Temperature gauge fluctuates after changing thermostat.

[JeffWS]

I changed the thermostst (using MotorcraftRt-1137) and now the temp gauge moves from just above cold to just before the middle of the gauge, and back again. I suspect air in the system, but I have tried various ways of getting it out. Ran engine with the cap off, filled system. Also pinching upper hose with cap off. Anyone have this problem, or know the fix?

 

[FireFighter431]

Park it on a incline with the front end up hill? Or run up on some ramps. Then run it with the cap off or just loose for while and all the air should come out. Add coolant as needed.


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[RonD]

What engine do you have?

Most good t-stats have, or should have, a "jiggle" valve, it is a small hole in the t-stat plate with a metal pin in it that will "jiggle", this allows any air to come out passed the t-stat when refilling the system.
The t-stat should be mounted so this hole is at the 12:00 position.
I drill an 1/8" hole in the plate if I can't get one with a jiggle valve.

Yes, park facing uphill and have heater set to HOT, with rad cap off, put cap back on when coolant starts to warm up and is at the top of the cap opening.
Make sure overflow is at its proper "Cold" level, so after cooling it won't suck air back in.

 

[JeffWS]

The engine is a 3.0, and the stat has a jiggle valve. The stat is shaped that it fits into the housing only one way.

 

[JeffWS]

Update...

There was a leak at the hose where it attaches to the coolant tank. I replaced the hose.

Jacked the front up, ran engine for about 20 minutes with heater on, and with funnel in radiator. I revved the engine several times and did see some bubbles.

I'll drive in later today.

 

[RonD]

Good find

That leak would allow air to be sucked back into the system when it cooled, so cycle would be coolant out on warm up, air back in on cool down.

 

[JeffWS]

Still fluctuated on the way home. I am going to run it for a few days, and check the levels. If it still moves I will put a scanner on it and look at the engine coolant temperature as the gauge moves to see if the temp is really changing.

 

[RonD]

My '94 4.0l did a random temp up and down, never overheated, it would be at normal(just below 1/2) then go up above 1/2 and then go back to normal, didn't matter if there was a load on the engine, i.e. going uphill, just seemed to do it randomly.

Came to find out Ford used the heater core as the by-pass in my year, the core was clogging up and so didn't pass coolant as well, which caused the random up and down temp.
Core was about $25, took 20min., very easy to change on that year.

I think yours has a temp control on the heater hose so it is not a by-pass, but if the heat control has 4 hoses(2 from engine and 2 from heater core) then it is a by-pass and that could be your problem.

 

[JeffWS]

I saw there was a TSB for that issue, but it was for the 4.0, not the 3.0

I noticed that the gauge is staying at 1/3 (a little under half) while I am driving on the bypass, about 60mph. When I slow down at an exit, and stop at the traffic light the gauge drops to 1/4. This is confusing me because there wasn't an issue until after replacing the thermostat and changing the coolant.