1995 GMC Sierra compression issue

IHC Red

Member
The pickup has the 5.7L and about 163K miles. It needs tires, has just a little rust, but otherwise isn't too bad. It's also an extended cab 4x4, which is more desirable around here. It doesn't leak anywhere.

It has started to miss a lot in the last couple weeks. Sometimes (after it warms up) the miss isn't bad and it runs almost normal. Other times it misses bad.

I've narrowed it down to one cylinder (2nd from front on the drivers side). Compression test shows 20-25psi at idle max 90psi when revving slightly.

I'm trying to figure out it it's valves or rings. I squirted some clean oil into the cylinder and that didn't really change anything.

Any ideas ? If it's rings, I'll probably sell it for whatever I can get for it, since that's going to take some cash to fix. It would only be worth $2000 if I fixed it anyway. I really don't need it anymore (but I will miss it for mostly sentimental reasons because I like the roomy cab).

Thanks in advance.
 
You can get a cheap Cylinder Leak-Down Tester for around $40. That will tell you if you have a bad intake, exhaust, or ring problem.
 
I"ve seen this before, on a friend"s truck of a similar vintage. One of the stamped steel rocker arms had broken at the pivot socket, and that valve was not opening.

Pull the valve covers, disable the fuel pump relay, and observe all the valves as the engine spins over with the starter. A valve that operates differently from the others will easily stand out.

socket that engages the pivot stud
 
first thing to do is get the compression test done right, till then its anybody's guess at the problem. remove all plugs and check each cyl. with about 4-5 compression strokes. a good engine and you will be at top compression in about 2 strokes. if that cyl. is low then pull valve cover as mentioned and look at rockers while cranking. a worn lobe will cause valve to not open or just barely move. nothing unusuall for the cam to be worn on a 350 espessially ones that idle lots.
but it can be any thing from a bad plug, wire, cap , rotor,ext.
you said yourself sometime it runs normal, and valve train will not do that. when its worn its worn.
 
Also block open the intake (throttle plate) or hold down the gas pedal. With it closed there will be less air entering the cylinder giving the same problem originally discussed. Jim
 
Your compression test readings don't make any sense. Cranking is always higher than running. Do dry cranking and record the readings. If that isn't sufficient do wet cranking. If that helps but isn't conclusive, do running compression. You have to do them in order to understand what is happening. Gerard
 
i knew you would come through jim, thought of that and didnt want to repost, then thought jim will catch it.
now i wish i knew if i will win the lottery tonight...haa ha.
 
I find a lot of broken valve springs on these . Pull valve cover ,back off rockers , check springs. while you have it to that point ,do a cylinder leakage test . You will be able to pinpoint exactly where your lacking if the valve spring is ok ,,,oh ,also make sure the rockers are moving ,,bad cam lobes possible ..
 

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