base time, rather it generally retards until no more knock is sensed. So if fuel quality had degraded somewhat to lower octane rating level, timing might be retarded only several degrees but would limit it ( and thus rpm) there due to knock consistently sensed at higher load . That could sure explain a lot.
Not saying this is necessarily what is happening at all , but after all your efforts so far , if it was my boat I imagine I'd be very interested in seeing what performance a portable clean tank of fresh "high test" might allow.
(But then on my MPI's with MEFI-4 ignition it would be easier to just quickly scan the engine to see if knock has been sensed previously> code tripped, or presently and to what degree timing had been automatically retarded due to sensed knock.)

If that eases any timing retardation, I still don't know whether that would affect manifold vacuum to better draw fuel through the secondaries.

It's might even be the case on some engines if knock was sensed the code would have to be cleared before the engine would get back into full "good to go mode." But I have no idea how a scanner could even hook up to your '95 carbed (T-bolt v?) engine.
 
The more I read here the more I think you have an electrical or control system problem. Intermittent problems are usually not mechanical or even fuel though trying known clean fuel is a cheap and easy test.
 
Another update: We did some more tests yesterday. We haven't fixed it yet, but I think we are closer than we've ever been.

First, we removed the hose from the vacuum port on the carb (the one that went to the now-disconnected mechanical fuel pump diaphragm housing) and plugged off the port completely. I had high hopes that this would solve the problem, but alas - it didn't do anything. (Darn...)

Without knowing what else to look at, I kept coming back to ignition timing. And like others have posted above, I feel that an intermittent problem is almost always electrical in nature. So I connected the timing light again, and we made a few runs while I watched the timing advance. But this time I carefully positioned myself very low in the engine compartment so I was able to get a much better look at the timing marks and I could sit down there and watch the timing for longer periods.

Sure enough, above 2000 RPM, the total timing advance is exactly 34 degrees, which is dead on. (8 degrees base plus 26 degrees from the ignition module.) I think my earlier readings were simply off a bit because I was in such a lousy position and/or I may have bumped the adjustment knob on the timing light when trying to pull my hand away from the engine to read it.

Also, unlike previous tests, I kept the timing light on and had my son continue to throttle up (while on plane) until we hit the wall at about 3000 RPM, and that's when I noticed the timing started to retard all the way back to base timing! Holy Crap!

I never bothered to watch the timing for more than just a few seconds in the past. Once I saw that the advance was working, I figured that was that. But this time I kept the light on, and that was the key.

I immediately thought: "Bad Knock Sensor!" So we disconnected it... But that didn't change anything. Grrrr!

We made several more runs and we actually got a couple successful runs to 4300 RPM. And when that happened, the timing stayed advanced to 34 degrees. But after a short period (30 sec to 1 min) the timing would fall back to base and the RPM would drop back down to 3000 or so. We could also hear the audible change in engine sound through the carb.

So after scouring through the Mercury Manual # 16 for the 454 with the Thunderbolt V ignition system, I finally found what I was looking for on page 333... It turns out that the knock sensor sends an AC signal to a separate knock control module that is mounted on the distributor (it is right behind the ignition module). That knock control module then outputs an 8 to 10 volt DC signal to the ignition module if there is no knock present.

I didn't realize that there was a separate module just for knock control. I thought all that was integrated into the ignition module. Guess I should have read the manual a little closer!

Anyway, if the knock module detects a knock signal coming from the sensor, then it drops out that 8-10 volt signal to the ignition module. This causes the ignition module to retard the timing in order to prevent engine damage.

Also, either disconnecting the knock sensor wire or shorting it to ground will effectively remove any signal to the knock control module, so it won't drop that 8-10 volt signal to the ingition module, and the timing should stay advanced. (And obviously, if you do this and you're running cheap gas while accelerating hard, you can damage your engine...)

