Low Compression - fix and re-survey?

B

BCSaltchucker

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Jan 1, 2000
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1095
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Hi, great forum

I have a conditional contract to buya boat, but the survey was just done today and one cylinder is under spec for compression.

The broker claims that the repair will be done at the broker's repair shop (one of the largest companies in the pleasurecraft business) under the current owner's warranty. He suspects it may be a bad valve and a new head may be installed. I am prepared to continue with the purchase if the engine is good after the repairs. Very clean boat overall (6 yrs old, 7.4 mpi B3, maxum 29 express)

When the boat comes back, should I pay the surveyor to re-check the engine compression, or just trust the service record?
 
easier to ask to witness the compression test after the fix is done
 
Yep .. I will do that. THanks for the tip.

Update is that the mechanic did a proper compression check today and all 8 are OK, but #3 is the lowest (all are within 12 psi of 165psi). The Surveyor did not do a true compression check, just used an analyzer on the spark lines that flagged #3 as below spec.

cheers,
SRoss
 
Just courious--how does a spark wire analysis determine compression?

Thanks
 
Bob,

I'd like to hear your opinion once you've reviewed the write-up on this Pico Diagnostics -Automotive Diagnostics Software (Beta) process.

The fact that I've never heard of such a diagnostic tool or process does not mean that it is not a valid indicator.
 
Sounds like a bogus surveyor. If the engine ran fine otherwise with plenty of power, I'd go ahead with the purchase. One thing about surveys, sometimes you get too much information and you have to take everything in perspective.
 
Hi Bill,
I had never heard of this either. Apparently it is a UK based company. I just find it hard to believe that a single set of leads to a battery will do all of these diagnostic studies....but I suspose it is possiable. By measuring voltage fluctions, one could get a great deal of information . If you hooked up to the engine's computer, or used a vacuum guage or compression guage more likely, that an analysis would be valid....
They give a "engine is OK if compression is within 30% on all cylinders"--your surveyor had questioned #3, which turns out is within 12 PSI (or 8%)--and is certainly within normal range. I wonder about the 30% number--that would bother me.

It seems to me that the "hash" (interference) from the alternator would over ride any valid measurements--when the engine was running. How does the computer decide which cylinder is which during the cranking phase (it turns out it doesn't)? (one assumes that the amount of current taken by the starter motor is proportionate to the amount of force needed to drive the engine into compression phase). So one is meassuring only the voltage, (although there are is a 60 amp DC and a 600 amp AC/DC probe) how is it determined that the one cylinder is low? BCSaltchucker wrote of spark plug wire analysis--the compression test involves the "engine disabled"--which suggests to me that the ignition is disabled at the primary circuit, as well as disabling the fuel pump system. Even the Pico manual suggests that an abnormal
"cylinder Balance" test can be compression, spark plug, or injector defect--and does not differentiate. Also during the test Pico has you turn on the headlights, fog lights, window defroster and airconditioner (ie it needs high current draw off the battery). Further more, The manual states that with only the connection to the battery the analyzer does not know which cylinder is number one! It states this is a rough estimate, and you should then do a manual compression test. The cranking test has the same limitations--not knowing which cylinder it really is.

I suspect that the Pico Diagnostics, is something different than what was used on this boat. But is certainly an interesting concept and for a rough guaging tool it appears to be valid. I would much rather have a full compression or leak down test if I was concerned--and that is the reason I hire a mechanic to do the engine survey. I think I'll stick to ultrasonics--they seem more "logical"...

Incidently I understand the insullation piercing probes, but the acupuncture --that is going a bit far--but I hope it is not human body related...

I would ask the surveyor what he did and educate all of us.
 
Got specific psi per cylinder, ad the #3 is at 152psi, vs 182psi for the #8, so even if the compression is high enough, there may be too much variance. The range is 20%, when it should be 10%. The #2, #4 and #5 are all in the 150spsi, suggesting to me there might be leakage in the head gasket. Will get an mechanic's opinion on this.

BC S
 
taking compression readings off a gas engine isn't hard and doesn't take long. it's a critical step, if the surveyor can't do it then any mechanic shoudl be able to.

i think this raises questions about the rest of the survey ?

