Stumbling/Bucking around 2k RPM

[smitty1976bus]

Hi all,
Hope everyone is enjoying their labor day weekend. I planned having on. I was going to take a nice drive and do some camping, but the bus said "nope".

Heres my problem, for the past 2 weeks my 1978 Westy, 2.0L fuel injected, stock engine, has been stumbling/bucking when I reach 2000 rpm, it usually goes away once I go by 2200 rpm. At first it was only when I was maintaining speed, not accelerating or anything. But it's progressively become worse. Up to the point were it does it no matter what load the enigne has... it's un-drivable now.

First thing I thought of was a fuel delivery problem. So I hooked up my fuel pressure gauge and went for a drive. At idle it was at 28 psi, when accelerating it went up to 32 psi. All looks rather normal there.

Then I thought it was the peksy TSII sensor, so I checked that out. That's brand new'sh, and was putting out 103 ohms when the cyl. head was at 196 F. So that looks good.

My FI series resistor block first failed on me at the beginng of summer, so I replaced it with my back up unit. (the engine wouldn't fire, so I put the new block on and all was well, so I assume the old block just went bad) So I reached out to a local VW dude, and purchased a resistor block off of him. It looked to be in much better shape:

This seemed to lessen the stumbling/bucking, or maybe it was all in my head. Anyways, the stumbling/bucking was still present. So I moved onto the ignition system. Timing is at 7.5 BTDC and the points are a 0.16 inch. And my plugs look to be in decent shape:

I went out and bought a Duralast Coil, and new rotor, just to see if it was that. And that made the stumbling/bucking sooo much worse.... The coil looks to be within spec: the + and - have a resistance of 3 ohms and the coil tower and + have 8700 ohms.

Is there something I'm missing, or another test I should try? I've had a couple guys and myself trying to figure this out, and we're all stumped....

Thanks
SS

[busdaddy]

Try another condensor.
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[smitty1976bus]

Just tried another condenser. No change.

I even put the old coil and rotor back on, problem still present, worst than ever...

[busdaddy]

Damn, aside from plug wires I'd be considering popping the lid off the AFM and looking for a worn area on the wiper track.
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[smitty1976bus]

Dang. I knew I'd have to crack open the AFM one day...

[smitty1976bus]

ok, took the cover off of my AFM. That thing came off super easy, there was only a little adhesive holding it on. Not sure if this the first time its been off or not though.

Not sure what I'm looking for:

It all looks pretty good to me. Or are those lines on that track bad?

-SS

[busdaddy]

I don't think I see any of the white board showing through from here so there goes that theory, a little gentle cleaning with a Qtip and alcohol wouldn't hurt just bacause you are in there, also do the contact under the round thing with the three dots hovering over the center. When it's time to put the cover back use an electronics safe RTV or fishtank silicone, regular RTV gives off corrosive fumes.

So why is this happening at a consistant RPM regardless of load?, hopefully other minds will have suggestions soon, I'm beginning to wonder if you have a lifter pumping up (although that usually doesn't go away above the RPM where it starts). How sloppy is the shaft in the distributor?, any side play perceptible?
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[curtp07]

I had a somewhat similar event after my rebuild. Bus was mostly smooth..bucking around 2-3k...

Took my smoke tester to the vacuum system. Surprise. A few leaks. Fixed the problem.

Also check your intakes for proper torque...

Curt
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[timvw7476]

the ignition wires should be measured (ohmed) no more than 35k resistance.
the coil to cap lead as well, some like to leak powdered graphite & make more trouble than anyone knows what to do with. Misfire galore.
EDIT: check the grounding 'braid' on your points plate, they fray to almost
nothing & cause the same troubles.

[SGKent]

I would do smoke test to be sure you don't have a vacuum leak. I would also replace the fuel filter just because. Then I would go find someone who can put an air to fuel meter on it and see if it is going lean when it stumbles. Your plug looks lean.
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[smitty1976bus]

[busdaddy]

Wooo, that black plug isn't just carbon, that's a sick cylinder causing that buildup, done a compression test?, it's not likely the cause of the bucking, but it's something to put on the someday soon list.
The rest sure look like there's a vacuum leak going on, far too much variance between them, they are all over the scale.

Do measure those plug wires too, leaks and bad rings don't usually cause an RPM specific crap out, but bad wires can, so can a bad distributor cap.
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[smitty1976bus]

Damn... sick cylinder meaning the rings are toast? Or maybe bad valve seals? I take it that black gunk is burnt/caked on oil.. As for the compression test, the last time a did one was 2 years ago, but it came up 123 psi back then. I'll have to do another soon, but I feel it'll just depress me. Dare i ask if any of those 'mystery oil' products could actually work with helping my problem.

One problem at a time though. I'll check on doing a real smoke test, and look at the ignition components (feel like it the ignition system, the issue fluctuates everytime I change a piece to it.)

-SS

[Tram]

Replace the plugs with NGK non resistors first.
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[Abscate]

Those AFM tracks look poor to me, but get some expert eyes or a known good AFM to confirm.

That black tar cylinder is telling you to fix now while still cheap.
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[bigbore]

a type 4 is never cheep.
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[smitty1976bus]

[smitty1976bus]

I put everything back together and went out for a test drive. I can't call the problem fixed yet, but the bus did drive pretty nice. There was no hard bucking like before, but I couldnt' tell if there was some light bucking/stumbling or not. It could have all been in my head; I was trying to make it stumble while driving, I kept it right around 2k rpm in 3rd and 4th gear, where it would really start to act up. but nothing happened. I thought it stumbled once, but since I couldn't make it happen again, I figured it was my imagination.

Not 'mission accomplished' yet tho, I need to drive it more, to see if the issue comes back or worsens. It could be that the bus knows that weekend is over, so it doesn't need to be broken no more, ha.

As a plus, the bus has developed another oil leak. Believe this leak is coming from the pushrod tubes of cylinder #1. Nothing too crazy, just enough to be noticed. 2 steps forward, 1 step backwards... or 3.

-SS

[wcfvw69]

It would still be prudent to do a compression test in all 4 holes. That one plug looked concerning to Busdaddy's point.
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[Abscate]

Smitty...get on Colin’s or Robbie’s self help schedule. Best $5xx you can spend and learn the ins/outs of your fuel injection
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