Hey I just bought a 91 F150. I feel as if my Thermostat may be stuck open. The Heat guage rarely passes the "o" in the Normal area of the Stock Sender.

I suppose this may be normal.. but I've had some condensation on the oil cap when filling/checking the oil. If my truck isn't getting hot enough this may be the problem. Since it may not burn off all the water in system in this Cold weather

The other issue might be that the condesation crap is Coolant leaking into the oil (I hope not). But what are steps to check if this is actually happening?

If this is the case.. are there things I can check before I assume the Head Gasket is bad. I'm running on borrowed time if I have a coolant leak.

 

Same thing was happening with mine exept for the water under the oil cap. I replaced the thermostat with a 180 Napa one and the truck heats up to the "R" in normal. It was only $6 so give it a try and see what happens.

 

You can tell if its stuck open by opening the rad cap while its cold starting the truck and lettings it run. You should see it flow. Normal would be no flow until the thermo opened.

You might want to check with a real mechanical gauge to be sure and dump the thermoresistor to check.