safulop

Here's one for all you sages and gurus:

My S4's AC is in generally good order, but on hot days I've noticed that the console airflow decreases or changes somehow whenever I accelerate. Holding a steady pedal (or especially dropping back to idle) allows the good strong flow of cold air to resume.

I'm supposing this is some kind of vacuum leak issue related to the intake flap; no apparent problems with the car's running condition.

AC might need some charge refresh, it was done last year but my shop refuses to put a full charge in older vehicles because they are afraid of blowouts.

Hold On

Vacuum leaks in HVAC system. Not holding a strong vacuum to keep center vents open. Default position is Windshield vents. When you change RPMs, engine vacuum rises and falls, which causes vent diaphrams to move open and closed, which causes vents to open and close, which causes flow to increase and decrease.....you get the idea.

safulop

OK I get that; if windshield is the default setting, shouldn't the observed effect be the opposite of what I'm seeing? I'm getting strong console air at idle, then it changes when RPM increases. From your description, I would think that the symptom would be weak console air at idle, becoming stronger at higher RPM.

davek9

First thing I do when Troubleshooting this system is to pull vacuum at the Firewall where it all starts and see if the under dash modulators and solenoids hold a vacuum when they are asked to do so.

Wally P. did an excellent writeup on the entire HVAC system, and it's always my goto guide, thanks WallyP

safulop

Oh I forgot the most important tidbit of info though. The effect is observed only with the AC running. The airflow directions etc. are stable when the AC is off. So this can't really be just the vent flap solenoids and vac actuators can it? It must have something to do with the AC operational mode and its interaction with stuff.

Randy V

Check/replace the vacuum check valve off the brake booster.

Likely culprit.

safulop

Will do!! See, I knew that some wizard would know a fancy trick to this.

safulop

I looked into this and learned that vacuum leaks at the brake booster are a common cause of smog failure due to high HC. My car has had this problem ever since I've owned it, and low city mileage too. I'm hoping if I replace this valve it could solve several issues at once.

Alan

No - vacuum is high at closed throttle (idle) and very low at WOT. The only way you have good vacuum at WOT is to store it and have no leaks...

Alan

Alan

Then it likely isn't defaulting to defroster mode - its probably just the comb flap actuator dropping out. It could be a general vacuum leak - but its rather likely IMO that this one actually leaks. The actuator is a little hard to get to - of course.

The comb flap is a bypass flap - it is what directs air from the AC evaporator straight to the console vent in AC mode - but only when on sufficiently cold settings. otherwise the evap air goes through/around the heater core first (some % through based on the mixing flap positions). Airflow drops (esp. at the console vent) in non-bypass mode and usually gets warmer too.

Alan

safulop

Thanks for more food for thought Alan.

safulop

Bump...

replacing the brake booster vac check valve seems to have solved the comb flap fluctuation with AC on.

Thanks for the tip Randy!

-Sean