1989 GMC S15 low compression on two cylinders but still running

vikasintl · November 9, 2015

1989 GMC s15 automatic 4 cylinder ...

I found that there is very low compression in 2nd and 3rd cyl. like about 5 psi but 1st and 4th cyl. have 125 psi

is 125 psi ok or it is sill low?

also, truck was running ...can it run on 2 cylinders?

and is there any other reason why it shows 5 psi in 2nd and 3rd apart from either valves or lower engine problem?

00Silverado4x4 · November 9, 2015

Can it run on 2 cylinders, yes, but it'll run like crap. The part you need to find out is if it's your valves or your pings or some combination of both. You squirt a little oil into the cylinders that are low and recheck. If compression comes up it's likely your rings, if not, then it's likely to be your valves. That's where I would start.

Badass69 · November 11, 2015

Head gasket is blown between 2 and 3.... almost a guarantee.

vikasintl · November 11, 2015

Head gasket is blown between 2 and 3.... almost a guarantee.

You are absolutely correct...found that head gasket is damaged in between 2nd and 3rd cyl. ..you did not say if 125 psi is ok on other 2 cylinders? ...head gasket was ok on those 2 cylinders...

Badass69 · November 11, 2015

Good enough to run if it's consistent with 2 and 3 once you repair the gasket sure. Would not surprise me if you find the deck surface of the cylinder head to resemble a banana thus the reason the gasket blew.

Sent from a potato

vikasintl · November 12, 2015

Good enough to run if it's consistent with 2 and 3 once you repair the gasket sure. Would not surprise me if you find the deck surface of the cylinder head to resemble a banana thus the reason the gasket blew.

Sent from a potato

No its not banana shape...I dont know why head gasket blew?

Motor City Rick · August 25, 2017

When any head gasket blows, you always look for the cause. Part of the "cause process" is to look for warpage on the cylinder head (especially aluminum) or the block. Have the machine shop check for surface alignment. Another thing is "always" replace cylinder head bolts (on any motor) and torque them the way the manufacturer want's it done. Seen too many "stretched" reused head bolts let loose not keeping the proper torque in place causing the head gasket letting loose a second time.