Low Compression

Mizz · September 23, 2007

I recently bought a 92 GMC extended cab z71. They told me the motor had a miss (its a v8, 350 i'm assuming but not positive), which it does so I did a little trouble shooting on it. #5 cylinder has about 35psi where the other 7 have at minimum 115 psi. There was no oil on the #5 plug and I visually confirmed it is getting fire. I also pulled the valve cover to watch the valves and both valves are opening and closing as they should, which rules out a worn cam lobe and bad pushrod. I'm assuming a burnt valve is the most likely problem.

With that being said, the motor has 196k miles on it and VERY low oil pressure. I changed the oil and put a quart of Lucas oil in it which raised the oil pressure up to around 15 psi (max) for normal driving. There are absolutely no knocks, pings or rattles coming from it at the moment.

With a motor this old would it be worth just having the heads redone or should I go ahead and have the whole engine rebuilt? Money isn't that big of an issue as I can do most of the work myself. Just want a few opinions from you guys! Thanks in advance.

bobcat5544 · September 25, 2007

A Burnt Valve, Carbon up Valve, or Stuck Piston rings is most likely the cause for low compression. The low oil pressure can be caused by Normal wear or timing chain being streched.

In truth it would be better to rebuild it or get a reman engine. It will problem cost less for a reman unit

ZZ327 · September 25, 2007

have it rebuilt at a good shop, youll have no problem. If money is no problem get yourself a FI 502 crate engine!

Zembonez · September 26, 2007

First: I'll agree with the burnt valve diagnosis. With that many miles a rebuild is in order.

Second: I believe your oil pressure gage is wrong. At 15psi maximum oil pressure you would certainly be hearing "not so nice" noises at idle.

thecraft · September 26, 2007

Second: I believe your oil pressure gage is wrong. At 15psi maximum oil pressure you would certainly be hearing "not so nice" noises at idle.

I'd hope so. 15psi while driving... holy crap!

Mizz · September 26, 2007

Thanks for the replies guys.

At 15psi maximum oil pressure you would certainly be hearing "not so nice" noises at idle.

^Thats what I thought but its still quiet as can be. I've put about 400 miles on it with no problems so far aside from oil shooting out the dipstick tube hehe