Anyone with big block knowledge?

alk

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ive got a 1999 7.4 mpi - so I think that means its a gen vi, with ‘peanut port’ heads. but not sure on the heads, some say they are ‘peanut port‘ in this motor, others say they are vortecs.

I’ve given up on the MPI, can’t get a steady idle, or above 3,800 rpms with a load. Plugs, wires, cap, rotors, iac, map sensor, tps and all filters have all been replaced. Have had it at a shop. They got nowhere either. Ran with brand new gas from an aux tank, no diffference. Fuel pressure is steady 40+ psi.

My thinking is that I’ll pull the motor out of the boat , and if the compression is good, over the off-season replace the intake manifold, throw on a carb, a new fuel pump, and a delco ignition. I know I’ll burn more gas, and won’t get better performance than a properly running mpi motor. But I don’t have a properly running mpi, so this may be my best option.

what I’m having trouble figuring out - does anyone make a cast iron marine intake manifold ( for carbs) that is compatible with the peanut port ( or vortec) heads? Or do I need to replace my heads too? If replace, which heads are compatible with the gen vi block? Small blocks are so much simpler, with more information out there. Not finding much on the big blocks.
 
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Golfman25

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Can't help you with the block, but did you look at possibly the ECM for the MPI? Sounds like that is the only thing left.
 

alk

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I thought about replacing the ecm - but those aren’t easy to find for these old motors. By 2005 they were already getting scarce. if you can find one, they are expensive - I may be able to do my entire old school retrofit for less than the cost of an ecm.

if I had to bet, I would say the ecm is messed up, or some wire somewhere is broken or corroded, or not making good contact some other reason. But I’ve lost pretty much this entire summer, and even the pros I took it to made no progress - so not confident the current setup is going to work.

only four choices I could come up with...
1) retire from boating
2) replace the entire 1,500 hour 23 year old motor
3) put a carb on this thing and hope for the best
4) trade in the boat on something new(er).

today my preference order is 1,4,3,2 - but i suspect half way through winter I’ll feel differently.
 
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Bill D.

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No advice here, but dang alk.. boat engine, trans-am restore, I’m thinking you need some big projects to keep you busy!
 
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alk

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Tell me about it. I did sell the sailboat, so down to two projects. if only someone produced marine exhaust manifolds for a Pontiac 455, then I could at least standardize on motors! This mpi motor had run perfectly for me, for nearly forever, so was not at all expecting this project. But pulling into a slip in early July, motor stalled. Probably first time this boat had ever stalled in the 23 years I’ve owned it. Haven’t recovered yet. Freedom boat club may be in my future!
 

Bruce Herrington

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FWIW, I retired from boat maintenance 1.5 years ago with no regrets. We'll rent one occasionally.

So I completely understand why retiring is #1 on your option list.
 

alk

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Looking into this for past two days. seems like I don’t have the peanut port heads, those were sold with the gen v motors. With the gen vi, it came with vortec oval port heads. At least that’s what came on the trucks. I wonder if Merc went with different heads? I am betting no, because I assume they are using all the same intake components as the trucks; would be hard to do that with different heads, I would think.

So if I pick up the manifold and ignition below, plus the Rochester 4brl in my garage, and a fuel pump in theory i can pull this swap off for about $900, plus some gaskets and misc. parts. I will probably pull my injectors out, and send for servicing. There is a slim chance that will fix the problem. But if not, the mpi is coming off. Pretty good chance I will be able to sell my ecm, newly serviced injectors, and rest of mpi parts for at least as much as I’m going to spend on the parts below. I wonder if I should put a different cam in?



Or maybe the DUI distributor setup, which seems a little simpler. Has anyone used one of these ?
 
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Bruce Herrington

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Replacing both carbs and installing DUI distributors on the twin 80's model 454 Crusaders solved a lot problems.

Did the work myself. Ran the boat for almost 2 years before selling it.
 

alk

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Last night i ran it for about 30 minutes in my driveway - as I never really got a good test run since I put the new map sensor on. Worst possible result, it idled just fine, rock steady at 605 rpm. So now I don’t know if the problem is fixed, or if instead of a motor that wont idle correctly, I have a motor that usually won’t idle correctly.
 

Bill D.

