Preface: The gauge in question is actually in a 1988 Mustang GT, but I am assuming that the F-Series used the same gauges in the late 1980s to early 1990s...

Problem: The temp gauge reads inaccurately.

When the key is off whether cold or warmed up, the needle sits below the Cold mark, and when I turn the key to Run, the needle jumps even further down to the peg on the Cold side.

When I start the car and warm it up, the gauge never lifts off of the cold peg, even though the thermostat is a 192º. Then when I shut off the engine, it slowly rises up off of the Cold peg very slightly, not even reaching the Cold mark.

Then as a test, I connected a jumper wire between the sender wire and the negative battery post and turned the key to Run, and the needle jumped right up to the Hot peg - exactly like it's supposed to.
But then when I turned the key to Off, the needle only dropped to just under half - about 1/3rd of the gauge up off of Cold.

So then I reconnected the sender wire to the sender, and as soon as I turned the key to Run, the gauge went back to doing what it was doing before, being stuck on the Cold peg with the key in Run, and then lifting slightly with the key turned to Off.


Some more background: I'm pretty sure I applied only antiseize to the sender threads when I put it in that manifold >10 years ago, and it's always worked fine, including in my previous Stang.

I have also seen this same problem occur in at least one other Ford, a 1994 Mercury Topaz years ago, although it only popped up after that car got severely overheated. The Mustang hasn't been overheated. I think I had it happen in a Ford Tempo after that, which also hadn't overheated, and replacing the gauge itself fixed it. It's been a while...

On all of these cars, it was only the Temperature gauge which did this.

On the Mustang, the Oil Pressure gauge works perfectly. Its needle stays where the oil pressure was when the engine was shut off, and then drops quickly to the Zero peg when the key is turned to Run without starting the engine. And of course when the engine is started, the needle goes to wherever the oil pressure is. I am pretty sure that the temperature gauge is supposed to behave that exact same way.

I once saw a thread in this forum, from a fellow who replaced a resistor or something inside of his truck's EGR vacuum solenoid, (EVR) restoring its operation!!! So I know you guys have some deep knowledge of the parts of this era. The Dentside forum is the same way, but Dents use the old 1960s to mid-1980s gauges, which are different than the late 1980s and later gauges.

Anyone else seen this problem, or know the cause?

 

From your test, I’d say the wiring and gauge are good. My guess is that the anti-seize is interfering in the sending unit being able to ground though the body of the unit. Just a little dab of thread sealant is all you need.

 

The gauges on my 92 GT does essentially the same. They drop from last position (including temp) but then it climbs with temp.

I'd say the sender is failing or bad grounding. Motorcraft replacement is cheap. I use Teflon paste, very thin coat after cleaning the threads with wire brush.

To test some before, I drilled a hole in a can & threaded the sensor tightly/gently in, filled with water & stole my wife's candy thermometer.

Hooked up my meter for ohms, threw it on the stove & watched resistance rise with temp. Could probably give you a good idea of it's health if you wanted to test it.

Resistors on the EGR don't fix, only fool... when it's sucsessful.