I'm having a very strange issue with my truck.

SHORT VERSION BELOW

First, a little background.

I bought my truck in 2005. Previously it was owned by a dealership for parts/title runner. It had 38,000 miles on it. The service record did not show any maintenance on the cooling system. Fine for a 3 year old vehicle.

Fast forward to now. It's 4 years from the purchase date and I have ~92,000 miles on my truck. Since I have owned it nothing has been done to the cooling system. No coolant changes, no nothing.

Recently, the temperature gauge has been acting funny, or at least indicating strange information. The truck will warm up to around operating temperature, then the gauge suddenly shoots down to quite cold then to very warm and eventually settles at normal operating temperature. It's as if the coolant is frozen then passes by the sensor then hot coolant follows and then it equalizes. Or the thermostat isn't opening properly and allows cold coolant to flow followed by hot. Who knows.

I've been in contact with my mechanic, Justin (Toreador4x4) and we have come to a conclusion on what we think it is. What would you think it is? I'm not saying he's wrong, but before I replace parts or do unneeded work I want more opinions.

SHORT VERSION

The truck will warm up to around operating temperature, then the gauge suddenly shoots down to quite cold then to very warm and eventually settles at normal operating temperature.

 

You're running low on coolant, or the gauge is grounding out somewhere, or the sender. Prolly running cold on coolant and your sensor is picking up air pockets.

 

You think that's the only problem? I guess it would make sense.

 

Well its easy to diagnose. When the engine is cold, make sure the radiator is topped off with 50/50 water/coolant and fill the tank up about 1/2 way too.