RPM wave when cold - engine wants to quit

[beetleman217]

Hi all,

I have a recently rebuilt 2.0 fuel injected stock engine in my Westy. I've driven it from MD to VA and PA and it performed beautifully.

However recently I'm experiencing a problem when starting it. When the engine is cold, it starts, revs nicely for a couple of seconds, but then the rpm goes down suddenly and I have to press abruptly on the gas pedal to keep it from dying. If I succeed, it revs back up nicely for another few seconds but then wants to die again. This goes on for 6-8 times and if I manage to keep it alive by pressing the pedal when it wants to die, finally it cooperates and stays on, still with moments of very low rpm but it holds. After driving around a bit, it holds nicely and drives ok.

This is a new phenomenon and sort of feels like a sin wave in rpm when the engine is cold. I know the cold start valve (aka 5th injector) is good because it fires right up on the first try.

In those moments when it wants to die it feels like there's complete starvation of air or gas, and when giving it gas it sort of stutters and 'coughs' violently (by violently I mean it kinda shakes the bus a bit).

The only way I can prevent that wave is if I give it a lot of gas upon startup, I mean really raise the rpm (and even then I still can feel the rpm changing a bit), but that is not the way I want to start the bus every time.

Is this issue familiar to anyone? What could be causing this all of the sudden?
_________________
If all else fails, stop using all else

1977 Westfalia
1961 Beetle

[Tcash]

Test the Auxiliary Air Valve
http://foxed.ca/rx7manual/manuals/BOSCH%20L-Jetronic%20Injection%20Manual.pdf

http://www.vaglinks.com/Docs/VW/Misc/VW_L-Jetronic_Fuel_Injection_Workshop_Manual.pdf

[beetleman217]

That make sense... Since the problem goes away when the engine gets warmer, it has to be one of the parts that only operate when engine is cold. The thermo-time switch signals the cold start valve when to work, but the auxiliary air valve is kinda independent. I should probably first test that it gets power when starting, but even if it does it still might be broken... Is there some other way to test it before obtaining a new one?
_________________
If all else fails, stop using all else

1977 Westfalia
1961 Beetle

[Tcash]

In the links provided.^^^^^

[beetleman217]

So everything I read and researched (including this fine thread) pointed to that the auxiliary air valve was the culprit. So I ordered a good used one. Imagine my dismay when I put it in just to find that nothing changed. I was starting to worry it's the ECU or a similar nightmare. Luckily I had a spare Air Flow Meter that I bought at a swap meet just as a spare. I had no idea if it even worked or not, but decided what the heck let's give it a try. I swapped the AFM and OMG, the motherscratcher is purring like a kitten! Starts on the first try every time and holds nicely. So it was a bad AFM, which worked ok for a few months and then just broke down for no apparent reason. The AFM is a critical component that can wreak havoc and cause you to suspect a million other things. You just gotta have a good one. I hope this info helps out the next guy.
Cheers and thank you Tcash
_________________
If all else fails, stop using all else

1977 Westfalia
1961 Beetle

[Wildthings]

Don't rule out a vacuum leak. Your replacement AFM might just be tuned a little richer which compensates for the vacuum leak. AFM are pretty simple devises. You can remove the lid and inspect the wiper track. If it is ripped up looking it is bad. There are way to compensate for a damaged track.