6.9 gauge reading cold

Weldingman

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I have an 86 6.9 idi new to me by about two months. The truck has never gotten up to the middle of the temperature gauge as long as I’ve had it. I drove it home from Massachusetts’s to NJ on a hot July day and the gauge never got above the C mark on the dash. I figured it was a faulty sender so I flushed the coolant put in a new motorcraft thermostat and changed the sender but the truck is still reading on the C mark on the dash. I took a temp gun and shot the thermostat housing after driving the truck around and it was reading in the 190 range give or take a couple degrees. The radiator cap is a 13lb motorcraft. Any reason why the gauge isn’t reading in the middle of the normal range? What should the thermostat housing be reading? The PO changed out the radiator to an aluminum one with plastic ends is that the culprit or just a bad gauge on the dash?
 

IDIBRONCO

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The factory gauges are useless for anything other than to make the driver feel good. They are not even remotely close to being accurate. The only way to be sure is to install a quality aftermarket gauge, either electric or mechanical. The Motorcraft thermostats start opening around 190*-195*. Your temp reading of 190* on the thermostat housing is accurate. Despite common myth, your radiator will not make the engine run cool/cold as long as it has a properly working thermostat. When the coolant gets too clod, the thermostat simply closes and shuts off coolant flow to the radiator, partly or completely. On the flip side, a radiator that's not functioning properly (for whatever reason) can make the engine run warm/hot.
 

Big Bart

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Ditto! These gauges are useless! They took idiot lights and half baked them into guages. My factory coolant gauge runs all over the place. Hills different, summer different, winter different, same highway going east X reading, going west Y reading.

If you want to know as IDIBronco said put in some good aftermarket guages. If turbo then put a Pyrometer in too!
 
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