This leads me to believe that the ignition module is probably working correctly and the problem is localized at the knock control module. (Not the sensor though: If it was the sensor then disconnecting it should have solved the issue since there would be no signal present.) It could either be a bad knock module or a wiring problem (intermittent connection) between the knock module and the ignition module. There is also an edge-case scenario where interference between the spark plug wires and the wire from the knock sensor to the module can induce a false knock signal on the input to the knock control module.

So the plan is to put the boat back in the water today and focus all our efforts on troubleshooting this knock control module and associated wiring. My first instinct is to simply disconnect the signal wire from the knock module to the ignition module and wire a constant 10 volts DC to it. (Easy enough to do with a little wire from the battery and a pair of resistors to make a voltage divider.) If I do that and she revs to 4300 or better, consistently, we know we're on the right path.

I'm guessing that the knock control module has an intermittent fault and needs to be replaced. But before we buy another expensive (and non-returnable) electronic part, I'm going to go over all the wiring and make sure it's not just a loose connection somewhere.

Still, I feel very confident that we've finally closed in on the source of the problem. I hope to be able to post some good news later today or tomorrow! (Crossing fingers...)

By the way - during my research I uncovered some information that could explain the low manifold vacuum readings I saw when the engine was idling. (I measured 13" but was expecting 15" to 18".) It turns out that chasing that may have been a red herring.

While it is true that late valve timing will cause poor manifold vacuum, it turns out that changing the cam profile will do the same thing. The idea here is that you accept poor manifold vacuum (and thus less efficiency) at idle speeds in exchange for more power and efficiency at high RPM. Basically it means you get better scavenging of the exhaust gases at high RPM and can thus pull in more clean air and fuel for the next cycle so you make more power. Since boat engines are often operated near their upper RPM range for long periods, this is a trade-off that makes sense in a marine engine - particularly in a big block mounted in a heavy boat!

So now I'm less concerned about the manifold vacuum readings at idle. When the engine is at 4200 RPM (near WOT) the manifold vacuum is only 2 inches, which is pretty close to ideal. That tells me that the valve timing is probably correct and it's just about got to be the cam profile causing the slightly lower manifold vacuum at idle.

I'll be sure to post back here after we take the boat out today and test the knock control module. Would really be nice to finally put this issue to bed! :)

Adam
 
Now that is one beautiful and very informative post, ....and very encouraging!

However, I would caution you ( as you touched on above), if the knock-sense timing retard system is working correctly because of fuel octane-rating degradation, bypassing that system on a test run might have undesirable consequences.

I don't have any idea how long knock (detonation) can take place before the high pressures involved cause damage, but might it not seem wise to take a run on a clean portable tank of fresh mid to high octane gas as a test 1st?

Granted, since timing is being retarded all the way back to base level rather than to degree at which detonation likely subsides, that could tend to indicate a faulty control sensor. But there could be some risk in not using known high quality gas in your "work-around" test run.

If/when you do get timing to stay at full advance, it will be interesting to hear what effect if any that may have on the carb secondaries' spray pattern.

You are sooooo close. We're all rooting for you two, and the engine!
 
Thanks for the words of encouragement, Sandy!

Unfortunately, I've returned from the boat landing with bad news. The knock module is not the problem. It's definitely the ignition module itself that is malfunctioning.

So when we put the boat in the water this afternoon, the first thing I did was hook up a voltmeter to the test lead (purple-white wire) that comes off the knock module and goes to the ignition module. This is the same connector that you ground out to lock the engine in base-mode timing. Turns out it's also the wire that carries the output signal from the knock module to the ignition module.

Basically I wanted to see if the knock module was sending a spurious "knock present" signal, which would explain the timing spontaneously being retarded all the way back to base. But I had a solid 8.3 volts on the lead, even when we throttled up, hit the wall at 3000 RPM, and noticed the timing start to retard all by itself. Darn it!

Just to be thorough, I disconnected the entire knock module and hooked up a voltage divider between the battery terminals so I could put a constant 8.5 volts right on that lead that goes directly to the ignition module. Then we tried it again. Same thing... Timing advanced right up until around 3000 RPM, at which point it started retarding again. CRAP!!!

