I recently bought a 1990 F-150 with an I-6 engine. The temp gauge rises briefly, but then barely comes off the low mark even when fully warmed-up. The heater doesn't have much output. The 195 degree stat is working properly as is the standard 13# radiator cap. No hose leaks are evident, no smell of antifreeze and nothing funky in the antifreeze or oil. During a cold start the high-idle seems to come off very rapidly (less than a minute). Ambient temp. outside has been between 10 and 45 degrees f.

I'm stumped. Any thoughts appreciated.

 

How do you know the thermostat is working properly? I've seen them open really early before, and the engine never really gets warm.

 

Hmmm. I must confess I removed the original aftermarket stat and placed it in water, heating it along with a new one. They both opened at the same time. Now, had I done it (as I know better) by monitoring the temp with a thermometer maybe I would have had a more effective reading,

Bye the bye, the outlet was in very bad shape so I replaced it with a new one. The new one, Chinese, was actually cast iron,the old one, what we call "white metal". I've since read that the Motocraft stats are made specificly for the vehicle and lock into place. There was no provision for that in the new casting so I glued it into place with sealant which I allowed to set before assembling the outlet to the head.

Yeah, they do suck fuel for a six.

Thanks, I'll check it out further. G.

 

I'd say the stat is not working properly, as it does not keep the coolant at 195F. It should open and close as needed to keep the coolant hovering near that temp. If the stat came from the same place as the housing, I'd be replacing it with a Stant or Motorcraft. The new stat should close tightly enough when cold that you cannot see light around the circumference of the moving disk. It should seal tight.
There are two 'legs' on the stat that are bendable, I think, to push out against the inside of the housing. That should hold it in place until it is attached to the engine. If the stat does not form a good seal with the housing to block the flow of coolant, it will not function properly, and regulate the coolant temp.
If you get it running at the right temp, the fuel mileage will improve. You should be able to get close to 20mpg without trying REAL hard, as we used to get 17-18 towing a 17' Ranger boat with a '78 with an automatic. (if memory is working close to reality, anyway..)
You might want to consider getting a FoMoCo outlet if this one does not allow the stat to seal properly...??
tom

 

The new stat did seal effectively upon closing, no gap between the mating parts. I was indeed concerned about the aftermarket but it was purchased from a reputable long-term local parts house not a "big box" chain outlet. I checked Stant stats after the purchase to see if there was any visible difference in construction and at least visually there were absolutely none. I suspect both were from the same vendor. The factory-marked original outlet had a cast rim on the sealing surface (gasket surface) of the casting and there were corrosion pits crossing the rim. The aftermarket outlet was machined perfectly flat. Both had a machined indent for the face of the stat.

The fit in the outlet is a concern of mine. According to Chilson the stat should turn and lock into the outlet which holds it in place during assembly. The FoMoCo outlet clearly has two flats cast on the interior, the aftermarket has none. the stat that was in the vehicle was also an aftermarket item and not Motocraft. With all else equal I'll find a Motocraft stat and see what it looks like and then go with a FoMoCo housing to see if it turns and locks.

I agree on the mileage factor. In this (upper midwest) climate any sustained cold really chops the mileage. I also own a '02 Toyota Echo and it drops from nearly 42mpg. in summer to 35 or less on occasion in the winter.

Thanks again, back to the board with this one
G.