F150 not warming up

I have a 1991 Ford F150 XLT Lariat. On Monday everything was working great and the heater was nice and toasty. On Tuesday I let the truck run for about 10-15 minutes to warm up before going to work. The defroster warmed up enough to melt the frost on the windshield, but the temp gauge barely moved. As I drove the 10 miles to work, I noticed that the warm air the heater was blowing started to get cooler and cooler and the temp gauge never got into the “normal” range.

The radiator and heater core were both replaced in the last couple of months, and all of the hoses are warm after I get home, but the truck doesn’t seem to be getting into the normal temperature operating range.

Could this be a thermostat issue?

I put a piece of cardboard in front of the grill to try to limit the air cooling the radiator, but that didn’t seem to help.

When a vehicle fails to warm up to normal operating temperature, the thermostat would be my first step in trying to fix it.

You can pull the thermostat, maybe it’s not closing at all. Put it in a pot of water on the stove and see what happens when you heat the water. Use a thermometer to see if it’s opening at the correct temp.

Thanks. This is was I was expecting to hear. I picked up a thermostat yesterday, and hope to replace it tomorrow.

Replaced the thermostat on Friday and it seems to have fixed the problem!