! piston on a compression test shows 30 psi!
Thread Starter
Le Mans Master
Joined: Oct 1999
Posts: 6,366
Likes: 34
! piston on a compression test shows 30 psi!
I have a very sick 350 here. over the last week it has been missing when I start it up, but it would go away after about 30 seconds and was running fine. About 2 days ago it stopped clearing it self up and I cant stop it from missing. I spent half the day replacing the usual ignition suspect. nothing changed. I checked the valves for broken valve springs and movement. they looked fine.
So last I decided on a compression check. I only checked 4 pistons, the first 3 put out 165-170 PSI, the last one I checked put out 30 tops. I tried it over and over and sometimed came up with no compression.
I ask you all, a cylinder with 0-30 psi compression must have a big problem right? What should I expect to see? A hole in the piston? Busted valve? Although it turns over and runs? Would dead rings be my problem?
Seems strange that it kind of started out as a minor problem....
I wont have the heads off till probably friday night, so I wanted to hear what you all think.
So last I decided on a compression check. I only checked 4 pistons, the first 3 put out 165-170 PSI, the last one I checked put out 30 tops. I tried it over and over and sometimed came up with no compression.
I ask you all, a cylinder with 0-30 psi compression must have a big problem right? What should I expect to see? A hole in the piston? Busted valve? Although it turns over and runs? Would dead rings be my problem?
Seems strange that it kind of started out as a minor problem....
I wont have the heads off till probably friday night, so I wanted to hear what you all think.
Race Director
Joined: Nov 2000
Posts: 11,872
Likes: 12
From: Waterloo ontario Canada
Re: ! piston on a compression test shows 30 psi! (bill mcdonald)
Guessing is not going to help. You will have to pull the engine down to check. You could do a leak down or add air to the cylinder to see if it's comming out the intake, the exhaust or breather. I have a fitting that screws in the spark plug hole and lets me put the 135psi air line in. Maybe it would help pinpoint where the leak is.
If you get lucky only the head needs rebuilding but if it's a piston you are out of luck.
Maybe now is the time for that rebuild.
If you get lucky only the head needs rebuilding but if it's a piston you are out of luck.
Maybe now is the time for that rebuild.
Le Mans Master
Joined: Oct 1999
Posts: 9,713
Likes: 3
From: Sachse Texas
Cruise-In I Veteran
Cruise-In II Veteran
Re: ! piston on a compression test shows 30 psi! (bill mcdonald)
I agree, do a leak down test to determine where the air is going, and do it bofore you pull the head. I had a ring go out on mine from Nitrous use once (stock shortblock) and just by looking at the ring, I would have never know it was bad. There were absolutely no markings on the ring. However, I used a leakdown to determine the air was getting passed the ring, so I simply reringed the piston and all was well. I did not even pull the motor.
Race Director
Joined: Mar 1999
Posts: 10,258
Likes: 1
From: Calif.
Re: ! piston on a compression test shows 30 psi! (bill mcdonald)
Cracked/broken piston.
Broken ring(s).
Burned valve(s).
My guess is a piston and/or rings since it came on slowly. When it runs do you get a poopieload of blue oil smoke from the valve covers? (crankcase blowby?)
Did you do your compression test "wet?" Did the psi come up or stay the same? Do it wet and you ought to have your diagnosis right there. Confirm with a leakdown.
Either way it's time to start collecting shop rags, I bet.
Broken ring(s).
Burned valve(s).
My guess is a piston and/or rings since it came on slowly. When it runs do you get a poopieload of blue oil smoke from the valve covers? (crankcase blowby?)
Did you do your compression test "wet?" Did the psi come up or stay the same? Do it wet and you ought to have your diagnosis right there. Confirm with a leakdown.
Either way it's time to start collecting shop rags, I bet.
Pro
Joined: Nov 2000
Posts: 678
Likes: 4
From: wilson,nc,usa
Re: ! piston on a compression test shows 30 psi! (The Dude)
i had the same thing happen on my BB about 30psi in one cylinder. When I pulled the head I discovered a blown head gaskit. It blew between the steam holes into the lifter vally. I had no indication of a blown gaskit except a lot of oil consumption.
Jeff
Jeff
