[Scotter21]

98 K1500 5.7L has low compression on one cylinder. 110 across the board dry, except #5 is 85 dry and 105 wet. Should this be enough drop to give noticeable performance issues? Rough Idle and Low RPM performance, fine at top end. Have decarboned intake, replace plugs, wires, cap and rotor. Decarboned IAC. Check engine showing random misfire and left bank O2, or on #5 side. Possibly IAC? If I unplug while at IDLE, no change. Ideas/Opinions?
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[ASTE25]

Anything more than a 10% difference is a problem. Low compression can cause your symptoms. Hopefully the cause is up high (like a burned valve) and not low (like a bad piston or ring).

 

[Rogue205]

We just went over this a little whole ago in my engine repair course. Usually a big change for the better indicates a problem with worn or defective piston rings. If it only helps slightly its likely a defective valve.

I am only a student mechanic at the moment, but that info I got straight from the book.

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[swingpress]

Sounds like a bad valve to me. If your able remove the valve cover then rotate engine to put #5 at TDC. Take your compression tester hose and apply air to the cylinder. Place some lite weight paper like tissue paper over the throttle body and tail pipe. Watch for movement that will tell you which valve is the week one.

 

[Scotter21]

Ok, that's my next step is leak testing the cyl. I'm also being told to check with new compression tester. Mine is probably from 1984. I'm told that the performance is too good for the compression numbers I'm posting. I also think I may have a leaking fuel regulator or #5 injector line due to blackened plug. Will leak test and recheck with new gauge. Research is saying comp should be around 175-190.

 

[swingpress]

Your on the right track. Keep up the good fight.