So with the timing light on, we just sort of played with the throttle for about 20 minutes, up and down. And the more we played around, the worse the timing got! I started to see the timing advance well beyond 34 degrees total. This should never, EVER be possible. Then I saw it retard below the 8 degrees of base timing. What the deuce?

I think I was watching the ignition module in the process of failing. As the afternoon wore on, the timing seemed to get worse and worse. Finally, after about 30 minutes, I decided to hook the knock module back up and head for the landing. My thought was that if the module failed in an extremely advanced mode it could damage the engine. But if the knock module was hooked up and still working it should pull the timing back and stave off any harm.

So now with the boat back on the trailer my son and I have been researching the ignition module. The news is not so good. Turns out the original Thunderbolt V ignition module (Merc part # 806960T4) is no longer in production. Instead, they have a new part (806960A01) that incorporates both the ignition module and the knock control module in a single housing. So you have to re-wire the connectors. Oh, and it's $550. And it will take a week to get one. Sigh...

My son is still looking on-line. He found one of the original modules on a website that sells surplus parts, and it was listed as new, so we'll see... But worst case, if we get this updated ignition module kit, that should solve our timing issues. Just going to take a while for us to re-wire everything. (Yay... Not!)

BTW, I should have mentioned earlier that we have completely ruled out fuel as a source of the problem. One of the first things we tried last fall was a portable tank with good gas in it, and we repeated that test earlier this year after the new carb, fuel lines, and fuel pump were installed. No luck there.

I was really hoping that the ignition module would be OK. It's the most expensive component in the entire ignition system. And of course we've already replaced everything else (including the coil!) earlier in this process. Sigh. Rotten luck I guess.

One more test run after we get the new module, and then maybe I can post a happy epilogue to this thread!

Adam
 
Adam- I'm very sorry to read that the amazing saga continues , but very impressed with (especially) your most recent diagnostic attempts to nail the sucker down. Really impressed with your recent research and all the things you have carefully checked and tested. Sorry to have nagged about the possible fuel quality issues but good on you that you had already ruled that out.

I wonder, have you ever contacted Mercury Marine to check whether there is any means on that '95 carbed engine to hook up a diagnostic scan device which might provide an electronic check of the Thunderbolt V ignition control module?
Unfortunately , I've have seen my Rinda Diacom software for laptops and Rinda TechMate hand-held spec that they scan Mercruiser carbed engines with T-Bolt V ignitions only from 2006 -up.

Looking at component wiring diagram here, is there any possibility any part of the timing lead could be arcing to ground at higher rpm/ & possible alternator output?

http://www.boatfix.com/merc/bullet/94/94_02.PDF

But... one might expect that to only have momentary timing retard back to base level, not sustained. . Plus there is a note saying " The jumper wire must be connected to the Timing Lead and ground BEFORE turning ignition key on, otherwise the Module will not go into the “Base Timing Mode” ". So according to that at least, it should not revert to Base Timing Mode if that lead was somehow grounding out underway.
Which tends to lead you back to the TB-V ignition control module. If you spring for that, I sure hope it works.

FWIW, I get nothing back in a quick search for Thunderbolt V module re-flashing, but wonder if anyone or Mercruiser ever does that.
 
Hey folks! Sorry for the lack of an update, but it took us a while to get the time to revisit this issue.

Short version: After the previous runs showed erratic ignition timing at higher RPM under load, we decided to replace the ignition module with the new style that Mercury sells now. This required some minor re-wiring of the ignition harness (just 4 wires had to be changed). Replacing this module solved the problem!

Lengthy synopsis: My son ordered the replacement ignition module kit from an on-line vendor (cost $500). This was the new 806960A01 part number, which required us to cut a few wires loose from the old module and install new bullet connectors on them so they could plug into the new harness connector for the new module. Total installation took about an hour or so. Note that this new kit re-uses the old knock sensor and knock control module. (Which we proved were functioning correctly during previous testing.)

