GPfamlee

Hi all, I recently (last week) got daughter #2 a 1991 5-speed XJ. AX15 (non-magnetic middle plate ?) manual transmission, D30 front diff, Chryco 825 rear diff, 231J T-case. Overall the Jeep is in great shape. Paint could use some help, has usual clear coat fading for its age, but most importantly NO RUST ! Transmission shifts smoothly, and I pulled the diff covers to swap fluids and have a look inside. Good2go. 155k miles, and from what I can tell responsibly adult-driven (daughter #2 will likely change that, haha).

New plugs, plug wires, battery, air filter, 4x4 service, Royal Purple in the transmission, and I checked compression along with the plug/wire change. 3 attempted starts each, about 3-4 cranks each go-round:
1: 90, 90, 95
2: 90, 90, 95
3: 85, 85, 85
4: 80, 85, 85
5: 85, 85, 85
6: 85, 85, 85

From a quick google search, it appears the spec for compression is 120-150. Max variance for my test was 15psi, i believe the max allowable is ~30psi. Wondering if I had a faulty tester (it was an Auto Zone loaner), or user error: i hand-tightened the adapter into the threaded spark plug inlets.

XJ is otherwise very healthy. Starts up without complaining, idles nicely, shifts well, no sticks or clunks when shifting, and goes into/out of 4WD just fine. I'm handy, but not a mechanic. Initial thought is it might be time for a new head gasket ?

fb97xj1

Hold the throttle open (if you werent already) and try it again. If its still low, try another gauge.

BlueRidgeMark

Yeah, that's pretty consistent for a toasted engine, and pretty low for one that's running well. I'd get a second opinion. You wouldn't be the first to get a loaner that's badly off.