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How close should compression stats be?

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crendel

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We are looking at buying a 2006 Larson with a 5.7 gi motor. The compression stats com back 170, 170, 140, 170, 145, 165, 170, 170. The engine surveyor was concerned about the lower compression on cylinder 3 & 5 and suggested it might be a head gasket. The owner took it to another shop that did a carbon test and a leak test and could find no problems. The surveyor is saying that a 25% difference is normal. Should I be concerned about buying this boat? It has 259 hours and the survey has come back very good. The boat had no problem getting up on a plane.
 
The Surveyor saying that 25% is acceptible is mistaken.

For a rather healthy SBC engine, the spread between the highest and lowest cylinder pressure reading should be no more than 10%.
10% of 170 = 17 psi.
170 - 17 = 153.
The lowest reading found was 140.
That equates to 13 psi out of the suggested allowable tolerance.

Do you know how the readings were taken?
Was the engine warm or cold?
Did the surveyor give the cylinder being tested at least three complete engine cycles? (this may mean 7 crankshaft revolutions per)

.
 
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FWIW, GM changed their recommendation on what constitutes acceptable compression test results and also endorsed "cold compression tests". Like Rick asked, "how was the test conducted" is really critical to assessing the results.
 
The readings were taken both cold and warm. They did at least 8 revolutions. The engine surveyor is thinking it may need a new head gasket.
 
crendel said:
The readings were taken both cold and warm. They did at least 8 revolutions. The engine surveyor is thinking it may need a new head gasket.
Click to expand...

Of course, he needs the money! I think a new head gasket would show larger differences that what your showing. Could just be crud build up on your valves. Try decarbonizing it first. Then do another comp test as described. Dry first and then wet and post your numbers back. Also try and correspond the PSI number to the cylinder.
 
crendel said:
....... The engine surveyor is thinking it may need a new head gasket.
Click to expand...


Without performing a cylinder pressure "leak-down" test, he is guessing at this!

As with any proposed work scope, ask for and be willing to pay for full and proper diagnostics up front.

Only upon the diagnostics clearly definining the issue, and only upon a written work scope "quote" (not an estimate) that also defines that the scope will correct the problem, would I be willing to move forward.
Otherwise, I may be paying for their mis-diagnosis that lead to unnecessary parts and labor.



.
 
A leak down test was done. No leaks. They decarbonized it and took it out on the lake. Cylinder 3 & 5 was 140 and 145, after decarbonizing it is 145 and 150.
 
crendel said:
A leak down test was done. No leaks. They decarbonized it and took it out on the lake. Cylinder 3 & 5 was 140 and 145, after decarbonizing it is 145 and 150.
Click to expand...
Your #3 and #5 cylinder pressures came up a tad bit. I'd be willing to bet that #s 1, 2, 4, 6, 7 and 8 also came up a tad bit.
IMO, #3 and #5 are still unacceptible when you consider that your highest reading may now be 175.

kimcrwbr1 said:
Are 3 and 5 next to each other?
Yes, on a 5.7L SBC engine, cylinders #3 and #5 are adjacent to one another.


If so the head gasket should cure the issue?
A well excuted cylinder leak down test would have revealed a head gasket breach between these two cylinders.
However, the OP mentioned that the test revealed no issue.

I find that the NO Breach between cylinders may be accurate, but I find it hard to believe that the test did not point to piston rings and/or valves.
Reason: something is causing the low cylinder pressures at the #3 and #5 cylinders.

Either way you need to at least pull the heads to go any further on the diagnostic in my opinion.
Before removing the cylinder heads, do the cylinder pressure leak-down test again.
This time around, pay more attention to escaping pressure that may be getting past piston rings, and/or past the valves!

IOW........ if there is NO head gasket breach, something else is causing the rather low pressures.
Click to expand...
 
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