mrrecon4

need help have a 99 sport runs just below 200 degress dosn't overheat. the problem is i will turn the heat on while idleing and get no heat but when i start driving it will come on, then when i stop to idle it goes back to cold air. the fan never changes speed I know a little about mec. and understand the coolant is what heats up the truck but what would make it go to cold air at idle and back to normal when driving ( thats in gear or not)
thanks for any help

landude

Low vacuum to you're heater controls maybe.

PDEM87

You may have a vacuum line that is collapsing. Your engine produces higher vacuum at idle and it lowers as you step on the throttle. A collapst vac line will be noticeably flat during idle. Good luck.

djb383

.....and heater core could be partially plugged as well as water pump impeller could be half gone, or both.

mrrecon4

Does anyone know were i can find a vacum diagram

djb383

I thought the "blend door" (sends more/less heated air to the cabin) was cable operated, no? Vacuum controls which vents air (hot or cold) blows from.

RTorrez1

^^More then likely this is your problem^^

mrrecon4

i will check out both is there a way to check the heater core without pulling it out

RTorrez1

Just flush it out in both directions several times.

annastefy

Perhaps due to Low vacuum to you're warmer controls..

bulrid8

Have you checked your coolant level? That is a classic symptom of low coolant level.

mrrecon4

looks like its fixed, flush core both ways until it was clear some crap out not to much, also low on antifreeze. not sure about water pump mainly wondering where antifreeze went (wheres it leaking) thanks for all the help and i too have a 11/16 hose on the inlet

CCKen

^^ This!

His impeller probably looks like a Chinese Throwing Star. LOL

Edit: OOPs, looks like he fixed it. That will teach me to hit the refresh button before posting.

Roler

And yes (I just got this out of my own mix up). Blend door is servo motor on 99-01 and cable in earlier years.
Vents are vacuum operated .