I have a 91 F150 with the Inline 6, AC (though not working). I have replaced all of my hoses, my T-stat, my t-stat housing, and my water pump. When it's hot outside, the Temp gauge will move a lot, depending on if I'm moving or sitting. Now that it has begun to cool off, my temperature gauge doesn't move at all. It sits pegged out on the very bottom. This wouldn't concern me if my heater was getting warm, but currently the thing barely puts out any heat. What should I be looking into as far as fixing this. I would really like to have a heater that heats, and a defroster that'll defrost this winter. Initially I thought it was an open stuck Thermostat, but this problem remains. I'm scared to go to a higher temp thermostat because of how high the temperature can get in the summer. I'm contemplating rigging some electric fans to this, which may help with the warm up, and maybe the cool down as I don't think the fan clutch is working right.

 

I would guess the thermostat is at fault. I know you already changed it, but maybe you should remove it and test it with a thermometer and a pot of water. Is it installed the right way around (copper pellet towards the block)? 180 or 195 degrees? When it is cold, remove the pressure cap (remove some coolant so the level is down a little bit), start it up and see if the coolant appears to be coming into the rad from the top hose. Coolant shouldn't be flowing in the rad until the thermostst opens up. Good luck. ...Terry

 

It happends, been there. Brand new product doesnt work straight out of the box.
Yep, I agree, take it back out and boil that thing.

 

I can't remember any of the details about how I did the Tstat as it's never been a real issue until now. I know the thing looked cheap as hell, and was difficult to keep in place when I installed it. I asked the Autozone 'technician' for a Tstat for a ford f150. I didn't specify a temperature rating or anything. I guess I'll grab a 195 high quality tstat Friday and get to ripping it apart.

 

Took it apart, and I found that the cheapo Tstat I put in had actually come apart. The pin in the center was completely dislodged. I put a 195* Fail-safe Tstat in, and it's already more stable on a hot day. Temperature stays at about the 'N' of Normal, which is good. I can definitely tell when it closes and reopens, because the gauge will fluctuate just a bit. Now I can't wait for it to get cold.