Hey all im having problems getting my truck to heat up to what i think is operating range. It started a few months back when i notice no matter where i was the temp guage would be just past the C right as the normal indication line starts. It usally is around the N or O where normal is written on the guage. So i tried the obvious and put in a new thermostat from the dealer. I idled it in my garage for a bit and it warmed up fine. I took an hour long trip and it was fine the whole way there. on the way back it went back to being on the colder side and is now still hanging around what i think is cold for that engine. I have an infrared heat gun but i have noting to base it on (no indication of what temp should be in degrees). What should it be? Also could i have a faulty temp sensor? i know where it is and couldnt be any easier to get to but its rusted on good and if i go that path i mite end up replacing more than i want but maybe some one has an idea of how to test it in the truck? The temp when is staying on the cold end isnt affected no matter if its warming up in the a.m. or running down the highway.

Specs:
Its a 94 f150 long bed std cab, I6 300, 2WD, 160K, 5 spd,
I live in boston area so right now its very cold out. I dont mind being chilly in the truck but it cant be good to run it cold. Heat does work no matter what and i can stay warm but the cab doesnt get hot unless parked

 

The dash gauges in these trucks are fed by a stand alone sensor.. it's not the one in the thermostat housing just in case you don't know.

I'm a bit confused by your post, I can't figure out of it's just the gauge reading you are having problems with or if you have no heat in the cab. If it's cab heat then that's related to the heater core just behind the blower motor. It could be clogged or the diverter door may not be working properly and letting in cold outside air. Check both heater core lines for temperature, if they are both hot then fluid is travelling through the core fine. If they are not both hot, disconnect them and force water through the core with a garden hose to dislodge built up gunk. To access the divert door and controls, you want to remove the glove box and core cover and look for a foreign object preventing the diverter door from operating properly. Check that the heater controls function properly while you got it all open.

 

The factory tstat should be 195 deg. On my 92 and 94 it warms up to the middle of the gauge within 5 minutes. When the tstat opens the gauge dips down but returns to the middle position within a few minutes. All tstats are not created equal. I have been using the Robertshaw High-flow balanced model on my trucks after having a new cheaper brand fail on the way to Maine (stuck open) in the winter. Also if the the coolant level drops the heater core will be cold. As Paul O says check the temp on the lines in/out of the core. Running too cold is not good for the engine or your feet, especially in New England.
regards
rikard

 

i get heat in the cab no problem, i dont think it has any thing to do with the heater core, i am worried mostly on why this truck isnt gettng hot and staying hott after i had just replaced the thermostat

 

Bad temperature sending unit if you've got good heat in the cab, most assuredly. It's the single wire sensor. I think your sending unit is down on the head on the I6, but someone more familiar with the I6 will have to confirm this. All I have around is V shaped engines (well, one flat opposing 4, hehehe). It is NOT the two wire sensor on the tee where the heater hose attaches. That would be the ECT. If it moves at all, the gauge is probably still good. Check that by using something to connect the wire on the sending unit to the block as a ground. The gauge should peg to the hot side. Don't leave it there long, you may damage the gauge. The sensor can be replaced without draining the coolant, but you gotta work fast or you're going to make a big mess. Obviously, do this on a cold engine. We don't wanna read about a percentage of 3rd degree scald burns. If you can't put your hand on the block and keep it there, you don't wanna be opening up anything on the cooling system. It WILL have pressure. Vent the pressure by opening the radiator cap before you start the job. If you want, you can drain some of the coolant down into a clean bucket and put it back in. That may prevent a larger mess. I recently put one on my son's '90 bird that was showing cold with NO coolant in it. I knew the gauge/sender was lying when I saw that. Quick test of the gauge told me which it was. New sender brought the gauge right up where it should be, about mid scale.

 

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