My heat blows cold idling are at slow speeds but when you get up to 35 to 40 and your foots on the accelerator it blows hot. How do I get heat sitting still? My 1998 F-150 hasn't hit 70k yet but I'm wondering if it could be the water pump issue also because it sits alot.

 

Who's NoHeatDave ? Never heard of him..

 

Just do a search!

https://www.f150online.com/forums/me...oheatdave.html

 

Can be one of two things, I think. Either you are not getting water circulation through the heater core at low rpms, or the plenum door is partially closing.

What do you think jbrew?

 

Would the door be an issue if you can sit still and rev the engine and it goes from blowing cold the blowing hot?

 

Honestly, I don't see how, unless it's vacuum operated and I don't know if it is or not. Sounds as if mabe you have some blockage in the heater core or water hoses. If the door is electrically controlled, don't think it could be that. Hopefully someone that knows more than I do about the system will chime in.

 

I would suspect the water pump. The impellers are worn out and do not circulate enough coolant at low engine speed but when you speed up it is turns the pump faster. Either that or a slipping serpentine belt.

 

Sounds like a water pump issue to me. When there's a blockage you usually don't have any heat.

 

Circulation, -JMC nailed it most likely. I've seen where there's nothing left of the impellers. They rot away, almost completely in some cases.

 

Especially if it sits a lot

 

Yep it is your water pump, same thing was happening to me, so I pulled out my water pump which was easy anyways, and it had barely any fins on it, just enough though at higher speeds but barely heated. Once I seen the fins I knew for sure, replaced it and now more heat than ever before. Hope this helps and good luck.