I have a 98 F150 with 250000 kms. that is throwing warm air not hot. The last few winters I have noticed a progressive decline in the amount of heat blowing out. Now I can drive 15-20 minutes and the interior will still feel some chill. The temperature gauge shows that the truck is running in its normal range.

Is this a thermostat or heater core problem?

 

Or the blend door. It most likely is not the thermostat, since I'd expect the truck to eventually overheat. If you have 100% water in the cooling system instead of radiator fluid, then it is most likely the heater core that is clogged with rust from the block.

 

I have always used the correct antifreeze/water mix in the rad. How does one "fix" the blend door?

 

This is unlikely to be a blend door issue. Blend door failures usually manifest as a complete loss of temperature control or by wide swings in outlet air temp as the vehicle starts or stops.

Most likely, it is one of two things:

1) Insufficient flow of HOT coolant through the heater core. Might be due to restricted core, weak water pump, thermostat, etc

2) Possible buildup of dust or other contamination on the surface of the heater core. Much less likely since most gets caught by the evaporator core first and causes greatly reduced volume of airflow though the system regardless of the selected outlet.

I'd investigate #1 first. Check both heater hoses to see if they're both hot and the same temp.

Steve

 

my old 97 used to be like that all the time, the only way i could get heat was by driving and then it would only blow warm, at an idle the air would be cold as hell.

every couple weeks i had to top off the coolant in the reservoir then the heat would work perfect. so i would just check that for now to see if thats the problem.

i have no idea what would happen to the coolant, like i never had any leaks but for some reason it would just burn away or something.