My 92 I've noticed doesn't seem to ever hit operating temp. I live in mississippi so it's been 80+ degrees and the heat still doesn't blow warm until a good 20 minutes of driving. It's got the 4.9 5spd. Radiator looks to have been replaced, I flushed the heater hoses and core. New thermostat and rad cap. Temp guage seems to work and won't get into the word normal on the IP. This winter I'm gonna want my heat to work. How long do your trucks take to warm up with the oem 195deg tstat? What should I do next?
On the six, I've seen the thermostat housing do what you are experiencing. They are made of pot metal, and when they are worn out, they will allow enough water to flow past the thermostat flange to literally keep the engine from getting good and hot.
When your motor is running at the required 195, your computer becomes very happy.
Another thought is whether you tested the t-stat before installing it. I only use Motorcraft and test any t-stat before installing.
Also, is the fan clutch in good shape? It could be failing and not slowing down so it's not allowing the engine to reach operating temp.
Holmes had a good point on the housing wearing out and not creating seal where it would allow coolant to pass and not heat up. Though I've never heard if that happening. Motorctaft t-stats have a small hole in them for air to escape when burping the system. Anything is plausible.
I installed a gates thermostat, definitely no motorcraft although I got what I could on rock auto to bundle shipping. The fan clutch felt normal when I turned it but I didn't inspect it much yet. I was fearing water pump impeller maybe. Figured the temp guage wasn't too accurate but I think it is somewhat worthy because only when it has creeped up a little off C will the heat put out any warm at all. I didn't even think of the housing, good idea
I wouldn't be surprised if its a bad tstat. I can try the one I ordered for my other ford. I just let the truck run for 20 mins straight with the infared thermometer reading an accurate 94 outside. The thermostat housing where the coolant temp sensor is only got up to 180 and the top hose had hardly any pressure on it. I'm leaning toward my $2 thermostat being bad.