No compression in two cylinders

JBROO222

MEMBER
1994 GMC SIERRA
5.7L
V8
2WD
MANUAL
180,000 MILES

Following an issue of a clogged muffler, and then cutting the muffler off, I performed a dry compression test. I found out I have no compression in cylinders 7 and 8 and all the other cylinders have approximately 140 psi (on a cold test). I then did a wet test on the two in question with no change in the results. What could be the most likely problem? The truck will not start and it backfires through the TBI. I have since tried blowing into it with compressed air and can hear a hissing sound when I open the throttle.

7 Replies

BMDOUBLE

MECHANIC
1,139 POSTS

The issue is likely a valve-train issue on the head associated with those valves, burned, sticking or bent intake valves is what it sounds like.

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Saturday, October 12th, 2019 AT 12:46 PM

JBROO222

MEMBER
4 POSTS

Thanks for your reply. I posted some more information on my question. But I think you may be right.
I am going to start taking the valve covers off. What should I be looking for?

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Saturday, October 12th, 2019 AT 1:32 PM

BMDOUBLE

MECHANIC
1,139 POSTS

The only thing you can verify with the valve covers off is the valves being stuck, the rockers will be loose when they are all the way off of the cam lobe. You can also remove a spark plug and with shop air and a nozzle, blow into the plug hole with the valves closed and listen for air escaping sound coming from the intake and exhaust. Any other test will require head removal and with 0 psi compression I assume that the inspection will reveal something obvious. Let us know what you find!

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Saturday, October 12th, 2019 AT 4:23 PM

JBROO222

MEMBER
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Hi, I do plan on taking the heads off, upon removing the valve covers I found that the rockers and push rods on the two back cylinders, were in bad shape from rust, and gunk build up. It does not look like they were getting any oil. The motor has sat for awhile before I bought it. I will let you know what I find after I remove the heads. Thanks.

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Saturday, October 12th, 2019 AT 6:31 PM

BMDOUBLE

MECHANIC
1,139 POSTS

Okay good! I worked on one last year that had sat for a long time as well and had stuck valves. I was able to get them unstuck by spraying seafoam on top of the valve stems and heated up the valve stem areas with a heat gun on and off for a few days in between other cars I was working on until they finally came up, but the owner didn't have the funds to remove the heads so I took my time little by little to help him out, and he just wanted the car to run good enough until he could get another one.