My '93 F150 will not come up to heat and the temperature gage is constantly moving. I just had a bad thermostat, leaking radiator, and leaking thermostat housing all replaced and it hasn't helped. The dealer says it is in fine working order, but in the cold midwest weather lately, I bet it doesn't get above 60 degrees inside the cab and that is with the heat at full blast. I am looking for some ideas to help me fix this problem. If you need more information let me know. I appreciate any ideas.

 

similiar problem

https://www.ford-trucks.com/forums/s...threadid=93396

 

Need more detailed description. What motor is in it? Is the motor original? What does "won't come up to heat" mean?
cab won't warm up? engine not getting hot? Temp gauge constantly moving between what limits? Is it overheating,
then cooling off or just cycling on the low side of the gage and for how long? The dash mounted temp gauge does not offer much for debug on cooling system problems. You might install a cheap mechanical gauge temporarily to get a better handle on the conditions. I own and actually like the fords, but they sure don't come without problems. I guess thats true of all mechanical beasts.

 

The truck has a 351 in it and it is the original motor.

Won't come up to heat, meaning prior to the problem the temperature gage would stand at the 12 o'clock position (sorry there are no numbers on this gage so I will have to describe using a clock face). Prior to these problems the temperature gage would get to the 12 o'clock position and stay there. It never moved no matter what the outside temperature was. Now it might only reach about the 11 o'clock position and fluctuate slowly between that and say the 10:30 position. All of the fluctuation is withing the 'NORMAL' range, according to the gage. So it cycles on the low side of the gage for as long as the truck runs.

The dealer says everything is in normal working order and the fluctuations are due to the cold weather (-12 deg F this morning) we have been having. They claim the air moving through the radiator is cooling the system so quickly that the thermostat can't keep up. I have owned this truck for nearly 4 years, and been through some pretty cold weather with it, and it never has acted like this. I appreciate the help. My feeling is if I can fix whatever is causing the gage to fluctuate, I will have my heating problem fixed.

 

Same problem happend to me and it turned out to be the sending unit gone bad. Replaced it and the guage has worked perfectly since.

The above is providing that your heater is still working the same as before and that the engine is actually coming up to temperature.

 

Sounds to me like you may have put in a bad or incorrect temp thermostat. If the temp guage fluctuates it probably means the thermostat is opening and closing. I have basically the same truck and mine does that in sub zero weather too. Check the thermostat

 

There will always be some cycling of the temp gage due to thermostat open/close and this will be more pronounced in very cold weather or with lower threshold thermostat. I think you'll want to verify you have at least a 195 thermostat for the low temps you are experiencing. 195 should put you around 12:00.
I think RichB may be correct. I think the thermostat is operating correctly but may not be hot enough for the cold weather. Since it
comes to only 11:00 seems to indicate it is of lower value than you had previously.

 

Is there an easy way to determing whether or not the correct thermostat is installed, without actually removing the thermostat?

 

No way to check without removing it. Your mechanic should know what he took out of it and what he put back in it. Another way is to put a mechanical gage on it. That's about the only way I
know to get a direct temp read on the engine. (10:00,
11:00, 12:00, who knows what the temperature is?)

 

Ok, the weather has cooperated some and warmed up (low to mid 30's now), and I have pretty decent heat in the cab now. The temperature gage is still doing the same thing. What can be made of this?

 

I recently had a similar problem in similar outdoor temps and it turned out to be that my thermostat was backwards! It was my screw up due to a rushed job. I yanked it apart, swore under my breath about how stoopid I am an gave it a flip. Not saying your's is backwards, but might be faulty. Another thought is maybe your fan clutch is siezed? The new fans are highly efficient and don't need to spin as fast. If your truck sounds like an early 90's Chevy then you know that the clutch is toast. Just a thought.
X0D

 

If you take it back to the shop someone there probably ha an infrared thermometer. It looks like a little gun that you can point at the thermostat housing. It will show 2 different temperatures until the thermostat opens, then the same temp on both sides. I use one specifically for thermostats, and it will show when it opens + or - a couple degrees.