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Dang alk, i’ve been impressed with your repair, replace, and restore skills until now. The rule is you now declare it fixed and use it. If anything, 8ncluding the same thing happens again it is a NEW problem.😁
 
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alk

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This is going to be the motor that kills me, or at least ends my boating career. Was out for a couple hours Saturday and most of the day Sunday. The idle problem, with the surging back and forth between 600 and 1000 rpms, seemed to have been solved. Still could not get over 3,800 rpms, however was glad that the idle was steady, and thinking about how much time and money I’d be saving not having to pull the motor and re-do it. But as I was pulling into the ramp last night, likely my last day boating this season, it was back. So pretty much back where I started.

and just for fun, the ‘up’ solenoid for the drive stopped working, between Saturday and Sunday - so I had the pleasure of replacing that in a west marine parking lot Sunday morning. And it looks like the outdrive is starting to drip gear lube out the seals. I’ve tried to maintain this boat as well as I could, but I guess 23 years of saltwater use is about end of life for a late 90’s Wellcraft with Mercruiser mpi & bravo 3 power. The list of things not working right and/or requiring major overhaul is getting pretty long. I dont know if there is even anyone left in business who can rebuild a bravo drive, at least not around here.
 

GeeBee

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alk, I got the Diacom diagnostic software you can use if you want. Your problem may be injectors or rather a bad injector, or your base timing may be off. It is on CD, I have the cable that plugs into a laptop. You can run it, record the results and analyze. PM me if you want it.
 

alk

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Thanks, that may help - but the half dozen or so laptop & desktop PC’s in my possession, I don’t think any of them have cd drives!

The shop I had it at ran all of the diagnostics and reported to me that no error codes were present, nor anything else notable. They were pretty shocked that I ran the motor for 23 years without having the injectors serviced; unfortunately I didn’t even realize injector servicing was a thing!

i think I’ll pull the motor out of the boat in a few weeks, as the access to anything is next to impossible. Even checking the compression requires a contortionist.

then I’ll check the compression again, and if that‘s fine, send the injectors for servicing. Apparently when they service them, they run a flow test prior, so that report should let me know if it was in fact an injector problem. If it was, I should be good. If not, I’ll have some decisions to make in November.

I should be able to replace intake and ignition with ‘old school’ for under a grand, and gain some hp. Big decision is if I want to do cam as well, that would be a nice HP increase, but big block cams are about three times the price of their small block counterparts.

can anyone tell me if replacing the seals in a bravo 3 is a diy job, or are too many merc tools required?
 
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GeeBee

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In 24 years, I've never cleaned my injectors on my 7.4 MPI. Flow checking the injectors will not tell you if they are firing off correctly at the correct time.
 

alk

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What would cause the Injectors to fire at the incorrect time? a problem with the ecm?
 

GeeBee

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Either that, bad wiring, bad connector or bad injector solenoid
 

alk

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So i finally pulled the motor out of the boat, and brought it home. Compression is perfect 152-155 across all eight. Pulled the plenum and intake off, the injection looks like crap - rust worse than a 1980 Pontiac. all the clips that hold the injectors in the cylinders were balls of rust, the rail, the connectors, hose fittings, all of it in very rough shape. I guess 24 summers of saltwater was too much for this setup.

i suppose it’s possible to clean all this up, but at this point I think I’m going to pull the heads to see how everything looks - and then start a reassembly with a carb, and dui or msd distributor. Only consideration is what to do about the cam. If I go bigger, I could gain a few dozen hp now that the mpi limitation of 500 or so cfm is gone. But that would mean new valve springs, I believe.

anyone have any spare big block parts they want to sell? New manifolds are a little under 400, hoping to find a used one for 150 or so.
 
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alk

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Not sure if anyone is following this thread - but if so, anybody have a marine carb laying around the are looking to get rid of? Preferably 750-850 cfm , square bore? I had a quadrajet and a holley sitting in my garage - but turns out the rochester is only 650 cfm, and the holley is even smaller, at 600. Neither of these will cut it on a 454. Hoping to find a used one, as the new ones are not cheap!
 

WALSHIE

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For over a year I had a 750cfm 4 barrel Holley I pulled from my Monte Carlo and replaced with a 600 cfm Edlebrock.
Unfortunately, I put it on EvilBay and it sold for nearly $250. Good for me but unfortunate for you, they are well sought after and expensive.

Having said that, I was talking to a garage gear head at a party a few weeks back about my progress. He said he only does fuel injection nowadays. Apparently the kits include manifold, fuel rail, computer, etc and are around $1k. He said it is getting easier and easier to swap from carb to FI.

Good luck!
 
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