I just did a compression check on my 1992 4.0 Jeep Cherokee and it read 90-100 on all cylinders ,what does that mean?

 

usually when they all ready about the same it means your guage is broken. and motor is fine

 

Usually when the cyclinders read close to the same all the way across, that is a good thing. You're looking for any one of the cylinders to have lower compression than the rest. This will give you an indication that you have problems in that one cylinder. Of course, low compression in all cylinders means your motor is worn out but, even compression and good running, I'd say it's good to go.

 

Yep, even compression is the goodness for sure. I haven't done a compression check on my XJ, but just out of curiosity, what would optimal compression be on a 4.0?

 

150 psi is what my 4.0 at 20k miles read

 

ya he's low 150-180 is good. next step would be a wet compression test. put a couple squirts of oil in the spark plug holes just before testing that cylinder. this forms a tighter seal around the piston rings. meaning if the compression shows signifcant increase the rings are shot. if it only goes up about 10 psi, its not the rings, probably worn valves.

that low of compression is getting close to not running if it is now. i've seen some run around 70 but not less than that.

 

Haynes says 120-150