1996 jeep Cherokee, 4.0, transmission cooler, 2 core radiator, stock fans, never overheated according to aftermarket temp gauge. Oil is clean, coolant looks bright green, no misses, stumbling or hesitation. Spark plugs look clean & no white smoke.

Backstory: Last week we go out on a trail run with some friends. Jeep is running great with no issues. We pull over due to one of the friends I was with starting to have heat issues with his jeep. Pull off the trail, the second I shut it down one of the heater hoses blows. I spliced the hose back together, add more coolant, burped the system, filled the overflow and I was off without any more issues.

Two days ago, after driving about 10 minutes, I shut it down and coolant starts pouring/boiling out of the overflow. Wtf? Let it do it's thing, waited for the pressure to bleed off, checked the coolant level and it was still good. Drove it around for 20 minutes trying to recreate this problem and it never happened again.

Today: Made the 20 minute drive to visit my grandmother, get to her place, shut it down. About 40 seconds later it starts DUMPING coolant from the overflow and spraying out of the radiator cap. Once again, let it bleed off. Seemed like it took forever with lots of pressure. Took it immediately to my buddy's shop, let it idle in his parking lot for 20 minutes, didn't put out a drop of coolant. Shut it down, still nothing. He pulled it in and tested the coolant for hydrocarbons, show 75+ ppm. We decide the system is being pressurized due to a bad head gasket however, the jeep is still running perfectly. So, I topped off the coolant and boogied home (about a 30 minute drive). When I get home, I shut it down, jump right out and pop the hood expecting coolant to be everywhere. Not a drop. Tried for an hour to get it to boil over again and still can't recreate it.

I'm absolutely stumped here. If it's a bad head gasket, why can't I recreate it? If I'm getting hydrocarbons in the coolant I would normally 100% say it's the head gasket, but this one has me scratching my head. What do you guys think???

 

Replace radiator cap with a good one first. Yours might not be able to hold the proper pressure in the system.

If that doesn't solve it, then a head gasket blown into the cooling system can certainly cause the stuff you are seeing.

 

I'm starting to sound like a broken record on this forum, but ...

I had the 0331 head crack that was opening intermittently under different loads. That made it very frustrating to diagnose. The system would work fine sometimes, but then the crack would open and spew exhaust gas into the coolant system, causing the pressure to spike. Mine boiled over from the bottle as you described a couple of times, but then the system would go back to normal. (I didn't even think to check for exhaust gases at the time because my brain was focused on the cooling system.)

If that's what's happening to you, you can't recreate it because you're not recreating the exact same temp/load situation that you had when it happened. When you let it cool down and heat back up again, it might not open next time even under the same conditions.

Since you're going to have to pull the head off for the gasket anyway, take it to a machine shop and have it checked for a crack. I hope it's not, but cracked heads seem to be a running them with these trucks.

 

The 7120 (91-95) and 0630 (96-99) were not Crack prone, the 0331 (99-01) definitely were.
That said it is not impossible to Crack an earlier head, on any engine if has overheated it is a possibility as is a warped or popped gasket.
As mentioned previously check the cheap stuff first... Thermostat, rad cap. Although the presence of exhaust gasses leads to head gasket issues for sure, the head itself may be fine.

 

Yeah, the only thing I can think of, I'd it seems my radiator cap isn't sealing properly. The neck on the radiator seems worn? Still doesn't explain the presence of HC though without the head gasket being blown.

 

While that's true, the symptoms described are similar to what I had. He knows he has exhaust gases in his coolant, so he knows the head needs to come off anyway. I would rather plan for the worst and hope for the best, because a pessimist is never disappointed.

 

Do a compression and/or leak down test, then yank the head.