I don't know if it's always done this, but I've noticed recently that when I shut off the engine I hear a hiss from the centre dash / console area for about 3 or 4 seconds. I imagine this is from the vacuum that drives the valve system that controls the heater vents. It happens about the same whether all 3 buttons are pushed in, or if none of them are pushed in.

Is this a normal pressure bleed, or does it likely indicate a leak that needs to be attended to?

I just fixed this last week. Check the black and white one way valve with a hose coming out of the injector manifold. You should pull it off and try to blow each way. Air should only flow one way.
I had to clean mine up with PB Blaster, and bang it on some wood to free it up.
Hissing is now gone when you shut off the engine.

Here's something I posted a couple of weeks ago on another post:

Each time you push in (or out) one of the buttons, you are transferring air from the bellows that controls that flap to the vacuum tank (reducing the vacuum). The white vacuum tank, which is right behind the passenger side of the console, gets evacuated when the engine is running, via the vacuum port on the intake manifold. There is a one way valve connected to the vacuum line that runs from that port to the white vacuum tank. When the car has stopped, this valve revents air from the intake manifold from entering the vacuum tank, thus preserving the vacuum. Therefore, when the engine is stopped, the vacuum tank will have enough vacuum to operate the flaps for any of the three buttons a few times. A quick check of the ability of the system to hold a vacuum is to disconnect the one way valve after the engine has not run for 15 minutes or so and listen for the hiss of the vacuum tank filling with air. A more sophisticated test is to remove the one way valve (engine not running), hook up a hand held vacuum pump and pull about 10 inches of vacuum. You should not notice any reeduction in vacuum for as long as you care to look at the gauge. Start with all the buttons out. Then, push in one of the buttons and observe the vacuum reducing and then stabilizing to a new lower level. Pull the vacuum back up to 10 inches and redo for the other buttons. At this point, all three should be in. Now redo the procedure by successively letting each one out. If any of the buttons causes the vacuum to go to zero, you've found your problem.

I had to replace the vacuum line from the manifold to the vacuum accumulator. The old one kept splitting. Cheap to replace and gets rid of a lot of problems.

I'd check it. Yes, it should hiss if you press a button, but if it hisses when you shut off the engine, I'd guess it was leaking, but unable to be heard when the engine is on.

There is one large hose from the manifold to the vacuum accumulator, plus a group of hoses to the buttons from the actuator flaps etc(4 or 5 hoses if I remember). The little hoses are numbered, as are the lugs to plug them into on the control switch. Mine had 2 which were reversed when I got the car, plus most of them were a bit loose. I cut the hose ¼ inch shorter on the loose ones 4 years ago and haven't had a problem since. The thick hose was loose also. I didn't notice the engine ran any better after, so the vacuum leak was possibly inconsequential. But mine didn't noticably hiss.


Alan Clarke
Nambour, Australia

Thanks for the information, Alan.

I had the console apart today. If I disconnect the grommet from the right side of the accumulator and plug the opening in the grommet, there are no leaks, so the problem is after the inlet to the accumulator.

The only other connection I can access easily is on the other end of the accumulator on the driver's side (it appears to go up to the 3-button vacuum control head). If I disconnect this grommet and plug the outlet hole on the accumulator, the hiss problem happens.

Do you know if there are only the 2 connections to the accumulator (the ones on each end) or if there are others?

If these are the only ones, then I'm thinking the accumulator itself has a leak (as I'm quite certain the seal on inlet grommet is good). If there are other connections to the accumulator, then likely the problem is downstream in one of them.

It sounds like the one way valve has ceased to function. It is in the line that leads from the manifold through the firewall and under the dash. It is near the manifold end of that line. I saw a post this week that someone had that problem and used a solvent to free up the valve- certainly worth a try.

Randy