Stumble -- it's not the ignition system... I think

[gatorjos]

I have an extremely intermittent stumble that doesn't seem to be related to anything. I thought it was rain and hills (water in the gas, bad spark plug wires?), but now that I have replaced the spark plug wires, and it hasn't rained in a few days and I noticed a big miss the last drive, I think it might be the EFI wiring.

Do you guys have any suggestions for likely suspects? Where do I start? Thanks!!
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1977 Westfalia Weekender Berlin "The Kitebus" aka " Colonel Mustard"

Previous romances:
1972 Ghia cabriolet (full body-off restoration -- stolen after five years of ownership)
2004 Passat 1.8T (manual)

http://21stcenturyhobo.com

[Randy in Maine]

Personally I would inspect all of the wiring to make sure the connections are all clean, tight, and secure. Start at the battery, starter and chassis grounds then move to the fuel pump, double relay, TSII, FI grounds and then all of the ECU connections.

That cost you nothing and only takes a couple of hours to do. Something might leap out at you, but at least you will know that that is not the problem.

[aopisa]

Take a look at the AFM contacts and tracks to see if they are worn/dirty. I had an intermittent stumble that turned into a non-running condition. It finally took a significant adjustment to the AFM to cure it.

Just start by taking a look before doing anything else. Many here will tell you to leave the AFM alone if you don't know what you are doing. Good advice since you can quickly make things worse. I had the help of Amskeptic when we made the adjustments.
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1977 Westy 2.0L FI

Leap into the boundless and make it your home! ---Chuang-Tzu

Please let me know if you do not receive this message...

[MidWesty79]

How intemittient are we talking? Once every mile? Once every 5 miles? Once every 50 miles? Once a week?
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1979 Westfalia, 1975 BMW 2002, 1996 Mazda Miata, 1979 Jeep CJ5, 2006 325 XI Wagon, 2021 M240i (all manual transmission... as god intended.)

[SGKent]

did you add a can of gas dryer? If it is moisture it can slosh around in there and cause that kind of miss.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[gatorjos]

So the stumble will be gone for a week, and then all of a sudden return once, or repeatedly for a while. It does "appear" to be related to hills (up or down, so not load-related), moisture, and moon phases (just kidding -- not moon phases)

I replaced the fuel filter, I do have a new fuel cap, and know my tank is sealed well. I like the gas dryer idea -- will try that SGKent.

Randy, I have checked here and there, and have replaced some wires, but I will check though more systemically through your list here. Do you use an Ohm meter when doing this? What value do you look for?

aopisa, The AFM is suspect -- I changed my original (that actually looks good, but turned out non-op) to a another used one with pretty worn tracks, but works. I recently had Buslab in Berkeley tune it with their CO sniffer.

It's certainly possible that the AFM is an issue, but looking for a bad ground and adding some gas additive is cheaper for starters. Thanks for the suggestions. I'll post any updates. It's a very intermittent issue, so it might be a while
_________________
1977 Westfalia Weekender Berlin "The Kitebus" aka " Colonel Mustard"

Previous romances:
1972 Ghia cabriolet (full body-off restoration -- stolen after five years of ownership)
2004 Passat 1.8T (manual)

http://21stcenturyhobo.com

[Randy in Maine]

I use both my continuity "beeper" function on my VOM and then I do an ohm meter reading to make sure I am not hanging on to a single strand of wire.

Buy a tube of this stuff to put on the electrical connection that live a hard life (like on the starter, battery and chassis grounds)...I got my 4 oz bottle from a HVAC supply house....

http://www.ilsco.com/ProductsDetail.aspx?kfjkff=YY...AduPeME%3D

[gatorjos]

Is that sorta like liquid electrical tape? I used to use that back when I built boats...

Thanks for the tip...
_________________
1977 Westfalia Weekender Berlin "The Kitebus" aka " Colonel Mustard"

Previous romances:
1972 Ghia cabriolet (full body-off restoration -- stolen after five years of ownership)
2004 Passat 1.8T (manual)

http://21stcenturyhobo.com

[gatorjos]

and oh, I'm planning on driving 1800 miles over the next three weekends, so I'll figure this thing out somewhere between here and LA...

Pismo Beach Ho!
_________________
1977 Westfalia Weekender Berlin "The Kitebus" aka " Colonel Mustard"

Previous romances:
1972 Ghia cabriolet (full body-off restoration -- stolen after five years of ownership)
2004 Passat 1.8T (manual)

http://21stcenturyhobo.com

[SGKent]

in the center of the late tank there is a baffle. It allows fuel to flow into the outlet a little at a time from the top and some holes in the sides. If you have water in the tank, possibly condensation or picked up at a gas station and it does happen, a small shot of it may only get into the line when you are at certain angles. That is why you want to run a fuel dryer. Basically it is isopropryl alcohol which picks up the water and emulsifies into the fuel. Any FLAPS will have some - HEAT is one of the brands and it is inexpensive. This will rule out water.

Another issue with the baffle is that some of the bays had a type of unleaded nozzle restrictor called a bird's beak. It falls apart and a piece about an inch in size can slide around in the tank and momentarially block the outlet.

You also have the possibility that there is a worn spot on the AFM. That can be inspected visually.
_________________
“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin