Hey there, looking for some help diagnosing the problem with my GF's cherokee.

It's a 1999 cherokee sport with the 4.0. She's owned it for like, 7 years, without doing much maintenance aside from oil changes. It has about 140,000 miles. She's never had a problem with it since she owned it.

A few nights ago she picked me up from the airport late at night, and on the way home, steam suddenly started coming from the hood. We stopped immediately, I checked under the hood for leaks, saw residue but couldn't find an actual leak, we tried to nurse it home but after about 3 more miles more steam started coming so we pulled over and got towed home.

Now I'm trying to figure out what the problem is. I still can't find an actual leak. Also, the temperature gauge: when she picked me up, the gauge was normal, and AFTER we saw steam coming from the hood the temp gauge went IMMEDIATELY went from normal to max, and the "check gauges" light went on.

Similarly, after we stopped the first time, we started the engine again and the gauge read normal and no idiot lights were on. Then after driving a couple miles steam started coming out again, the temp gauge jumped immediately from normal to max and the "check gauges" light went on. That's when we decided to get a tow.

EDIT: Sorry about the discrepancy between the title and the test, the jeep is a 1999. I thought it was a 2000 then checked while I was writing the post and corrected the text to read 1999 but forgot to fix the title. It's a 1999 jeep cherokee sport with a 4.0L.

Any ideas as to what went wrong? Did we screw up by trying to get home after the first instance of seeing steam?

 

From your description the radiator is shot, probably a small hairline crack somewhere that opens wider when hot. A pressure test of the cooling system would be the best option, it replicates the pressure in the system without the heat, not sure if you can still rent those at parts stores or not.

I wouldn't recommend running it until you get the leak fixed otherwise you risk seriously overheating it, resulting in a blown head gasket and possibly other problems. The reason the gauge went from normal to hot so fast is because as fluid escapes, air/steam takes it's place. Steam will be WAY hotter than fluid, so that's probably what the sensor is seeing. Or the sensor itself got fried from the heat.

If you don't see any coolant on the engine itself (like in the front where the water pump is), then it's probably the radiator. Replace that with any auto parts store one, along with the upper and lower radiator hoses. A new thermostat would be in order too.

 

I would start with pressure testing the cooling system. You can rent them at AutoZone ect. Hook it up where the radiator cap goes and you can pressure test the system while the Jeep is off so you can look at listen where it's leaking.

Likely culprits is the radiator or water pump/gasket. The cooling systems on these things need good maintenance. When you find and fix the problem, I would suggest replacing some cooling components for preventative maintenance. Such as the water pump, gasket and thermostat. Hoses, radiator and fan clutch.

For a thermostat, it is recommended to buy OEM from a dealer. The radiator can be bought from Autozone and a few online retailers if you search the site. I got mine at NAPA.

Good news these parts are cheap but it's well worth the time and money since you're going to be in there anyway. Good luck and post your results.

 

If you replace the radiator keep all mounting components that come off with the original, autozone and the like do not carry direct (OE spec) replacement radiators they are somewhat standardized to fit more vehicles and will fit a little loosely if you don't throw some of the original bits back on. Remember the transmission cooler lines are on the back of the driver's side of the radiator.

I'm going to take a guess and say that you have a hairline crack in the plastic end piece on one side of your radiator, but it could be any component within the cooling system. Pressure tester is your friend, should be about an $80 refundable deposit at your local parts store, pumped to 16 psi should simulate normal operating conditions.