PCM 5.7L Compression-High Pressure Test Questions

[GTNY]

I had a compression test done on a PCM 5.7L engine. The results were as follows:

1 190
2 190
3 195
4 230
5 220
6 220
7 190
8 190

Two things about these results I have questions on:

- Master Engine Specs of this engine (attached) show compression pressure range from 130-215 psi. Are my 220/230 readings too high, and why?
- The differential in pressure between cylinders is greater than 10% in several instances (I had read somewhere that the variation shouldn't be more than 10%). Is this bad?

I would be interested in knowing anyone's thoughts on this.

Thanks.

[AirTool]

Did you run two tests? one dry, then squirt oil in to lube the rings and test again? You would like to know if your pressure goes up w/some lube.

Are your 1 to 8 numbers right ? 1,3,5,7 and 2,4,6,8, matching the plug wires?

How did you run the test?

[GTNY]

Mechanic ran the test, I assume only once. I do not know if mechanic ran one dry and one lubed.

The test results did show 1,3,5,7 and 2,4,6,8. I simply renumbered them for convenience when typing them.

Should I have the mechanic run another test? I have not bought this boat yet, but am very close. Is this something I should be seriously concerned about?

Thanks for your help.

[Nautiquehunter]

The only thing I know of that will increase the compression readings is carbon buildup in the cylinders. This can happen if you lug the engine or if you are running too rich. Are you having a running problem?

[GTNY]

I will be taking it out for a lake test soon, so am not sure how it runs yet. If there is carbon build up, what is the best way to address this?

[AirTool]

The 230 seems a bit high. But I would think it would take a lot of carbon to displace enough volume to raise the pressure that high.

I've not checked my boat before, but my Chevy Tahoe 5.7 showed all between 195 and 205 on a dry only test....did not do the oil. I didn't do the wet test because the numbers were so good.

A squirt of oil might get your 190's up which would indicate some wear in the rings which everyone gets eventually. If he ran two test, he probably would have given you both sets of numbers. like 190/205, 205/205

Another question is whether he had the adjacent plugs in or out.

Except for 3, it seems odd those interior cylinders are high.

Was the engine hot when he ran the test?

Does this engine have a fresh water cooling system?

What year and how many hours?

There are some old school tricks for the carbon, but let's not go there yet.

There is a very good post somewhere on compression tests...maybe the best one from Mikeski...can't remember.

[GTNY]

Thanks AirTool.

I doubt the engine was hot since it was done at the dealer, a decent distance from the lake.

This is a 2005 engine with less 170 hours. I can't answer your question on the fresh water cooling system, and don't know whether adjacent plugs were pulled.

Should I have the mechanic do a wet test?

[AirTool]

Thanks AirTool.

I doubt the engine was hot since it was done at the dealer, a decent distance from the lake.

This is a 2005 engine with less 170 hours. I can't answer your question on the fresh water cooling system, and don't know whether adjacent plugs were pulled.

Should I have the mechanic do a wet test?

You could ask to see the spark plugs to see if they are oily or black...but he might have (should have) cleaned them before he put them back in.

My gut feeling (for this boat) is if the engine doesn't knock on cold start up, doesn't knock under load, doesn't obviously emit blue smoke....then it is just fine. Likely the mechanic doing the test was not striving for perfection and may not have consistently done the same procedure for every cylinder. The block and heads not being hot can also make a difference.

I pressure tested a cooling system on a chevy car one time when the engine was cold and antifreeze very slowly seeped out the heads. With the engine warm (not boiling water hot), it would hold pressure for over thirty minutes without budging. The block and heads expand when hot and the head bolts become very stiff springs. The engine is a different animal hot than cold.

[Nautiquehunter]

Agree with airtool. I would be more concerned with low readings rather than high ones. Its possible the compression gauge that was used was not working correctly. Test drive it and see how it runs .

[GTNY]

Thanks Airtool and Nautique Hunter. I have test scheduled for next week and will listen for knocks and look for blue smoke when I test. I truly appreciate your insight. I am very excited about getting this boat, just want to make sure not getting a lemon.

Thanks!!

GT