Ran into something I have not seen before today. Working on a 350 in a 91 Suburban. Had two new compression testers that read way different from each other.

One was a generic no brand one very similar to what Harbor Freight sells and the other one was a new OTC one.

The no name one was showing 68-75 PSI for 7 of the cylinders and 85 for the 8th cylinder. My OTC was showing 145-155 for 7 of the cylinders and 175 for the 8th cylinder.

I then did a leakdown and 7 of the cylinders were between 22-28% and the 8th cylinder was 12%.

I am leaning on trusting the numbers from the OTC over the no name one.

Both testers read 12-15% higher on the one cylinder. So it's probably safe to assume that cylinder has more compression the the other 7, regardless of which tester you want to believe. Or, of course, the other 7 are reading lower for some reason.

A '91 truck engine with 175-185lbs of compression sounds awfully high...like 93octane would ping high...

Depending on why you were testing the cylinders to begin with I would say you found 2 issues.
One is the 12-15% off on the one cylinder.
The much bigger issue, imho, is the unusually high compression IF the 2nd gauge is to be believed.
One thing to consider is that era of Chevys were famous for the plastic timing gear teeth falling off and playing he11 with valve timing/compression.
A quick check for that is to pull the distributor cap and turn the damper back and forth to see how much slop you have before the rotor moves. You ARE going to leave the plugs out from running a compression check right?

I would suspect the “8th” cylinder, the one that reads high having camshaft lobe(s) going south…most likely an intake lobe.

I wouldn’t doubt the need of a new timing set, but that problem would have the same effect on all eight cylinders.

I would probably trust the OTC gauge, their products are usually pretty good quality.

You could rig up some adapters and check the suspect gauge's accuracy with a compressed air and a reliable air pressure regulator for comparison.

This message has been edited. Last edited by: Jerry Kathe,

^^^^both are good advice ^^^^

I'm guessing a wiped cam lobe is what you'll find.

quote:

Originally posted by Jerry Kathe:
I would suspect the “8th” cylinder, the one that reads high having camshaft lobe(s) going south…most likely an intake lobe.

I wouldn’t doubt the need of a new timing set, but that problem would have the same effect on all eight cylinders.

I would probably trust the OTC gauge, their products are usually pretty good quality.

You could rig up some adapters and check the suspect gauge's accuracy with a compressed air and a reliable air pressure regulator for comparison.

Jerry,
That was a interesting test. I did as you suggested and rigged them up to my Mac tools leak down tester. The OTC gauge was dead on all the way to 100 psi. The other gauge was within 5 psi of the leak down tester's gauge, so both gauges are pretty close to each other with air pressure.

This motor is running rough at idle has a hard time not stalling when you first start out, but smooths out above 2000 rpm. It is getting awful fuel mileage and the exhaust has a very nasty smell to it. Vacuum is also low around 12" at idle.

I am also concerned that there is so much leakage in the motor. 25% average is not good even for a truck motor.

The timing chain seemed pretty tight as the distributor moved back and forth with the crank.

The cam is very suspect as are the rings for the cause of the high cylinder leak down.