I hope you picked the surveyor on your own, not use a broker recommended survey and that you were there during the survey.

did he check the drives ?

if any work is done by the seller/broker, i woudl make sure that I or my surveyor stop by to see the work in progress and then retest compression / do a full sea trial after the work is done.
 
Thanks for the analysis, Bob.

My reaction, after looking at the web site for Pico Diagnostics, is that this is just another one of those ideas, or fixes, that belongs in a file entitled PANACEA, and as we all know, there's no such thing as a panacea, except for when it comes to boats and boating, and then there's something for everything.

If the diagnostic capabilities of this tool were so reliable, then everyone in the yacht and boat industry would be using it, and that is just not the case. Why go through that process when one will or must compression check the engine anyway. It just does not make any sense (to me).

It seems that a "proper" compression testing was performed and that all readings fall within the generally acceptable 10% range. Should be good to go now, eh? :)
 
I use an engine analyzer when doing engine surveys. Not the Pico scope mentioned, but an OTC Vision Premier PC based engine analyzer. They are expensive and very few techs have one, but that does not mean they are not without value as a diagnostic tool. In fact, they are a "must have" tool (in my opinion).

Using an engine analyzer, "virtual" compression can be tested a couple of different ways. In these tests the analyzer knows/tells you what cylinder it is measuring because you select the firing order of the engine your testing and there is an inductive clamp that goes on the #1 cylinder.

First method for testing virtual compression would be veiwing the secondary ignition firing voltages. Compression in the combustion chamber makes it more difficult to iniate a spark across the plug gap, and the firing voltage required will increase. So it follows that a cylinder with low firing voltages may be low on compression.

I have found that firing voltages alone are not an accurate way to test compression. While looking at firing voltages it is easy to see a cylinder with no compression, but a cylnder with med/low compression cannot be reliably identified. There are too many other factors that can affect firing voltages (worn plugs, wires, ect.). For example, which cylinder on the pic below has 100PSI while the others are between 140 and 150 PSI?

Scope01


The second method would be Cylinder Balance Cranking test, which does not measure voltage, it measures amps through an inductive clamp on the starter cable. More compression = more amps required to drive the piston up.

You could also do a Cylinder Contribution test, which kills each cylinder individually and then measures the RPM drop (only valid with engine under load). But, as mentioned above, there are more factors than just compression that can contribute to individual cylinder performance.

All of that said, I still do a conventional cranking compression test on every engine. Even if compression could be measured with 100% accuracy with an engine analyzer, a visual inpection of the spark plugs is important in detecting water intrusion and other problems.

Dave
 
Ive been using a 12 volt powered scope for many years now. The scope i use performes live data testing on clyinder balance and wave form testing on alternator voltage. Clean voltage is a wonderful thing, An alternator can be charging and have spiked voltage that can harm electronics. By looking at a parade of clyinders you can see where there maybe a problem. Some of the vessels out there today are a nightmare to get to all the plugs. If there is a clyinder that shows low or thats out of range, you can go to it and perform further testing there. Also you can take a compression test on a clyinder which is easy to get to and make a determanation on where your at on pressure. There are many companys out there that sell these diagnosic tools. Some of us have been using them for many years now.
 
Newportdave,

Assuming each plug has slightly different gap, wouldn't that cause an error in determing compression differences by measuring the voltages?
 
If what you want is a compression test, do a real compression test. Anything else can be easily fixed ie. bad plug, wires, cap, etc. Will the electronic testers show something, yes they will, but it doesn't always mean there's low/bad compression. They will detect a problem, then you have to do a manual compression check in each cylinder to eliminate bad compression as your problem and work on it from there.

Gary Washienko
Sound Marine Survey
 
Follow-up. Well I did have a proper compression test done. The psi and range is supposedly 'within Mercruiser Spec,' but the mechanic and surveyor says the engine will need a valve job or rebuild within a short period. I negotiated a price reduction and am gettin the manifolds and risers replaced right now, which should keep the problem (possible sea water invasion to affected cylinders) in check until winter, when I will have the heads pulled and all findings addressed.
 
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