01 XJ bizarre temp gauge behavior

[1988JeepXj]

Coolant temperature dropping out while driving, but not fully. Drops from operating temp to around 110-120 degrees, and back to operating temp seemingly at random, for random lengths of time. Not the gauge, engine idle changes with temp sweep. I doubt it's a mechanical problem like a thermostat, the heat doesn't change, and my trans temp gauge remains steady. It sweeps fairly quickly, it seems like a loose or corroded connection, but I couldn't replicate it in the driveway by wiggling connectors and harnesses. No codes.

Anyone ever have this happen before? ECT going bad? Cruddy ECT connector? It looks okay, I'm probably going to deoxit and grease the connector and see if that helps. Usually they just fail open or closed, but I've never had one go bad on a XJ.

Weeding through all the threads and forums about XJs with actual cooling system problems makes researching this very difficult. :looney:

 

[tech1001]

I would start with checking and cleaning all the grounds .

 

[Tim_MN]

Last time a thermostat failed in my 2000 it did exactly what you describe. Thermostats are not expensive, and easy to replace. Cheap ones don't last long, so you might as well buy a good one and replace it every or every other coolant change. I would also replace the Coolant Temperature Sensor while you are checking the sensor wire plug, wire harness, and the engine grounds.

 

[RCP Phx]

The only "grounding" is done thru the PCM so also check that on the PCM itself. I use "No-lox" on all the electrical/grounding connections I touch.

 

[halmca]

My oil pressure has apparently been hanging out with your temp sensor. In my case -- I have already replaced the sensor twice -- I suspect the wiring harness in the immediate vicinity of the sensor. Since it would be a b*tch to get down in there amongst the starter, oil filter and pressure sensor it self, I have at least in the short run learned to live with it.

Mine too lingers around quarter scale sometimes.