I have a bone stock engine in my 99 Cherokee. I dozed off in the parking lot the other day waiting on the wife. When She came out the car was running pretty dang hot. I figured I get some air moving so I started down the road and in about a minute the temp came down but not a little at a time.....all at once. It drop to normal in about 2 seconds. Sticky thermostat? Any other thoughts? Mark

 

If the coolant temperature creeps up ONLY at idle but stays normal when you're moving down the highway, the #1 suspect is a bad "fan clutch". Often overlooked, inexpensive and easy to install.

 

Fan clutch seems OK. Not locked up, and has a little drag when cold. Found it unusual that the temp dropped in a couple seconds when it finally went down.

 

Unfortunately, testing of the fan clutch the way you describe is not conclusive. Again, if it ONLY happens at idle, I'd still suspect the fan clutch.

But always review all cooling system components when symptomatic.

 

+1000 on tj's advice.....it's cheap, easy and u don't have to drain/re-fill coolant.