jgarza1004

I tried searching through some of the threads but can't seem to find any similar issues. When I have my heater on and the truck is moving it'll blow hot air like it's supposed to but the moment the truck comes to a red light or is still, it will start blowing cool air. I tried messing with the air temp to see if something would change and it wouldn't make it start blowing hot air. I have a 2008 xlt, any ideas?

Toshbar

What if you rev it up in neutral while stationary?

You checked coolant level I would hope.

Mod (Ret.)

Strange, as the heater core gets water circulated to/from it via the water pump. If you do not notice the engine temps increasing at idle, then I'd say the waterpump is fine; perhaps the valve that opens the water flow to the heater core is not opening all the way, or the vacuum line that controls it is bad.

techrep

iTrader: (1)

Answer this question .... to enable further troubleshooting...

Toshbar

As I understand it, coolant is always circulating to the 04-08 heater cores. Isn't that why people install ball valves in the heater core line to shut during the summer?

techrep

iTrader: (1)

True...the only thing that would prevent this circulating would be a clogged heater core or a bad water pump..

Martian

You mean like this one?


https://www.f150forum.com/f12/heater...3/#post4436077

Dirttracker18

or low coolant level/air lock in the system.

OP the question was asked if you checked the fluid level. Have you?

If that is good then you need to purge the system of air. If you are certain that you have ALL the air out of the system we can move on to other issues.

Diagnosis always starts with the basics and moves to more complex.

While a partially blocked core could be the culprit, both of the first two issues need to be ruled out first.

techrep

iTrader: (1)

Sounds like a worn water pump...while at higher RPM's he gets heat...ie pump spinning faster and at idle little to no heat...

Mod (Ret.)

In my younger (and perhaps foolish) years, I owned a Pontiac Fiero. This thing would overheat as indicated on its temp gauge, then drop to normal, then overheat, all at different times and RPM ranges. It was finally discovered that the water pumps impeller was slipping on its spinning shaft.