Porsche Enthusiasts Club Forum

The heater/air distibution system seem to work fine on my 928 S4, when you alter the distribution control you can hear the mechanism moving, and the air distribution does change. However very little air is directed to the footwells, even on the setting for footwells only. It does work, but not properly, as if there is a blockage. The air stops coming out of all the other vents but not much comes out of the footwell vents and there is a whistling noise, as if air is trying to get through.
Is there anything I can do about this?

This is without the aircon turned on, which has been repaired, and now works. When you turn the aircon on the distribution mechanism alters on its own, all the air is directed to the screen and wont go to the footwell vents at all.

Hi

It sounds like one of the footwell air pipes has become partially disconnected.....the problem being that to get to ANY of that system is a pain in the proverbial as its deep behind the centre console.....


All the best Brett

The whistling sound sounds like a vacuum leak in one of the actuators.

Coincidentally was checking all mine this afternoon!

Without undoing anything (hardly anything) you can perform basic check of the heating/ventilation system to see if you have a vac leak.

Do you have a vacuum tester? If not you can still do really basic test. Idle car for a few seconds to build vacuum then turn off. Now pull off the vac line that goes from under driver side fuel rail to the solenoid which switches vacuum to the resonance valve (aka "flappy valve" - the thing in middle of intake with rubber cover).

As you pull the line off do you hear a hiss of esacping vacuum?

Let me know outcome, and whether you have a MityVac vacuum tester (invaluable - you'll need one to track down leak if you are losing vacuum), and we can take it from there.

If you have lost vacuum then your hvac could still be ok - you may have leak in vacuum reservoir in passenger side wheel well.

To me it sounds like the air is not being directed anywhere, like when you close all the vents on your car and then turn the fan up...the air has nowhere to go so just seeps out from the closed vents. In fact that is what is happening.
If you move the distribution lever to footwell/screen it certainly turns the screen air on, it comes out like a hurricane. There's also no problem with the centre air vent, although that seems to be independent of the distribution control.
I'll check the system for leaks as you suggest though, I guess if there was a leak the distribution flaps may not operate correctly.
Where is the actual air distribution unit located? All I can see under the footwell is an outlet duct.

No distribution unit as such, rather a collection of vacuum actuators, a setting motor and mixer flaps dotted about.

The manifold where vacuum is switched to the various actuators is centralised though and lives behind your radio,

There's a fantastic write-up on testing the HVAC system here, including replacing failed actuators:

http://forums.rennlist.com/rennforums/9 ... -pics.html

Thanks, I've downloaded the article you suggested, and it is very good. I imagine you are right, and there is a leak somewhere.
I need to buy one of those vacuum pumps and then I can get on to it.

OMG the guy who wrote that article has SIX 928's.
I am green with envy!

Great advice from Adrian again.
Good luck Lightning.

Stephen

SIX 928's? That's six HVAC systems with obscure faults and leaks, six faulty aircon units, six leaking sump gaskets, six pairs of collapsed engine mounts, twelve jammed electric seats, 48 injectors that could fail.....I think ONE 928 will do for me!


Looking on ebay with the word "Mityvac" there are all sorts of pumps available. Can somebody possibly have a look and tell me which one I should buy?

One just like this, with pressure gauge and decent collection of adaptors:

http://cgi.ebay.co.uk/Mityvac-Automotiv ... 2556ae7d9b

Thanks...it's on it's way to me.

You'll find it's invaluable Lightning.

Not only is it essential for checking any vacuum actuators on the car (heating/ventilation, flappy valve, fumes diverter, air pump diverter...) but you can do quick check of intake vacuum quality (proxy for gaskets/hoses) just by hanging it off a vac line and seeing what vacuum you are achieving.

And it helps you on your way to 928 destitution - spending more on tools than on parts.

'fumes diverter', oh please tell me more about that, what would be the symtoms if this was faulty?

If it's anything like I've experienced so far, the symptoms will be no money left in your wallet.

Er you would smell fuel fumes.

Collecting fumes into an activated carbon canister allows the fuel cap to be fully sealed - the vapours have to go somewhere though if it gets warmer or when tank is filled.

Fuel vapours from the fuel tank are hoovered up by the intake system then dumped into a large carbon canister bolted into the wheel well, under control of the fuel injection computer. If the vacuum actuator that routes the fumes fails then the system won't work, though a more common failure mode is for a plastic y-piece (£1.26 worth) to crack and allow fumes to escape before they reach the carbon canister.