After installing the new ignition module, we tried to put the boat in the water to test it, but we found a new problem - the trim pump would only trim down, but not up! Fortunately we hadn't launched the boat yet, but we were in the water on the trailer when we figured this out. We had only trimmed down about 2/3 or so at this point.

We decided to haul the boat back out right then rather than risk getting the drive stuck in the full-down position, which would have forced us to pull the trim rams off and rig a rope sling to lift the drive in order to trail the boat.

A few days later we were able to troubleshoot the problem: Bad solenoid on the trim pump unit. Found a local supplier that had it in stock for just $25, so we were back in business by early yesterday afternoon.

The moment of truth was yesterday afternoon. We got the boat back in the water and tested it out. This time the engine responded like we expected! (Finally) No hesitation when throttling up, no real change in the engine sound (apart from the throaty roar when the secondaries opened up) as RPMs climbed past 3000, and no issues at all getting to wide open throttle. Peak RPM is still a tad low at 4200 (should be 4500 or better), but the bottom paint is 3 years old and very chalky, plus the props could use some attention, so we're not too worried about that. The main point is that the engine now revs up normally (and smoothly) throughout the entire throttle range.

We stayed out on water for a good hour just playing around. Throttle up, throttle down, hold steady, dance up and down a bit, and even a few holeshots just to see how it performed. No issues whatsoever, and my son was a very happy boater for the first time in about 14 months! :)

Manifold vacuum is unchanged from previous readings, which supports the theory that Mercury may have used a different cam profile on this engine to get slightly better performance at higher RPM. For sure the engine is very smooth now, and throttle response is back to what it was before this whole adventure started over a year ago.

Thanks again to all who offered advice and suggestions. To anyone reading this in the future, the take-away that I have from this entire process is to check your ignition timing early in the troubleshooting process, and be sure to watch it for several minutes while you change throttle settings so you are sure you have been watching the timing while the engine problem manifests. Had I done that earlier, we could have zeroed in on the ignition module as the source of the problem before we replaced a bunch of other expensive parts.

And finally, to address your comments Sandy: The older Thunderbolt IV and V ignition systems from the 1990's pre-date any sort of engine scanning tool, at least in the marine market. (Which really sucks!) While some newer engines do support the OBD engine codes and scanners, the manufacturers have added extra electronics to accomplish that. The basic Thunderbolt ignition circuit doesn't even have any sort of port to connect a scanner to.

As for any wiring arcing to ground - we did look at that too. We monitored the output voltage from the knock module to the ignition module continuously, and even when we saw the timing start to retard back to base, we still had a solid 8.5 volts on that line.

Interestingly, the line you found in the manual about needing to ground that purple-white wire before you start the engine (to lock it in base timing mode) is a bit misleading. The module will retard the timing all the way back to base timing *anytime* that lead is grounded. (That's actually how the knock circuit works... When the knock module detects a knock signal from the knock sensor, it grounds that wire instead of sending the steady 8.5 volts.) However, even with that lead grounded, the ignition timing will still drift around a bit to maintain RPM at idle, and I think it also has a mode where it will slightly increase the timing periodically to see if the knock problem clears at higher RPM. So while it is true that it won't be "locked" in base mode when grounded, it will retard the timing close enough to base that it will cause the same sort of performance problems we saw with the failed module.

Once again, thanks to everyone for all the help! Now I need to schedule a weekend's worth of work helping my son clean off all the old bottom paint... (Ugh.) At least I have a pressure washer! Fair winds and following seas, folks!

Adam

PS: J-D must have seen my edit to the first post in this thread before I finished this lengthy one. :) Rest assured, we have already planned out several trips for the near future. And this fall we'll probably take the boat down to the harbor for some more sightseeing with some relatives from out of town. Feels good to have finally put this issue to rest!
 
Glad it's up and running. Wonder how common this type failure of the ignition module is? Enjoy the Boat!!
 
Thanks for the rest of the story!

