What problem is causing temperature gauge needle to go to next line above middle when vehicle is idling, and return to middle when driving?

Question

2003 CHEVY MALIBU

Water pump, cooling fans, and serpentine belt replaced. No leak during coolant pressure test and engine block test.

Answer 1

Favourite answer

Usually you replace the thermostat first.

Answer 2

First logical thought is the radiator cooling fan or fans not working. 

If that is the case, the problem could be fan or relay or fuse or computer control signal from the engine coolant temperature sensor. 

Don't confuse the engine coolant temperature sensor that controls an instrument panel temperature gauge. 

The computer uses its own coolant temperature sensor. 

You say cooling fans were replaced, but have you witnessed them cycling while the warmed up car sits idling, with the hood open? 

Were the old fans defective?

Answer 3

I going to go out on a limb and guess that you still have Dexcool in the system, that stuff is BAD news. You are seeing the thermostat cycling, and by driving you are increasing the air flow over the radiator. I would lay dimes to dollars the inside of your radiator is full of a nice reddish brown muck that used to be Dexcool. Good ole GM side terminal batteries and Dexcool, idiots.

Answer 4

Did they look at the thermostat? Check that the fans are activating?