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Engine compression test Hot or Cold

S

scott_duerring

Regular Contributor
Can anybody answer that questi

Can anybody answer that question? A mechanic just performed a compression test on my engine when it was cold. Are the numbers he got of any value as I thought the engine should be hot when you test it?
 
"Some do it cold ,some do it h

"Some do it cold ,some do it hot,some do it cold then hot.What were the numbers when he did it???"
 
I do not have all of them but

I do not have all of them but he said the one and three cylinders were at 165 with the other six falling within 145 to 155 range. He said that he thought the higher numbers for 1 and 3 were caused by oil leaking past the rings as he said the plugs looked like they were oil fouled. I would think that the compression would be lower in those cylinders??
 
"The reason some want to do it

"The reason some want to do it after its warmed up is there usally hoping the numbers will go up.
those numbers look fairly respectble to me.
And I agree with you the compression would be lower if oil was getting past the rings,at least I would think so.
It sounds like you got pertty good compression to me."
 
"GM put out a notice in the la

"GM put out a notice in the last century that stated doing the comp. check cold was the way to do it, for all of their products.

No cylinder under 100 PSI. All cylinders within 30% of the max value. sounds like your numbers are well within the acceptable range."
 
"Thanks for the information, I

"Thanks for the information, I am wondering what then would cause the plugs to get oil fouled? Do I need to run a hotter plug in those cylinders? Is it wise to only bump up the heat range in a couple of plugs? I have a rough idle and am thinking that this may be contributing to that."
 
"Are you sure it is oil foulin

"Are you sure it is oil fouling? Oil fouling usually leaves the plugs with a wet look.

A rough idle due to an over-rich mixture will give you a carbon colored plug. If its on the verge of flooding, the plugs may be wet, too. Possible to confuse the two without good lighting and some first-hand experience. A sniff test can also help in distinguishing the two.

I wouldn't change the heat range of the plugs. You need to verify the problem and then select the fix(es). A good inspection of the combustion chamber can help too.

Besides the oil ring not seating, valve guides & their seals are known contributor to excess oil in the combustion chamber."
 
Is there any way to get the oi

Is there any way to get the oil ring to seat properly short of a rebuild? I had a complete valve job done several years ago so I don't think that is it. Can I have good compression and still have oil leaking into the combustion chamber? I have electronic fuel injection (MPI)so I do not think it is an over-rich mixture. The plugs do look oily and dont smell like gas.
 
"..."Can I have good compr

"..."Can I have good compression and still have oil leaking into the combustion chamber?"

You bet, via the intake valve guides. The seals tend to harden over time. Using an air chuck and the right valve spring compressor, you can replace the seals in place. And don't waste your time on changing the exhaust seals--won't do a thing.

Jeff"
 
"Yes, and if they individual t

"Yes, and if they individual that did it was on top of his game, he would have used a positive seal instead of the cheap umbrella seals. The guides would have also been replaced (vs reworking or knurling them).

My findings on big block chevy's different from Jeff's. The exhaust guide seals DO make a difference; more so on an used engine.

Pull your receipt(s) from the valve job and see if the seals were identified separately. If not, you received the default umbrellas and should follow Jeff's spring compressor recommendation."
 
"Maybe but not likely. You can

"Maybe but not likely. You can buy them in stand- alone sets (vs whole or partial gasket sets), if needed. I'd be willing to bet you could visit your favorite machine shop and get the umbrellas given to you. Same shop should be able to provide you a positive seal but not at the same price."
 
You can hold the valves up by

You can hold the valves up by feeding some clothes line rope into the cylinder and running the piston up against it to hold it in place. I also have some adaptors to use air if there is enough pressure/volume.
 
"I got the numbers for the cyl

"I got the numbers for the cylinders, in order from #1, 175,160,175,150,150,165,169,155, he noted that cylinders 1 and 3 were "wet with oil" ..."
 
"just an editorial comment fee

"just an editorial comment feel free to jump in, but the same mechanic that did this test just charged me eight hours of labor to replace a set of front engine mounts on one engine... seems to be alot...any one have info on what the "book time" is on this through Crusader?"
 
"Scott:

Sounds like Dave ha

"Scott:

Sounds like Dave has a deal...

I don't think you'll find a "book time" from Crusader...too many variables driven by the hull.

I can tell you on my 'open express style' I changed both front mounts but after digging out the sheared off 1/2" stainless lag bolts. I also renewed one of the mounting pads. I did this over the course of about 10 calendar days due to family commitments. I think i had about 6 hours, total, involved, including the alignment check. I could see it easily expanding if you had to work under a fixed deck or work around another obstruction."
 
"Thanks for the info, guess it

"Thanks for the info, guess it is always easy to "armchair" things, one of the mounts had very easy access but the outboard side would've been the bugger to work on.
Dave, whatdaya want for them?"
 
"how about 15 PostPaid? THese

"how about 15 PostPaid? THese are the tan, 454ci valve seals from Felpro. Full set. PayPal?"
 
The same valve will go in eith

The same valve will go in either engine as long as it is stock - no custom mods.
 
"Still confused over the issue

"Still confused over the issue as to where the oil is coming from, guess I want to believe the machine shop did its job right and the valve seals are in good shape. So one more question, With the cylinder reading 175 psi with a cold compression test and noted "wet with oil" does that reasonably rule out worn rings and point to a leak in the valve guides? Considering everything I would HOPE it is something as simple as the seals. Does the fact that the adjoining cylinders ie, 1 and 3 have the same issue have a bearing on the diagnosis? I understand when you have adjoining cylinders exhibiting the same problems that is an issue with the head gasket or the head itself, but again, two years ago everything came off from the head on up, new gaskets, valve job... so is it the valve seals or the piston rings?"
 
"With those compression number

"With those compression numbers, I'd say it is highly unlikely your rings are worn beyond the service limits...

Adjacent cylinders having the same issue(s) can be due to the head gasket but not always. You've got to look at all the clues you can collect and formulate a likely cause, verify it, and then fix it. Looking at your numbers, you could say #7 is high, too.

If 1 & 3 are leaking oil thru the guides, those cylinders will have carbon build up accumulate faster than the others. Do you know which type and brand of seals were placed on the guides?

Being as the seals are a much easier & cheaper job than the rings, the cost benefit trade indicates doing the seal swap won't cost you much more than time..."
 
I cannot tell from the invoice

I cannot tell from the invoice which type and brand of seals were used. My invoice is from the marina and it just lists a price from the machine shop that they used to do the valve job. I was told by the mechanic that the shop they used specialized in big block high performance engines so one would hope that they used the better seal...
Is it possible to remove the valve cover with the exhaust manifold and riser still in place?
 
You should be able to call the

You should be able to call the machine shop and ask....most are very thorough and keep records with excruciating detail.

50/50 on the cover...depends upon how much 'stuff' is in the way. If you need to remove the manifolds - don't. Just loosen the bolts and let them lean out. it usually gives you enough access without having to mess with the coolant. It may be helpful to remove the outermost two manifold bolts and temporarily use two that are ~ 1/4" longer.
 
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