Driving with the cruise control on going up a long hill. The noise from the HAVAC blower kept changing pitch as the car accelerated and decelerated after reaching the crest of the hill and continuing down. It sounds like a vacuum leak controlling one of the heater-A/C doors leaks letting either a door open or close dependant on manifold vacuum.

Anyone else seen or heard of something like this?

 

I do know that my heat or ac will do something like this. If i accelerate to where the engine may be struggling a bit, the air coming out of the vents will slow to almost nothing, then when i let up, it goes back to normal.

 

The HVAC doors are vacuum controlled and the vacuum pressure is supplied by the engine. If you have a vacuum leak in the engine bay, it will cause this kind of problem among other problems (4wd, etc). Start tracing all of your vacuum lines under the hood & replace any that look cracked even the least bit.

 

So i guess that would explain why if i switch to just the floor vents, it dosnt really change anything? It still comes out the top aswell.

 

Thanks guys for the quick response. I suspected a vacuum leak. It reminds me of the old vacuum wipers that would quit working in a rain storm when accelerating. When you let off the gas the wipers wiped like crazy. I guess I'll be looking for the leak.

Again Thanks.

 

I found the vacuum leak. The PO replaced the hood and didn't put the vacuum ball back on. Since I don't have a vacuum ball I put a plug in the pig tail after hooking up the vacuum line that goes through the firewall to the heater controls. I guess I'll be shopping the junk yards for a vacuum ball.

Also after looking around in the engine bay i noticed he didn't put the windshield washer squirters back on the new hood. I guess I'll need them as well.