Hey all, trying to figure out what is going on with my truck. I have done some looking around but wanted to see what you guys thought.

I have a 1999 Ford F-150 with around 110k miles on it. Having issues with the heater blowing cold and not sure if I should change thermostat first or do a heater core flush. Symptoms of the truck have been a little bit strange to me. Truck temp gauge seems to go up fine, maybe a little slower than normal, but sets right where it should when warmed up. Never fluctuates. Heater on full, the heat will be lukewarm at best if outside temperatures are above 35-40 degrees. Sometimes it will intermittently go from cold to lukewarm. If it is very cold out, it will blow cold air the entire time. Antifreeze level is good, probably needing a flush and new antifreeze. I started the truck with the rad cap off and kept an eye on the level of antifreeze at startup as well as when warmed up completely. Didn't seem like the level was changing at all in the reservoir and was pretty much not moving much at all. Should I just start with the thermostat and hope that does that trick. Also seems like it has started to burn oil more than usual.

Thanks in advance!

 

You should start by checking to see if you have hot coolant flowing through the heater core loop. If you don't have that, you'll never have heat. Check the temperature of the inlet and outlet hoses.

The temperature gauge isn't a definitive indicator of what's going on since its function was dumbed down back in the 90s.

If both are hot, the problem is in the temperature blend door function inside the dash.

If inlet is hot/warm and the outlet is cold to barely lukewarm, there is insufficient flow. Multiple possibilities including partial clog or restriction in that loop, airlock, worn pump impeller, etc.

If both are cold, then it could be almost anything in the cooling system (pump, coolant level, clog, thermostat, or more).

 

When the gauge is right in the middle after driving for a while, is the upper rad hose, and heater hoses hot to the touch, and under pressure? If not, and coolant level is full, then i would start with replacing the thermostat with a Motorcraft OEM one.

 

After a little trip I pulled into the shop to check out. Rad hose is hot to the touch. The inlet and outlet hoses were pretty hot as well. I'm not sure which is which but the one closer to the drivers side was hot like the rad hose and the passenger side hose was maybe just a touch under the other one for heat. It was more than lukewarm however. It was in a 60 degree shop and the heat seemed to be working fine tonight. Went for another trip and it seemed to be working okay, I would say about 80 percent maybe which is confusing me. The weather is a bit better here now. A few weeks back it was down near zero and it was blowing pretty much cold air.

Also thanks for the reply on the blend door. I think it is working properly as when I switch it from full hot on panel to full cold on panel it changes and gets cold. Switch it back and it sends hot again. Not sure what is going on. I'm sure when the temp drops again, it will start acting up.

 

Just for Grins and Giggles, i would change the thermostat, with an OEM Motorcraft one, and fill with new clean antifreeze, and bleed all air out, and then check heater performance, could have a thermostat intermittently sticking.

 

Could also be the blend door sticking when it's cold out, I'd move it to hot and leave it for the winter. It might have fully moved because you were in a heated shop.

 

I changed the thermostat and antifreeze this morning and it seemed to do the trick.

Thanks guys!