compression test using air compressor
Thread Starter
Drifting
Joined: May 2002
Posts: 1,509
Likes: 3
From: Hazel Park MI
compression test using air compressor
I have the plenum, runners, TB, VC's off. Purchasing the wrong springs led me to a few bent pushrods, and it was suggested I make sure the valves are OK. This forum (pete K & others) steered me in the right direction to check cam damage. I didn't hear any valve slapping or anything, just the RRs becoming loose and leading to the rough running condition. From there, I found out the seat pressure of the springs was too low. I have the proper springs in, so my question is can I do a compression check using compressed air with the aforementioned removed? I guess maybe maybe a leakdown test would be more conducive to a correct diagnosis....please educate me.....TIA, John
Le Mans Master
Joined: Sep 2006
Posts: 6,050
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From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
Yes, you can do a leakdown test without that stuff you took off. the cylinders don't care what path the air takes to get to the valves. Just be careful to ensure that the area around the intake passages on the manifold is clear of obstructions and hardware. you don't want anything to be sucked into the intake passages.
A leakdown test is a little hard to do unless you have a method of getting the cylinder exactly on TDC. (Hard to do without a propeller out there to lean on huh?)
If you can do that, you need a leakdown tester.
If you have 75% it is passable, but you really don't know where the leak is unless you can listen at the exhaust pipe or the intake pipe for that cylinder.
If you take the oil cap off, you can hear the air escaping past the rings.
The problem here, is that to do a good leak down test, you need a warm engine, and yours is apart at the moment.
Otherwise, the piston is not at expanded running temps, and the valves and seats aren't hot at expanded running dimensions. ( or at least warmer than ambient)
You will be in the ballpark, just not at home plate.
A leakdown test is a little hard to do unless you have a method of getting the cylinder exactly on TDC. (Hard to do without a propeller out there to lean on huh?)
If you can do that, you need a leakdown tester.
If you have 75% it is passable, but you really don't know where the leak is unless you can listen at the exhaust pipe or the intake pipe for that cylinder.
If you take the oil cap off, you can hear the air escaping past the rings.
The problem here, is that to do a good leak down test, you need a warm engine, and yours is apart at the moment.
Otherwise, the piston is not at expanded running temps, and the valves and seats aren't hot at expanded running dimensions. ( or at least warmer than ambient)
You will be in the ballpark, just not at home plate.
Last edited by coupeguy2001; Apr 15, 2010 at 08:22 PM.
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