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Adjusting Wiper Spring

AFM, Adjusting (experimental)
Year(s):  1986 -1991
Model/Type:  Gas, Digifant
Bentley Page(s):  24.57

From:  Darrell Boehler

Symptom(s):  surging, chugging, stalling, runs rich

-----Original Message-----
From: Martin's To: vanagon@VANAGON.COM Date: Thursday, October 22, 1998 7:52 AM
Subject: Digfiant being defiant
I have a 87 Vanagon with a digfiant afc. Now that the cold weather is upon
us in Chicago, the sytem is running terrible. The fuel consumption is
around 16.1mpg (automatic) and the tailpipe is very sooty. ( had been
18mpg) The engine is surging/chugging and stalling frequently. I
disconnetcted the O2 sesonrs with no better results. I also checked the
temp sensors and they were resonable,( as I did not have the exact temp).
Any ideas ? The O2 sensor is the 3 wire type, and the coolant temp sensor
were new in the summer (I know, that's meaningless)
Date: Thu, 22 Oct 1998 08:50:42 -0500
Reply-To: Darrell Boehler
Sender: Vanagon Mailing List
From: Darrell Boehler
Subject: Re: Digfiant being defiant

Hi Larry,
Your van should run near normal with the o2 sensor pulled. Possibly your
afm (air flow meter) main spring has relaxed a bit and is causing your too
rich mixture. I have seen afm off enough that the oxygen sensor / ecu could
not compensate enough and the vans ran very rich all the time. I would
suggest you attach a lcd meter to the black wire on the o2 sensor and check
the voltage after running a couple of minutes. Have the o2 disconnected.
Below 0.5 is lean , above 0.5 volts is rich. This may seem stupid as you
already see black exhaust but it will let you know the o2 sensor is

Now if you pop the black plastic top on your afm you will see a wiper
with a contact on it. If you move the wiper to the right is leaner to the
left is richer. See if you can get a reading on the o2 to go rich and lean
by moving the wiper left and right. If you determine the afm is set up too
rich or lean you can adjust the tension on the main spring you see in the
afm. Mark where the black plastic gear is situated before you move it . To
adjust the tension pull the detent, loosen the detent screw is easier, out
of the gear teeth and increase spring tension to run leaner and decrease
tension to run richer. One other little detail, the afm should be adjusted
around 3k rpm and I like slightly to the rich side of the perfect 0.5 volt
reading, about 0.6-0.7 volts. With the voltage adjusted at 3k using the afm
main spring tension now adjust the idle mixture using the idle bypass screw
on the afm box. Double back and recheck at 3 k and idle again, tweak them a
bit if necessary.
Good luck with your quest. One little word of caution it is necessary
that your tune up items be in good shape no bad wires etc. If you have a
leaky injector this will not be good either. So please mark the gear so
you can return it to the original position.

Darrell Boehler
Makanda Illinois


From: Malcolm Holser

Do this stuff first, make it all right before you go messing with the AFM.

*Check all the plugs and wires, rotor, distributor cap, timing -- general tune-up

*Check all the FI wiring harness, unplug the wires at each connector and look at the contacts, reconnect, check all the grounds. Be methodical.

*Check the fuel pressure and the fuel pressure regulator (you need a fuel pressure gauge for this, but it is a useful tool you should own).

*Check all five injectors for proper spray pattern and for leaks -- in your case pay particular attention to the "extra" cold-start injector.

*Check for intake air leaks

*Re-check the temperature sensors -- does the car run well cold and only give fits as it warms up?

*Check the AFM wiper track. These wear out, although the usual symptom is intermittent "cutting out" while driving.

While the info on the AFM spring adjustment is valid, this sort of thing usually is gradual (it can show up suddenly). Check everything in order, and then you might want to start messing with that spring. This is kind of scary work, as the tension here controls the open-loop mode fuel mixture across the whole throttle range.

A recommendation is to buy, read, and keep the Probst Bosch fuel injection manual in addition to the regular Bentley book (Probst is a Bentley book, too, I think).
Most of the list vendors sell it. The FI is not all that difficult to service, but it is something with many factors controlling it, and you really want to deal with them in a methodical proper order.

Good FI mechanics are out there, but are tough to find sometimes. I advise that you become one yourself. The tools (mainly a good VOM, a fuel pressure gauge, and the books) will set you back the cost of a single repair session
or less.

Malcolm H.

From: Darrell Boehler

Hi Malcolm,
Excellent advise, it is a must that the rest of the system be in proper
repair and tune before attempting afm( air flow meter) adjustment. I
didn't stress that enough on my original post. Thanks for emphasizing this
Malcolm. It is a rare thing when a digifant afm needs adjustment. The
bosch book "Bosch Fuel Injection & Engine Management " you mention is
especially good to have and review from time to time. It is like icing on
the cake and is relatively easy to understand. With lots of trouble shooting
and component analysis.

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