We bought a 1996 4.0 with 230k miles. Ran fine on test drive but on way home it overheated badly, I'm not sure how long the temp gauge was in the red as my daughter was driving but it lost power and wouldn't start until it cooled down. We got it towed home and now it's been running on less than 6 cylinders.

I pressure tested the cylinders cold and 4 registerd almost no pressure and the back 2 registered 150 pounds. I'm sure the head gasket has gone but given the almost zero pressure would you agree the head is cracked too or would the cold test give me this kind of result? 2 of the spark plugs were destroyed and missing bits of the tip. When it ran after the overheating it was smoking white from under the exhaust manifold and the exhaust pipe was pumping out white smoke.

Any thoughts or tips would be appreciated as I'm part way dismantling the engine to get the head off.

Thanks

If it ran for an extended period at ultra high temperatures, the engine began destroying itself from all the heat. You will need to look beyond the head to see if there is other damage. At 230k, that engine already had lots of wear and a good long overheat probably cooked a lot of parts. You may want to be ready to shop for another motor.

If it ran that hot for an extended amount of time the engine may well be in need of a total overhaul or replacement. It sounds like at least 2 cyd's are shot and need to be redone from part of the plugs missing.


Do you have any idea what failed that started the chain reaction?

I believe it overheated because of the thermostat. My Daughter described steam coming from the rad cap area, the expansion tank was full and when we drained the system so was the rad. So it didn't seem to lose much water which leads me to think the water pump is ok. She was driving at about 60 on the freeway for 20 mins after she bought it when the overheating occurred.

To be honest I'm not sure I've got the skills/time for a complete engine rebuild and was thinking about just doing the headgasket, new plugs, new thermostat and possibly new head and seeing how it runs. While I've got the head off would you recommend getting it professionally checked for cracks or will they be obvious? I'm assuming its the 0630 head as its a 1996 and the guy had owned it for 15yrs and had not indicated any head replacements in its past. Given its an 0630 head would it still be likely the head is warped/cracked?

While I have the head off would it make sense to carry out any top end upgrades/replacements if I'm not doing the bottom end

I'm a diy mechanic but have never done a complete engine rebuild so would rather not do that at this stage and any further advice would be appreciated.

Yes, it is likely that the head is damaged significantly. Your best bet is to replace the engine with a used one or scrap and buy another Cherokee. Honestly, I think replacing the t-stat and head alone isn't going to be enough. The prolonged exposure to heat has likely damaged the engine beyond just a top end repair.

In addition, and this advice is a little late now, I don't turn over a used vehicle to my daughter until I have personally driven it 1,000-1,700 miles.

Too many things can be wrong, and can go wrong, on a higher-mileage used vehicle. She is not mechanically oriented, so I don't expect her to be looking out for things wrong on a used vehicle. That's my job.

In your case, I would not put my daughter back in that Jeep without a top to bottom rebuild. You said you don't want to fix it right. Put it back together, get it to start, and sell it. Get her something else.

How are you going to feel when it fails on her again at 11 PM on a dark road? And she's by herself and scared?

Rob,
Sorry to hear about your overheating issue; that must have been pretty disappointing for everyone. Not that it would help, but did you let the PO know that the car he sold you you overheated before you even got it home? My guess is that he probably knew...

FWIW, I recently bought a 93 Cherokee and due to the engine's high mileage (over 307K) I have been researching how to replace it: rebuild, buy reman/crate motor, buy used, etc.) and I came to the conclusion that there is no cheap answer.

Personally, I decided on a used engine and was able to find a low mileage (73K miles) 4.0 not far from my house for $500. Before making the swap I expect to put at least another $200-300 into it in misc parts to make sure it's ready to go.

"My" Cherokee is actually for my son who will be heading off into the USAF soon. I am not sure where he will end up being stationed so I want to make the Cherokee as bulletproof as I can before turning it over.

If your daughter will be staying closer to home, going through her Cherokee more incrementally may be a more reasonable option, but based on the info you've provided, that motor may not be worth messing with.