KdB

Hi Guys, I've got an issue with my temp gauge I can't seem to solve (1994, F150, 302). I've done a bunch of searching on this forum and tried some of the suggestions in older posts as well as followed directions in the Haynes manual with out any luck.

The issue is that the temp gauge goes from cold to hot right away. In a short 10 minute drive the gauge will climb from cold to all the way hot. The check engine light does not come on, and I'm experiencing no coolant loss.

I've replaced the temp sensor this week. Ive tested the gauge by connecting it with an extra wire to the negative terminal. The gauge pings from C to H when connected to the negative terminal, so that should mean the gauge its self is ok?

The entire cooling system has been replaced in the last 2 years, I have all the bills for it, but was not the owner of the truck at this time however (new radiator, hoses, pump, thermostat, and I know the fan is working).

I believe the thermostat is working because the heat in the truck works as expected.

Any suggestions of what the issue might be? I will take the truck to a mechanic otherwise. Sort of hoping for one last hail marry here..

Any help will be greatly appreciated.

96obsf150

Have u tried running with no thermostat i didnt thay for my old '79 bronco.i live in cali so it doesnt get to cold

KdB

I have not tried it w/o thermostat, I do a lot of winter activities and often travel into cold climates. Not being able to warm up correctly may be a concern in this case.

Is there a specific reason to run it with out a thermostat? I take it that would basically ensure the cooling system was always free to flow as apposed to trapped inside the engine only due to a thermostat malfunction?

marktim

I had a similar problem like yours and only solved it by buying a aftermarket temp guage and sender unit , it niw sits on 195 degrees .
On the the stock temp guage it would be close to overheating.
Did the grounding out of the guage like you and it was ok but still registered hot
Hope that helps.

broncojunkie

Heater hoses bypass the thermostat, so just because the heater works doesn't mean the thermostat is good.

Chris_1

iTrader: (1)

You said you replaced the temp sensor. Unless that's a typo - what you need to replace is the temp sender. Sender feeds the gauge, sensor feeds the computer.
2 totally different things.
But if you meant to say you replaced the sender ;
For what a new thermostat costs it's pretty good insurance - overheating fries motors.
Problem could also be in the wiring or gauge. You can also get brand new parts that don't work right.
Or your gauge is working correctly and you do have a cooling issue - in that case you would start with the thermostat. And a system flush and coolant change would be an idea. You also want to ensure there is no air in the system.

fltdriver

^x2, Engine Coolant Temp Sensor is for the computer to use to regulate running conditions, Engine Temp Sender transmits signal to the dummy gauge on the dash.

ECT Sender, what you need


ECT Sensor, what the computer uses not the gauge on the dash

KdB

Thanks for the replies, this stuff is helpful.

Sorry, yes, that was a typo. I replaced the ECT Sender.

With the old sender the gauge would flip from cold to hot quickly (within 5 minutes of driving) now it takes slightly longer but still has the same result.

I've also checked the radiator hoses after a 10 minute drive and from what I've read if the top one is cold then there's likely a thermostat issue, but it was hot. Is there a trick to rule out the thermostat with out taking it off and testing it?

Since I did replace the Sender and not the sensor, does that change any of the recommendations? If not I'll start looking at the wiring / thermostat.

Chris_1

iTrader: (1)

Heat can transmit up the hose, even if the thermostat is closed.If it's actually going all the way to hot within 5 minutes, the lower rad hose should be hot too.
If the gauge says it's that hot, every hose in the system should be really hot to the touch if it's circulating.
I know you want to identify the problem before starting anything, but this really sounds like a stuck thermostat. Just change it - put a 192 degree one in and see if that doesn't fix things. if it does, you've overthought the the whole thing. If it doesn't, then you will have eliminated the most obvious thing and can start looking at more complicated possibilities.
Either way, a new thermostat is always a good thing, it could very well solve the problem, and it's not very much money. Don't cheap out - buy the best one you can get (might cost you $4 extra).

96obsf150

Tip hoes should be hot n the hoes on the bottom cold means thermostat is bad