My '97 4.6 has run cool ever since I've got it a year ago. Temp gauge only goes about 25% of the way up the acceptable range. Finally decided to deal with it and measure the top hose temp with an IR temp gauge - 136 degrees at the radiator.

Decided to replace what I assumed was a thermostat holding open, etc. and did so. No change in the situation. Some more details:

Truck runs great - starts right up and drives strong
No coolant leaks anywhere
Ran Bar's Leak solution recently after a flush and new coolant - fixed a heater core leak just fine
Top hose measures 125 at the radiator and 105 by the thermostat housing
Lower hose runs around 90-100
Radiator runs 125 at the top and around 90 at the bottom
Top hose fills with water but doesn't seem to flow through the block

A mechanic who works at the local O'Reilly's did all the temp measurements last night again - he's stumped. Couldn't come up with a single explanation.

Anybody got any ideas?

 

To properly see what the computer sees, use a Scanner to look at the live temperature.
It should be between 196 to 200 degrees.
Good luck.

 

The IR gun gave us enough of an idea after a full heatup to know things were wonky. I'll remeasure again to see the amount of change -see below.

I replaced the radiator cap with an official Motorcraft version (at twice the cost of the white bread versions) and now have 'correct' heat in the cab, which is to say plenty. The dash gauge hasn't budged, so I am assuming a faulty gauge or sender. No matter, the truck runs fine and I'm warm.

Case closed...

 

The dash gauge is only a glorified dummy light. It move into one position and prettymuch always stays there unless it overheats or is not heating.

 

When you start the engine, it's at full C. As it warms up, it slowly climbs to mid scale and stops there. It stays there until it overheats, then goes full H almost immediately.

 

I was afraid of that - can I wire up a real gauge off the sender or does it need to be changed too? (Think I know the answer to this one...)