sidewaysil80

I picked up a 2001 Cherokee with an immaculate/rust free body and interior. My plan is to tear it down and rebuild it as a camping/hunting truck and winter DD. I did a wet/dry compression test to see where the motor is at and I'm wondering if I should do a full rebuild or just the head. It's the 4.0 straight 6 and has 152,000 miles on it. For what it's worth it pulls really hard and the truck seems really reliable as is. My logic is if I'm going to be redoing the suspension and investing time/money into, I want the motor to be in peak condition as well.

(In order from cylinder 1-6, wet test was 2 teaspoons of oil)

Dry: 155 160 120 170 160 160

Wet: 165 170 130 180 172 173


Obviously cylinder number 3 is the runt of the litter. My confusion stems from whether its safe to blame it on the head or if it still could be rings. Each cylinder gained about 10psi on the wet test including 3. I would have thought if the rings were to blame for the low compression, the oil would have given it more than the average increase. If I separate the head and the pistons/cylinder walls look good...should I just rebuild the head? Or is this an instance of "buy once, cry once" and just do a full rebuild? The numbers seem really strong so I'm not sure if its a waste to rebuild the bottom end at this point...

Thanks for your time and I appreciate all input!

-JB-

Add a tablespoon of oil into cylinder #3 and re-run the compression test.

If the compression comes UP after introducing oil, you likely have a piston ring issue. (oil seals the rings)

If the compression stays the same, the problem is likely in the cylinder head.

Another extremely valuable test for diagnosing internal problems is the "leakdown test".

These motors can last forever. I wouldn't fully rebuild if you find the culprit for your issue now. I know several running 200,000-300,000 miles on them with no motor work at all.

sidewaysil80

I ran it with oil and it went up 10psi...but so did ALL the cylinders. So since it wasn't a noticeable increase in psi and on par with the other cylinders, I'm leaning towards a valve/head issue. I am going to run the leak down test to be sure though.