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[24.57] AFM

AFM, The Problem
From: Sam Walters - compiled from the archives
Year(s):  WaterCooled
Model/Type:  Gas, Digifant, Digijet
Bentley Page(s):  24.57
FileDownload:  The whole shebang
Symptom(s):  Bucking, Stumbling, Hesitation, Loss of Power, Cutting Out

Problem Defined - terms frequently used to describe it: Bucking, Stumbling, Hesitation, Loss of Power, Cutting Out, etc. At its worse, car will drive about half a mile, buck a time or two and then cut out, then restart just fine (after some period of time from a immediately to 20 or 30 minutes), only to repeat the process, over and over. Someone reported taking a long trip with this happening every half mile or so.

A Total Analysis of The Potential Sources of the Problem - Covers Several Major Systems of the Vanagon, While Most of What Follows Focuses on The Fuel Injection System.

A very small group of problems can cause the type of hesitation/poor performance you speak of. It seems that the remainder of your message was clipped, so we don't have a lot to go on, so I will explain the obvious things to may have one or many of the following problems. It has to be one or a combination of three things.
the fuel injection system, or
the ignition system,
a bad ground.

Most likely problem is the fuel system. Bucking here is often caused by a dirty wiper on the Air flow meter (AFM), but never have I seen this fault cause such a marked decrease in performance. There is a lot of recent information in the archives (a post in the past 24 hours) on cleaning the AFM wiper. Follow the info and clean it. This may help.

To diagnose the ignition system, look for an old condenser, ugly cap/rotor, or bum ignition wires/a high voltage spark leak. Change your plugs, rotor, points, and condenser if this hasn't been done recently. If your plug wires are old, replace these. If the ignition is the problem, this will probably fix it. The only thing that would remain questionable is the coil, which probably wouldn't be the problem, but also 'could' cause your problem, though it is less likely.

Finally, (III) check all the grounds. There is a group of grounds under the ERG valve, and MANY of the fuel related components rely on a solid ground. If, for example, the dual relay isn't screwed tightly to the firewall, it may not feed a clear signal, and cause 'bucking'. Check all of these.

Also, the "dual relay" (on the driver's side of the firewall) may be to blame (small black box with lots of wires). This is a semi?costly part, ($50?) and I am in favor of opening it up and cleaning the contacts thoroughly prior to out?and?out replacement. It is time consuming and requires patience, but may be worth the effort. Finally, your fuel filter 'could' be the cause, but that usually doesn't cause bucking when it is clogged, just horrible performance (like not being able to get above 40 mph).

[S.W. - the fuel pump relay is available for $5 from Bus Depot, when a mechanic swapped an old one from a dead van in his lot, my performance increased markedly, I say a post somewhere saying that if this relay is bad, that it could gradually fry the fuel pump. The same mechanic had also found poor output from my fuel pump and replacing it had also improved things substantially.]

Also, the battery MUST be firmly attached to the circuit in the L?Jetronic system (which you have). I had a 1976 bus that bucked wildly, but intermittently from NC to TX. At 2:00 a.m, after looking at everything else on the van for two days, and fed up with the bucking, we wearily stopped in the parking lot of a place called "Clown Around" in the middle of nowhere, and figured out that the negative lead to the battery was loose. Check that.

In the broad analysis, ignition related bucking is usually more "sharp" or severe than fuel related bucking, if than makes any sense. The cylinder will usually fire a little bit on remaining fuel vapor if the fuel cuts out for a microsecond, but a lack of ignition means a total failure to fire for that instant. Comprendez vous?

Please let us know what the problem turns out to be...
Best o' luck,
G. Matthew Bulley, Bulley?Hewlett & Associates Cary, NC USA

Summary descriptions of problem & alternative fixes:
This is a common problem...most likely. If you check the archives....other people refer to it as stumbling, bucking, etc. For some, it only happens at higher elevation. [For me, it happened much more frequently when the weather was hot. But was not completely restricted to hot weather.]
The cures vary....they are:

[Replacing the Air Flow Meter (AFM)];

VW makes a special wiring harness that replaces the old one at the air flow meter that is designed to cure this... takes about 10 minutes to install....about $90, I think, but that is not always the problem;

[The cheap capacitor fix - duplicates the special wiring harness from VW.]

Cleaning the carbon track in the sealed Air flow meter (AFM) has worked for many and is free....don't ask the dealer to do this, he won't and will want you to buy a new one for hundreds instead...see the archive for the procedure...fairly simple;

Replace your 02 sensor if it hasn't been for years (details in the archive)...make sure to get one from either the Old Volks Home or bus depot...or possibly volks motorsports cafe....for much less than 1/2 dealer price....about $25;

There is a small temp sensor, called the TempII Sensor, near your thermostat that sends signals about the coolant temperature to the fuel injection...some have found this to be the cause...about $23 from a dealer or any of the list suppliers;

[This is a recent post about the proper functioning of the Temp II sensor:

The ECU does not actually change fuel mixture in any relationship to temp except for cold start. The ECU basically uses the temp sensor to replace multiple temperature switches. During cold start, a range is set for cold start enrichment and operation. When the coolant reaches about 100'F, the ECU goes into "Closed Loop" operation. At this point, the final fuel mixture is determined by the oxygen sensor. This is where modifications can be made to alter the mixture. Even modifying the airflow meter will have little effect unless things are so far out of range, the O2 sensor can't compensate. Dennis 10/26/00]

[Sam addition: On my 84 Vanagon, the problem never occurred when the two leads into the Idle Stabilizer Unit were disconnected from the unit and plugged into one another. With this unit replaced, so far so good. It is less than $30 from list vendors.]

It never hurts to replace an old fuel filter, but probably not the problem.

Another summary:
Marshall Ruskin writes:
I cannot help you with the head gasket problem, but I may be able to help with the bucking. It is an extremely common problem, often, but not always caused by electrical noise in the Air Flow Meter (AFM). There are several similar ways to fix this:
(1) Most expensive ? replace AFM and pray that was the problem; [$150 list vendor]
(2) Next most expensive - Obtain $150 US AFM harness from VW, and pray that it fixes the problem; [$120 list vendor]
(3) Cheap solution - Attach (solder) a 22 microfarad 35V tantalum capacitor across pins 2 and 4 (somebody correct me if I got the pins wrong) of the connector to the AFM ($0.25 ? $3.00) ? no need to pray;[See below for the HOWTO.]
(4) Carefully pry the black plastic lid off of the AFM, (may need to cut the silicon seal), spray electrical contact cleaner liberally, move wiper a few times, let dry. Avoid touching contacting surfaces with your fingers, or getting dirt, dust or oil inside. This is my personal favorite. I pray to MTNGAL who discovered this trick ? works great, solves the problem, and is very easy.[More on HOWTO below.]

(5) Cheapest solution: Open AFM, move the potentiometer's wiper contact surface so the wiper contacts a fresh surface. Some serious skill required. Cost $0.00. [HOWTO below.] Hope this helps,
Marshall Ruskin 84 Westy Iron Igloo Winnipeg, Manitoba, Canada

See Comments on this entry (F.W.I.W.)

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