found out my coolant was low, after i realized my heat is not working right, so i added coolant to top it up. but problem still persists..the defroster and floor is only blowing heat when truck is idle...when i start driving it only blows very low warm air not cold but not very warm at all...was wondering if i have a air pocket in the sytem, thermostat acting up, or pluggged heater core anyone ever had this.....ive had the opposite happen idle cold air and when driving hot air coming out but this is weird one to me....2000 f150 4.2l thanks

 

There are several threads about bad heat. Do a search and you'll find all the info you'll need.

I'm working through a similar problem myself.

 

It's either air in the system, a clogged heater core, a bad water pump, stuck blend door, or a vacuum leak.

Im leaning on a vacuum leak

 

if it is a vacuum leak do you have an idea where to look?

 

Juast an FYI - vacuum does not control the temperature blend door. I'm not too familiar with the 4.2 liter, but on the modular V8's, they did not use a vacuum modulated heater valve. I do see one listed on Rock Auto for your truck though, so you can look for this. It will be located in the hoses going to your heater core. It will look like the picture below. Check the vacuum line going to the nipple for leaks.... but almost every one of these I've had to replace (on Explorers and Rangers) had coolant leaking out of them as well.

 

I don't actually know if these use vacuum either. Thats my bad

 

No worries. There is a vacuum valve that is higher up on the heater box, but I believe that one is used to activate defrost.

 

Is your engine staying in the proper heat range? I was thinking about an open thermostat...