boostd4

I had the owner of the car I'm looking to purchase do a compression test on the car after it appeared to lose a cylinder (This is a 99 LS1 shortblock with GTP stage 2 5.3L heads). There were some tapping noises, then it went away, and I'm guessing it was either a stuck spring/valve or siezed lifter. Anyway, before assuming it was something cheap like that, I wanted to make sure, and had him do a compression test.

Seven cylinders all came within 150-160psi, and one cylinder had ZERO psi. The owner's mechanic said he's 99% sure it was a spring hanging up and not letting the valve close fully. I have a few questions:

1) Shouldn't good compression on a 9.x:1 stock LS1 be 180-200psi?
2) Shouldn't the 10.x:1 C/R of 5.3L heads make you expect higher than 180psi?
3) Is 150-160psi a sign that it's got blowby and needs to be rebuilt?
4) Is it possible for a spring to hang up and not let the valve return to the seat without the piston smacking the valve?

Thanks. I still want to buy the car because it's setup the way I like. I don't mind having to rebuild the motor if necessary, but we need to negotiate a new price if that's the case.

Thanks again!

boostd4

Anybody?

Basically, would you buy a car with intention to drive it/race it occasionally with these numbers, or would you assume that it's going to need a rebuild soon?

ryarbrough

iTrader: (18)

I wouldn't be concerned as long as all the numbers were even (at 150-160).

I would not buy the car with one cyl down like that though. Holes in the pistons often cause zero to come up during a compression test. Let him fix it and then you buy the car.... Don't inherit someone else's problems.

bjamick

iTrader: (31)

A leak down test will tell the tale.

S1LV3R

iTrader: (5)

The cam design can effect the cranking compression. Obviously, it won't give you the zero you saw.