Had a small water pump leak last week on my 99 xj. Replaced the pump and refilled the system thinking I had burped it sufficiently. Got in it the next day to drive it and temp would not reach normal operating temp. Gauge showed 155 or so. Drained coolant and refilled in hopes of removing air that might have been causing issue. Didn't work. Tried a third time draining and refilling being careful to burp system really well. Matter of fact, ran at idle (front end lifted) for 45 minutes watching it burp slowly. Gauge reached about 190 at idle so I assumed I got the air out. Got in it this morning to drive kids to school and same ol same ol. At idle, temp will rise to about 175/180 but when driving down hwy temp drops down to about 145/150. Very luke warm heat coming from vents. Any thoughts?? Bad thermostat?

 

Sounds like it may be stuck open or wrong temp that would be the first thing I would check.


Your E-Fan isn't running is it?

 

Nope, E-Fan not running. How do you check if it is stuck open?

 

U should replace thermostat when replacing water pump anyway.
Mopar brand.
And it would be a good time to replace your temp sensor also.

 

A 99 will burp itself with approximately three full heat up and cool down cycles. They really are quite good at doing this on their own.

Check the level of coolant in your overflow; that is where it will pull in more fluid as it burps itself.

If the coolant level in the radiator/overflow are okay, then it might be time to install a new 195 degree quality thermostat. Stant Superstat or factory MOPAR stat are my picks. While you are at it, install a new radiator cap. Radiator caps are very inexpensive and often overlooked; they are very important for proper pressurization.

How many miles on the current thermostat and radiator cap?

 

Yep, had the same prob.... thermostat was fried, stuck open, not allowing it to heat up fully. Cold air when driving around would drop my gauge 30+ degrees. New stat and gasket fixed it. I also back flushed my heater core while I had the hoses off. heat works much better now as well.

 

Did just that! Installed new Dealer thermostat and radiator cap this afternoon. All seems fine at the moment. The thermostat was only about 15 months old. 12k miles. It was a "failsafe". Im done with those!! Im convinced I had a thermostat stuck open.

 

Well there's your problem. Those fail safe thermostats suck and routinely fail open for no reason.

 

Yes the fail safes were a great idea but they have their own problems just change the thermostat every 2 yrs like you do the coolant and flush the system.