Hey fellas,
I have a 1996 xj with the 4.0. Has 194,000 miles on it. I've had it for 4 years and it has been great.

I've been tracking down what feels like a slight miss when idling (almost feels like a heart beat). The jeep has done this since I bought it, but I didn't think much of it. I know have a little time this winter and thought I'd check it out. No codes other than a downstream 02 sensor. I replaced both 02 sensors, knowing the upstream could cause issues. Other than that no codes. I'll admit I got a little part happy and changed all my spark plugs, wires, cap & rotor, new 4 hole injectors, and cleaned all my grounds. My excuse was it's well needed basic maintenance parts. What I'd been neglecting to do was check compression.

I've now checked compression but will admit I'm not sure what all the numbers tell me when compared to each other. I did a dry test first, with engine warm, all spark plugs removed, and jumped the starter. Cranked 3 times before taking measurements.

Cylinder 1-90psi, 2-121, 3-119, 4-139, 5-125, 6-144

I then added a slight amount of oil to each and retested.

Cylinder 1-100, 2-135, 3-141, 4-157, 5-144, 6-n/a

I wasn't able to get oil in 6.figured it was the highest and probably fine. My question is what are these numbers saying? I assume several issues.

Thanks in advance

 

Isolating the cause of low compression without a tear down can be done with a Leak Down Test most of the time.

 

Just to get a more accurate readings...warm engine like you did, ...remove fuel pump relay....jam throttle body wide open. I usually crank each for 5 sec. See if your compression comes up a little. If your slight miss is the only issue and your oil pressure is still good. I would keep running it. Any and all repairs involve at least pulling the head for a head gasket change. I got a very nice rebuilt head for around 400 bucks to fix my constant #2 misfire code. I had excellent compression across the board...approx 180 But its a stroker. I figured for the amount of time needed to pull the head and my DD, it was worth the extra cost to do the head and gasket at the same time. Its a PITA to remove that head. No issues since...been 8 or 9 months now. One simple way to test blowby through your #1, or any cylinder is to remove both breather hoses and install some clear vinyl tubing a foot or so long. That much pressure loss thru the rings should increase crankcase pressure quite a bit and air/smoke /vapors should be blasting out of those hoses and easily visible. If no real blowby, then your low compression is not caused by ring bypass.