Hi everyone. First post here and hoping to figure out this problem. I'll start by saying that I had a bad heater core in my 99 4.6 replaced last fall. It started stinking and I recognized it as antifreeze. Then I started to get the fog on the windshield from the defrost so I read the writing on the wall. I have a good friend who is quite the part time mechanic and paid him to replace the core which I bought from an auto parts store. After the repair, everything worked great for quite a while. But I did notice that after the repair, my temperature **** was loose and it fell off. No big deal I thought, I'll stick it on later with a little dab of silicone or something. But then I noticed that when I turned the temp ****, it didnt feel right. As if something was putting up some odd resistance and then slipping. The **** wasnt stripped so it wasnt that. I still dont know what that is. But by the stem under the ****, I can tell that it's in the right position for heat or A/C.

So anyway, a couple of months ago, I occasionally noticed that my heat wasnt working. Generally my truck warms up fast but even though the temp gauge was showing warm temps, my heater wasnt blowing warm air, it was ice cold. But then I also noticed that if I got on the gas pedal a little heavy, the air would almost instantly get warm and then usually stay that way until I turned the truck off and restarted it.

Now it's summer and when I first fire off my A/C, it gets cold, but then as I start driving, it gets warm and even hot. If I let the truck sit at idle after just starting it, it seems to cool just fine. But as soon as I start driving, the air turns hot. Any ideas? Your help is very much appreciated. Thanks.

Eric

 

Sounds like you might have a problem with the blend door. Im not too familiar with the blend door operation on these trucks so maybe someone else will have some ideas.

 

Sounds like your blend door is broken and is flopping around when you stop and start, an all too common failure.

Your friend is going to need to go dash-diving again...

Steve

 

Is it expensive and difficult? Worst than the heater core? I'm not great with autos so much.

 

Almost. Replacing the core only added a few steps to his earlier job.

Steve

 

I'm having the exact same problem. I thought it might be a vaccume leak somewhere because when with the engine warm the heat wouldn't work and I'd get on the gas and turn it on at the same time it gets warm instantly it's the exact opposite with the AC though. It runs good until you get on the gas. If anyone knows where the blend door is please let me know.