Strange temperature fluctuations, where should I start?

I’ve done some research but would really like some input before I start taking things apart to replace the thermostat/sensor or anything else?

I have the 4.0l v6 with 6sp MT with over 200k and just noticed my temp gauge heading to redline while at idle yesterday. In the last two years living in the desert I’ve never had an issue with overheating. It actually went back to normal once I began driving it. (Of course I shut it off and checked the coolant level, but everything appeared fine).

Today I retested the truck using a scan tool to monitor the ECT temp and once it got up to operating temperature the problem repeated but only at idle. At idle it will stay at about 215 degrees, but every couple minutes, or less, it jumps up to 235-237, then will go back down. It cycled this way a number of times regardless of whether the ac or heater was on/off. The only change would be if I gave it gas and kept the rpms up a little that the temperature would actually go down to about 195.

Anyone have this occur or think it’s definitely the thermostat? Most other threads I found mentioned overheating while driving but that’s not my case here. Any help is appreciated tia!

 

Interesting.

Imo I'd say a malfunctioning thermostat. But it could also be the water pump. When you drive. The increased rpms could be helping the water pump work better. But in idol it might not be.. idk, spitballing here

 

How’s the radiator fan doing? Issue only at idle; maybe the radiator could use a wash?

 

That’s a good possibility, I have no idea if the water pump has ever been changed!

 

The fan appears to be in perfect working order and I inspected the radiator and see nothing out of the ordinary. Actually just took it through the wash last week for the first time in months.

 

Replace the stat it's cheap then go after the fan clutch.

 

Don't know about yours(OP), but the front of my grill comes off pretty easy. Rinse off the A/C Condenser with a garden hose(it's up front) and then do the radiator(reverse side). Not saying this will fix your problem, just eliminating one of many possibilities.
Some say all sorts of crud finds its way in between the a/c condenser and the radiator.
Your truck is old. That could be an issue.

 

This.

 

If you have a way to measure the rpm of the fan, I would check that. It can look like its spinning and still not be pulling enough air through the radiator

 

Replace temperature sensor and thermostat. Either could produce your symptoms, both candidates for failure at your mileage.

I chased a very similar issue to yours in another car. Temp gauge would be mid to mid hot, all of a sudden take off like a rocket, then down to normal range, just acting nuts. After a replacement thermostat and rad flush, I replaced the temp sensor, instantly fixed.

 

If the fan clutch is slipping it will get hot, if it's seized up it will run colder than normal and make a good bit of noise. When it's running and hotter than normal the fan should make a lot of noise also and stop as it cools.

 

This seems like a good place to start, I’ll replace both and see if it helps! If not I’ll start checking into the fan, then the water pump.

Thanks all!

 

Test the fan clutch:

https://www.aa1car.com/library/cooling_fan_clutch.htm