The thermostat is a good place to start as they are not too expensive and it's not unusual for it to fail with the mileage and age of your truck.
Next time you have the truck up to temp, open the hood and squeeze the upper radiator hose to make sure the system is holding pressure. Radiator caps can fail even sooner than thermostats or sometimes the seal can get contaminants on it from the overflow cycling fluid. At least check for pressure and when the truck is cool check to see if the cap seal is good and the mating surface clean. You might want to go ahead and replace it anyway.
The next thing to look at would be the cooling fan.
With the truck at temp, make sure the fan is coming on. The fan switch or relay could be bad. If you put the A/C on next time it heats up the fan should come on to cool the condenser, if you have 2 fans just the condenser will come on, and it should drop the temp. The condenser fan does not come on with the cooling fan switch, so don't rule it out yet.
Hope this helps and I will keep an eye on the post if you need anything else.
Tuesday, January 11th, 2011 AT 3:41 PM