What was the Part Number of that Solenoid? I tried looking it up and it looked more like a starter slave solenoid than what I expected. Would like to check that out. THANKS!
 
Adam- That's just fantastic news and I am really really happy for you and your son!

You guys have the patience of (Steve) Job(s). Truly impressive (mind-boggling?) perseverance.

You have also offered up what I suspect has been the most comprehensive and intelligent OP replies to the mountainous load of our mishmash of WAGs and other suggestions and questions I recall seeing on any boating forum . With good spirit right along. I expect most everyone who followed this thread has learned a lot right along with you. Sure, in retrospect this problem might?? have been nailed down by a professional much faster(maybe) , but now you two know about everything there is to know about your engine or at least fuel supply and ignition for sure.

You and your son will forever remember this so-often agonizing learning & bonding experience and well-earned triumph over mechanics and rat-b*****d unruly electrons . What a detective team! I'm thinking you guys should be semi-official BoaterEd Mascots... as this is what it's all about.:) :)

Good on you!!

With apologies, just 1 question- What is that #806960A01 conversion module intended to convert from? Earlier , less robust/reliable version of T-Bolt V, or from T-Bolt IV , or ??? ( Not that I understand the differences between versions, or need to as I haven't had Mercs, I/B & O/B, for 40++ years.)
I see it does have adapter cables to help convert old connections.
 
Thanks for the kind words, Sandy! I figure if someone asks for help and others take the time to answer, the least you can do is acknowledge their input and address their questions/thoughts. To do otherwise is just plain rude in my book. :)

Then too, more than a few folks raised the question of ignition timing throughout this whole affair. So more often than not the advice offered *was* on-point and not just a WAG. If only I had been a little more patient during those times I had the timing light hooked up...

Besides, even when chasing down a dead-end, you still learn things about your boat (or engine, in this case), and you may accidentally uncover other issues you might never have known about otherwise. We made the choice to try to do this ourselves, and that decision comes with the understanding that it might be a long, hard journey. (Just didn't think it would be quite this long when we started back in May a year ago!)

I do agree that it's been a great learning experience, and heaven forbid I ever have to rebuild another engine, but at least if I do I'll know to be more suspect of the timing from now on! :) And yeah, a professional mechanic probably would have figured this out very quickly. I need to remember that as well for the next time.

The original part number listed in the manual for the Thunderbolt V Ignition Module for my son's engine is 806960T4. That's for a 1995 vintage 7.4 L with a Bravo 3. I'm pretty sure this is the 330 HP version of that engine. If you're very familiar with the 454, this version is the one with the smaller "peanut-port" heads. (Which explains the lower horsepower rating.)

Anyway, that original part number for the ignition module is now obsolete. Everywhere you look for that number, you will be redirected to the updated number, which is 806960A01. This is sold as a kit that includes the module, the two new wire harness connectors (instead of the one large one like the original part), and some extra barrel connectors that you have to use to re-wire a few of the leads on the old wiring harness. It also comes with an instruction sheet that takes you step-by-step through the process of removing the old unit, making the changes to the harness wiring, and installing the new unit. They even include a new mounting bracket.

As for what else they changed with the new model, I honestly don't know, but it is listed as a direct replacement for the Thunderbolt V ignition module on all 7.4 L Bravo 3 engines in his serial number range. If they did upgrade the internals, they don't advertise it in any of the information we found about the new module. I'm thinking that Mercury probably just went with a new case design and standardized it across all models for simplicity.

Also, it still uses the existing knock control module, just like the original ignition module did. Newer versions of the Thunderbolt ignition system integrate the knock control module into the ignition module itself, so you only have one box hanging off the back of the distributor.

As to Michael's question about how common this failure mode is, I can't really say. But usually when someone talks about the ignition module crapping out, they have no spark at all. I know I've run across that failure mode quite often, in boats and cars alike. This was rather strange though with the thing spontaneously retarding the timing... The fact that the engine ran so well at low to medium RPM really threw me off and convinced me to look at pretty much everything else first. Live and learn I guess!

