Temp gauge climbs when sitting at idle. Goes right back down as soon as I get going at just about any speed. Fan clutch seems ok. Replaced the temp sensor and a leaky heater hose about two weeks ago. Did burp. 50% green antifreeze. Any Ideas?

 

Does the fan clutch spin freely if you rotate it, or does it have resistance? Sounds like when you get moving, and air is flowing through the radiator, this is causing the system to cool off some. Also, there is a possibility that there is still air in the system. I had a pocket of air in my 3.0 that took drilling a hole in the thermostat to get the coolant flowing and get rid of the air.

 

If the fan clutch is ok, your radiator may be on its way out. Did this start happening right after you drained the coolant or did it start gradually?

This should always be done if not using an OEM thermostat with a 'jiggle-valve' and for that reason.

 

Is your heat working? If not your water pump probably kicked the bucket. Have you checked the thermostat as well?

 

Water pump would almost certainly be leaking if it had failed and would cause an overheat situation whether or not the truck was moving.

Thermostat stuck partially shut would cause the truck to overheat under power, not cool down.

To me it seems pretty obvious it is directly related to airflow through the radiator since it cools down when moving. The radiator on my 80 wasn't clogged but the hard layer of corrosion on the inside reduced the thermal efficiency enough that it gradually began to overheat when sitting still.

 

Thanks for the opinions. Airflow is my guess as well. There is a small electric fan in front of the radiator that is not working. It really does not look big enough to do much. After a second look, I'm thinking the fan clutch is not 100%.

 

Heater core is bypassed for some years now. It"s South Texas.

 

That smaller fan is for the AC condenser. The fan clutch can be serviced and it is relatively easy to do:

http://www.4crawler.com/4x4/Docs/FanClutch.pdf

 

You can test the fan clutch as described below:
- http://www.barneymc.com/toy_root/techneek/fan_clch.htm

 

if its not the fan clutch it sounds like its missing a thermostat.

 

Not saying it's impossible but it's highly improbable. Neither one of my trucks will even register on the temp gauge at idle without a thermostat.

 

good to hear i only mentioned it because i had a piece of equipment with a stuck tstat. i removed it to get it back to the shop and when it idled it would heat up quick until i revved it a little. my next educated guess would be a fan clutch.