Weird HVAC / Engine Load/Vacuum issue...

[SilikonBlu]

As the weather has warms up and I actually start using the Vent and A/C option on my '86 Mustang I have noticed something weird happening. When the engine is under medium to heavy throttle (average to low vacuum) all the forced air for the interior HVAC system goes up to the window (defrost position). Idling at a stop light or under light throttle, I'll get good airflow from the vent (or floor) but under medium to heavy throttle, everything goes to the top of the dash!?!? Has anybody seen this before? The car runs great, no loping or surging idle - so I'm a bit skeptical as to a vacuum leak. Any ideas or where to look would be great!

 

[Dip In Traffic]

lol what cam do you have????? i just fixed my heat and i realiszed the same thing

 

[SilikonBlu]

Whatever the stock cam was for '86, I've got. From what I can gather, they used the same cam on '85 - '88 Mustangs...

 

[blk92stangg]

LOL, thats funky. You should check the vacuum lines under the dash and behind the ac panel. My friends ac was stuck at the defrost vents, and was fixed by reattaching a vacuum line.

 

[HISSIN50]

stick a vac gauge on it and drive it to see what gives. on my SD 88, i have 19 inches of vac at idle. it is normal to have near 0 inches when you mash the gas (in case you did not know and think something is weird).
once the vent is selected, it should not change (no matter what the vac situation is). with a/c, remember the WOT relay disengages the a/c clutch when you give it full throttle, which results in diminished or warm air coming out of all the vents.

your problem is kinda weird, though. hhhmmmm.