Dandy observation that increasing engine speed brings the temperature down. Be sure the shroud is in place behind the radiator and there are no places air can bypass the radiator.
A leaking head gasket doesn't always pump coolant into the oil. It can push combustion gases into the coolant. There's a tester for that. It is a glass tube with two chambers of dark blue liquid. Your mechanic will hold it over the radiator neck and draw air through it. If combustion gases are getting into the coolant, the liquid will turn bright yellow.
If you are brave, when the engine is cold, you can test the thermostatic fan clutch by holding onto the blade when a helper starts the engine. It will tug a little but you won't have any trouble holding it from spinning. As the engine warms up, you should feel it start to tug harder and harder. I've seen this done, but I'm chicken. In the rare event the clutch would seize up, it could be catastrophic. I'm rather emotionally-involved with my fingers. I use a piece of light rope to hold the blade. Don't use wire such as a coat hanger. If you let go, it could tear up the radiator and hoses.
As an alternative, you should hear the fan become quieter as the clutch slips less when it warms up.
Did you install a 195 degree thermostat? A 180 degree unit can cause problems by not allowing the coolant to stay in the radiator long enough to cool down. This doesn't exactly fit your symptoms, but it's worth mentioning.
You might consider buying a scanner that will allow you to read the sensor values the engine computer sees. The computer and temperature gauge use different sensors. Unlike Fords, Chryslers have very little trouble with coolant sensors, but it wouldn't hurt to check the gauge sending unit against the computer's.
Friday, February 26th, 2010 AT 4:57 PM