Compression TESTING!!!
Thread Starter
Cruising
Joined: Jun 2022
Posts: 12
Likes: 2
Compression TESTING!!!
Greetings all!
Just recently did a compression test on my 2004 Corvette,
All cylinders except for one read 180 PSI after about 4-5 cranks with throttle closed.
There was ONE that cranked to only 165, and quickly leaked pressure.
Should I be concerned?
Thank you for the assistance In advance.
Just recently did a compression test on my 2004 Corvette,
All cylinders except for one read 180 PSI after about 4-5 cranks with throttle closed.
There was ONE that cranked to only 165, and quickly leaked pressure.
Should I be concerned?
Thank you for the assistance In advance.
Melting Slicks
Joined: Jul 2005
Posts: 2,649
Likes: 863
The general rule is that cylinders should to be within 10%. 10% of 180 is +/- 18, so the 165 is "OK".
The compression gauge should have a valve that only lets pressure "in" - till you hit the button to bleed the pressure out - so the fact that it leaked down quickly is a compression gauge thing - not an engine thing.
I wouldn't be thinking about pulling the engine - I would probably repeat the test in 10K miles - and see if it gets worse. IMHO - the next step would be to do a leakdown test, and pay attention to where the air is escaping from - that should tell you if it's a valve thing or a ring thing.
The compression gauge should have a valve that only lets pressure "in" - till you hit the button to bleed the pressure out - so the fact that it leaked down quickly is a compression gauge thing - not an engine thing.
I wouldn't be thinking about pulling the engine - I would probably repeat the test in 10K miles - and see if it gets worse. IMHO - the next step would be to do a leakdown test, and pay attention to where the air is escaping from - that should tell you if it's a valve thing or a ring thing.
Thread Starter
Cruising
Joined: Jun 2022
Posts: 12
Likes: 2
The general rule is that cylinders should to be within 10%. 10% of 180 is +/- 18, so the 165 is "OK".
The compression gauge should have a valve that only lets pressure "in" - till you hit the button to bleed the pressure out - so the fact that it leaked down quickly is a compression gauge thing - not an engine thing.
I wouldn't be thinking about pulling the engine - I would probably repeat the test in 10K miles - and see if it gets worse. IMHO - the next step would be to do a leakdown test, and pay attention to where the air is escaping from - that should tell you if it's a valve thing or a ring thing.
The compression gauge should have a valve that only lets pressure "in" - till you hit the button to bleed the pressure out - so the fact that it leaked down quickly is a compression gauge thing - not an engine thing.
I wouldn't be thinking about pulling the engine - I would probably repeat the test in 10K miles - and see if it gets worse. IMHO - the next step would be to do a leakdown test, and pay attention to where the air is escaping from - that should tell you if it's a valve thing or a ring thing.
So it was purely the gauge that leaked the pressure? I'm not so sure, because it was the only cylinder that quickly dropped pressure immediately after i stopped cranking.
Again, appreciate the quick response and helpful insight.
Melting Slicks
Joined: Jul 2005
Posts: 2,649
Likes: 863
If the compression gauge did not "hold pressure" - the gauge readings would drop the instant that the piston headed back down after completing TDC on the Compression stroke.
Most of the time when doing a compression test - you'll see the gauge jump to about 70% of the final reading after the first revolution through the compression stroke, and move up a bit more with each successive two revolutions. Buy the time you've run through 4 - 5 compression strokes - the gauge doesn't move any higher. If the gauge didn't "hold" pressure - it wouldn't act that way...
It is certainly possible that the "bleed off" valve of the compression tester wasn't quite fully seated when you tested the one cylinder. That would explain what you saw,....
Most of the time when doing a compression test - you'll see the gauge jump to about 70% of the final reading after the first revolution through the compression stroke, and move up a bit more with each successive two revolutions. Buy the time you've run through 4 - 5 compression strokes - the gauge doesn't move any higher. If the gauge didn't "hold" pressure - it wouldn't act that way...
It is certainly possible that the "bleed off" valve of the compression tester wasn't quite fully seated when you tested the one cylinder. That would explain what you saw,....
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You should never trust compression test readings if the throttle is closed. Every publication I've ever read, and every quality mechanic I've ever spoken with, ALL say the test is not valid if the throttle is not open. Most hold it wide open, because it's the easiest way. It would probably be OK if you're at least 1/2 throttle, but why chance it. Just get a buddy, wife, girlfriend, etc, to hold it open when you crank it. With the throttle closed, say at the idle position, there's not much air entering the intake. You can't compress what isn't there!! 
Le Mans Master
Joined: Aug 2005
Posts: 7,392
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From: Merritt Island, FL
You should never trust compression test readings if the throttle is closed. Every publication I've ever read, and every quality mechanic I've ever spoken with, ALL say the test is not valid if the throttle is not open. Most hold it wide open, because it's the easiest way. It would probably be OK if you're at least 1/2 throttle, but why chance it. Just get a buddy, wife, girlfriend, etc, to hold it open when you crank it. With the throttle closed, say at the idle position, there's not much air entering the intake. You can't compress what isn't there!!
Burning Brakes
Joined: Nov 2021
Posts: 774
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From: Old Dominion - VA
Not sure how you were cranking it and I am no expert by any stretch, but if you stopped cranking and a valve was open it would leak down, but the gauge itself would hold the highest pressure achieved, unless the gauge itself had an issue and was not holding pressure. Personally I would recheck it and make sure the gauge is not leaking down.
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Yeah, but if there was enough flow around the closed throttle plate to allow for 180 psi to build in 7/8 cylinders, then a closed throttle plate wasn't the cause for the 8th cylinder to only build 165. All 8 cylinders had equal airflow, albeit somewhat restricted by a closed throttle.