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Odd overheating issue (99 Wrangler 4.0)

I love my Jeep. I've had it since I drove it off the lot new. That was 140k miles ago, and it still looks and runs nearly the same as my first trip...Except

The last few months it has been running hot once in awhile. It is generally on very hot and humid days, but in the past it handled them fine.

I suspected the thermostat, so I replaced it and the housing, power flushed the cooling system (it needed it). It seemed to do the trick, but then again it cooled off outside so probably was a coincidence. It happened again.

I checked the fan clutch and it seems fine, I cleared out the radiator fins, checked tightness of the S-belt. All good. I am thinking maybe it is the sending unit, because the way it overheats is very odd.

It will go up to 210 and just sit there like it is supposed to. When it overheats, it doesn't just gradually increase, it snaps up to just under the red line and the "Check Gauges" light comes on. Then a couple minutes later it will snap back down to 210. Then a bit later it will snap back up again to under the red and then snap back down.

No noises or noticable changes in performance. I was wondering if this is some kind of computer symptom of something else overheating, but I can't find anything on it anywhere.

Any ideas?

Reply to
Ice
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I think it is pretty hard for real temperature to snap back down so fast. It is probably a bad sensor.

Reply to
nrs

The sensors are not known to be very robust, they fail all the time.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail > I love my Jeep. I've had it since I drove it off the lot new. That
Reply to
Mike Romain

Thanks. I'll pickup a sensor tonight and give that a shot. I'll also check the wiring as best I can in case there's a short causing it. Already did a cursory check, but I'll look closer.

Someone posted to another group they had an identical problem, and in the end they had to replace the engine. Unlike him, I'm not under warranty 8^o

Reply to
Ice

Unless they changed something about those recently, an open would peg the meter. Can be as simple as corrosion or a broken connector.

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First thing I'd do would be to clean the contact and try again. Salt contamination is a possiblity. If you have a multimeter just probe the sensor.

Reply to
DougW

If the temp gauge "instantly" shows hot it could be a short to ground too. Wipe all that grease and road grime off of the wires, and make sure none is resting on an exhaust pipe or something like that.

Earle

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Reply to
Earle Horton

The YJ 2.5 has two sensors, one for the gauge and one for the computer, not sure about the 4.0 but just a heads up...

Reply to
nrs

before you throw parts at it it should be looked at with a good scan tool and all pids should be carefully looked at esp. temp sensors and see if the relationship between the sensor and the gauge is good also 99 clusters had a big time problem with the connectors to it when you say fan clucth is good what did you mean?? it should move by hand with a stiffness but freely if it spins easy it's not in good shape

Mike Roma> The sensors are not known to be very robust, they fail all the time. >

Reply to
man of machines

This is a long shot but it could be the problem; When I changed my thermostat on my 86 CJ-7 I didn't "burp" the system. So I had a pocket of steam trapped near the sensor and my temp gauge erroneously reported a hi-temp condition when in reality the engine was operating normally. Try "burping" the cooling system (get the air out) and that might fix it for nothing.

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Reply to
Jo Baggs

I have done this by removing the temperature sensor in the head. Then fill to the level of the sensor, install it again, and fill the radiator. Works like a charm on the vehicles where I have tried it. If the engine is already full of coolant, just unscrew the sensor enough to let the air out. If you are butterfingers, do not lose the opportunity here to drop the sensor and have coolant spew everywhere. ;^)

Earle

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Reply to
Earle Horton

Reply to
man of machines

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