Finally, to address J-D's question about the solenoid for the trim pump, the Sierra part number is 18-5817. It's actually listed as a Starter Slave Solenoid, but on my son's boat they use those same solenoids for the trim pump (one each for up and down, mounted right on the trim pump bracket on the transom).

Adam
 
Adam, your post above just reinforces how patient, methodical, and cooperative you've been through all this.

I just learned more about that Trim Unit. I'm used to an arrangement where a pair of SPDT relays are used to reverse a DC motor. There's also a heavy duty reversing relay called a Trombetta that accomplishes reversing with one part, and a few relays not as robust as Trombetta that accomplish reverse too. I was NOT expecting two SPST relays, like Starter Slave Solenoids, doing this. Looking at it, the arrangement I thought of, is to reverse a two-wire, permanent magnet, DC motor. Seems MerC uses a three-wire motor, so one relay powering say Blue, would run it up. Then the other powering say Green, would run it down. This works because the third wire is ground.

Shipmates!!! This is also why it'd be a great idea to have a Slave Solenoid in our onboard spare parts box! It's backup for not just a failure to crank, but also for a Trim that won't go Up or Down!
 
Did you peek at the carb secondaries to see if the flow at high throttle openings looked more consistent? Now that the engine is attaining high rpm I'd think the increased air flow and venturi effect would have to be pulling a lot more gas there than in the video.
 
Excellent news. Enjoy the fruits of your efforts. You deserve them! It's been a while since we have seen such a detailed investigation and everyone has learned from it. Thanks for sharing.
 
Hi Folks;

The weather conspired against us, so I didn't go out with my son this weekend, but he's already put 10 hrs on the boat since we changed the ignition module on the 11th. To say that he is a happy boater now is probably the understatement of the month! :) Hoping for some better weather soon so we can all go out...

Sandy: I didn't look down the throat of the carb when we took it out for that last test run, but I do plan to do so (and video it) the next time I go out with my son, just to be sure we've 100% got it. I've still got my vacuum gauge hooked up to the manifold too, and I won't pull that off until I've double-checked all the carb adjustments and checked the base timing one last time just for good measure.

EDIT: Here is the promised video of the spray pattern with the new ignition module installed and everything working correctly: https://youtu.be/Xj5LtUp6sW0 The rolling shutter of the cell phone camera makes the flow pattern appear more choppy than it really is. The important point is that the engine now revs all the way to 4200 RPM every time - smoothly and without hesitation.

At this point my son has put another 20 hrs on the boat, and everything is working normally. Really happy that the new ignition module solved all the problems! :)

J-D: Your description of the trim motor is spot-on, right down to the wire colors! But yeah, rather than reversing the current, this system has 3 wires on the pump, so presumably dual sets of windings inside, or maybe one large set with a center-tap for ground.?. Either way, Hot to blue = up, and Hot to green = down. Black is ground as you would expect. And the blue and green wires each have their own SPST solenoid, which are in turn triggered by the rocker switch on the throttle arm. Good point about keeping a spare in the boat toolbox though! (Although in a pinch it's nice to know you could steal one from the trim system to replace the one for the starter, or vice versa...)

Boatbum: Happy to share the story man. Hopefully it helps someone else one day!

Adam
 
Well, shoot. I didn't intend to bump this thread back to the top, which is why I initially just edited my previous post (instead of making a new post) to add the video link that I promised Sandy earlier.

However, I forgot that the forum considers an edit to be a new post, so it got bumped to the top again anyway. D'oh! Sorry about that! (And now this explanation/apology post will add more impertinent information to an already lengthy thread...)

Anyway, there are no changes from the previous update. Boat is running perfectly now that we have the new ignition module in place. My son is happier than he's ever been about his boat. (He had it out again this weekend, in fact, and last weekend he overnighted on the boat for the first time.)

Enjoy the rest of the summer folks... (And don't forget to double-check your ignition timing!) :)

Adam
 
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