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Tue Dec 27, 2005 7:05 am

When I got my 1990 automatic Vanagon, it would start, idle and run just fine, except at times the idle would stay at 1200 in gear. After it was fully warmed up, if I put it in Park, the idle would skyrocket to 1900!
I took a look at the throttle plate stop screw and found it screwed all the way in; I backed it up until it barely made contact and now my idle is OK at 900 in gear.
Here's the problem, when fully cold it starts and idles fine but after 20-30 seconds it dies. No stumble- just dead like someone shuts it off!
I cleaned the Aux Air Valve and the cold start system seems to work fine (it started right up and idled in 20 degree weather) but I cannot figure out why it's getting a "shut down" signal within a minute. It will only idle on it's own when the temp needle has reached the first white marker on the gauge. Could this be a temp sensor that has gone faulty?
Oh and one more thing, on occasions it still shoots RPM's to 1500 when I put it in Park!!!!!
What's going on?!?!?

Tue Dec 27, 2005 8:03 am

It sounds like the PO tried to compensate for a problem by changing the throttle stop. That is the first problem you identified. Its the screw with the red sealing goop around it. The next thing to do is adjust the idle with the air bypass screw, its the large screw shown in the other pic.

Tue Dec 27, 2005 10:14 am

I would follow the Bentley to set the timing and idle for a digifant bus. Unfortunatly, i've yet to start on the project syncro so i'm a little lost. On a AT bus, is there a system to over ride the idle when switching from park/neutral to drive? Could that be faulty?

r39o has an auto bus that had some similar problems I think. His is a digijet bus, that may make a big difference.

Tue Dec 27, 2005 1:35 pm

Well, today I took the throttle body out and cleaned the crap out of it, adjusted the microswitch and cleaned the throttle cable. Put it together and I got the same situation as before...but I have a new suspect.
After I drove around, the idle shot to 1900 in Park. I looked around and disconnected the Aux Air Valve- the idle dropped to 900, what I had originally set it to a few hours before!
So, with the throttle stop screw getting me low idle and the air bypass srew completely closed, then the only thing left is a malfunction of the AAV!! (no cold fast-idle, erratic base idle, no effect on idle at certain times)
Am I off the mark?

Wed Dec 28, 2005 11:14 am

That sounds like a likely culprit. Engine speed is partly related to air availiable. I think that AAV is common to other VW's if not many other cars with Bosch FI too.

Another link to the puzzle...what tells the AAV what to do?

Wed Dec 28, 2005 3:40 pm

On the 2.1 digifant, the idle air bypass valve is controlled by it's own control unit. I had one of these fail on me but the valve seems to poop out more often (actual moving parts and all). This looks like a large relay and lives behind the pass side tail light. To get to it just remove the plastic panel in the engine compartment. It's the panel attached to the short hose on the input side of the air filter housing.