My 1996 F150 has been having an unusual problem with it's temperature gauge.
It only occurs some days.. probably about half the time. - But when I am driving it and it does act up, the temperature gauge will fluctuate from the O (In NORMAL) all the way to the end of the L. This happens after the regular warm up cycle - It is fine all the way up to the N when it warms up.. I've noticed when I set cruise on 60MPH, it likes to start climbing. It can go from the O to the L in about 3-4 seconds, then it will go back down a little bit, climb back up, then go back down.. Just random fluctuations. (slow-delayed movements, not very erratic). The other day it actually climbed up in to the red, near the "H". I parked the truck, got out, popped the hood and put my hands near the engine and it wasn't radiating immense heat, it was just normal. I also felt the radiator hoses and they were all warm but definitely not HOT. The truck is also not sucking a bunch of coolant or anything, I've been checking. I live in northern Michigan, so the temps here are pretty cool at the moment. Could this be a sensor problem? Any help would be appreciated. Thanks.

 

I would suspect the thermostat first thing. Have you checked the coolant level?(only do this when the engine is cool) Does your heater work as it should?

 

Heater does work as it should. I just checked the coolant level. In the reservoir it was about 90% (to the cold fill line) and in the radiator itself it was just over 3/4 full - not to the top.

 

I assume you mean the heater blows out heat, not just that the fan works? I should have clarified when I asked the first time.

When you say the radiator was 3/4 full, do you mean somewhere like 2 inches below the fill cap?

Was the engine completely cold when you checked the fluid levels? I'm thinking if the radiator was not full the reservoir should have been empty.

 

Possibly the coolant temp sensor?

 

The heater blows hot slightly after warm up. I would say more like 3-4 inches below the cap. The engine was completely cold. And the reservoir was still at 90% full.

 

3-4 inches below the cap was probably enough for the coolant to circulate. You might try filling the radiator full and see what happens, don't forget anti-freeze if you need it(I don't know where you live).

I would check the tube going to the reservoir and make sure it is not plugged or has a hole in it.

I would still guess you have a bad thermostat.

 

I would check the temp sender. I just went through that on my '95, 302, Eddie Bauer.

After replacing the busted radiator, the leaking water pump, the loose fan clutch, top and bottom radiator hoses, and the NON Motorcraft t-stat, mine was drifting between N-L.

Finally, I used my DVOM and Ohmed out the sender. It was bad. I even Ohmed out the sender wire to the cluster while I had it out. That it left the sender.

 

I know this is an old thread, but I'm curious if 96fonefitty ever figured out what this issue was? My 96 5.8L is doing exactly the same thing. I bought the truck last summer, and it has done this since I've owned it. I replaced the timing cover gasket, water pump, thermostat, and radiator in October due to the radiator and timing cover leaking coolant. While I've stopped both of those leaks, the temperature gauge is still fluctuating as it did beforehand. The only issue that I do still have is my coolant level hasn't remained full. I've had to refill the radiator and overflow reservoir multiple times since October. No apparent leaks that I can tell. I plan on trying to burp the system again this weekend, which I've already done after finishing replacing all of the items that I did, to hopefully eliminate that possibility. What do you guys think? Could air pockets be causing the gauge to fluctuate? Maybe something with the temperature gauge sending unit?

 

Sounds like you have a coolant leak some place. Possible heater core, or cracked hose some place. Might not leak until system builds up some pressure. Your system should have worked all to most of the air out by now. Low coolant level will make your gauge bounce up and down. warm air pockets will drive the gauge up, and colder water pockets hitting the sending unit will drive the gauge down. Track down your leaks or leak. Ive had hoses not seal on the plastic radiator fittings. IF that's your issue Ive had good luck with using Indian head sealer on the fittings before slipping the hose on.

 

Thanks for the reply, OhioFordFarm. Makes sense. Whenever I'm able to check it out this weekend, I'll let it run until the system builds pressure and check all of the hoses out. I did that back in October when I replaced everything, but I suppose things can come loose, especially in the bitterly cold temps we've been having. I've also heard that gurgling water noise from under the glovebox before (heater core), but only a couple of times. I haven't noticed any leaks there, but I can't honestly say that I've thoroughly checked. So I'll be sure to inspect that as well.

 

Any update @96EddieBauerEdition ?

 

No, sir. I did check pretty thoroughly for more coolant leaks, to no avail (which is good I suppose). The radiator and reservoir have held the appropriate levels of coolant since I last posted. I did replace the radiator cap, and I also purchased a coolant temperature switch (perhaps unnecessarily) but have not been able to install it yet due to life happening. Other than that, not much has changed.

Not sure if this has been mentioned, but I can say that once the truck is warmed up and has been on the road for 10 minutes or so, the fluctuating gradually calms down and the temperature gauge needle mostly settles to the middle of “NORMAL” as it’s supposed to.