I have a 1981 f100 with a 4.9 300 6 cylinder that won't get up to operating temperature and takes an extremely long time for the heater to produce heat. I just replaced the thermostat and it seems like the fluid is flowing properly. Does anyone know what could be causing the low operating temperature?

 

Several things to check... obtain a hand held infrared thermometer and point it at the thermostat housing to get an actual reading of the temperature [the truck gauges are not to be depended on]; if that temperature is good [185-195F], then perhaps the heater core is plugged.

Does your truck have clutch type fan? When the engine is cold and not running, use your hand and see if you can spin the fan. If it resists turning, the clutch may be defective and moving too much air when not needed [cold].

 

That wouldn't effect anything unless the thermostat was stuck open, in which it would take a very long time to build any heat, even without a fan in play.
The fan is for cooling, not heating.


Rule #1 - Use a Motorcraft thermostat!!! They are the only ones lately I have found to work correctly.

Rule #2 - The factory gauges are useless unless you know where on the "NORMAL" the actual temps are. They vary and are more "guidelines" than anything.
If you want actual temps, install an aftermarket gauge, like an AutoMeter one.

Rule #3 - Make sure ALL air is out of the system.

The thermostat is the only device that will allow the engine to maintain it's heat, or let the engine build heat. The fan will not come into play until the thermostat senses the engine is up to temp and opens to allow cold coolant into the engine. The radiator fan is only for cooling the coolant that the thermostat allows out of the engine and into the radiator. If you are not building heat in a "reasonable" amount of time, the first thing I would look at is the thermostat. If you are just not getting heat in the cab, look at the heater core. If it's running way too hot, then check out the rest of the cooling system along with the thermostat, because if it's stuck closed or not opening far enough or at the right temp, you can overheat.

How long is a long time to get up to temp?
After it's been running for a while (what you think is an acceptable warm-up time):
How hot are the heater hoses?
How hot is the upper radiator hose?
What temp thermostat did you put in?
It sounds like the thermostat is opening waaaay too soon or it's stuck open.

 

It has been taking over an hour for it to get up to to temp.
The upper radiator hose is around 130f, the lower is around 110f, and the thermostat housing is around 160f.
The thermastat i just put in is a 180f one

After an hour the gauge on the cluster is just shy of the line where NORMAL starts when in the past it has been around the O and R.

 

Use a 195° T-stat w/13lb cap. I tried a 180° stat once and had same low-heat output at heater. Truck OE stat was 195°.