77 Bay window 2.0FI hiccups between 3000 - 3800rpm

[heiko910]

Hi there,
I have an issue with my 77 Bay window and I am running out of things to check.
1977 VW type 2 with a 2.0 Fuel injected type 4. All stock.
My Bay window runs and idles great, pulls strong. It has been perfectly reliable until it started getting a highspeed hiccup between 3000- 3800rpm.
It is not miss firing, at least it is not making the typical miss fire noises.
It is just bucking in that RPM range (very intermittent). It will keep accelerating even through those hiccups. It only does it when the engine is under heavy load (i.E. up hill, strong headwind). It runs fine and has no signs of hiccups on level road. Neither will it show any indications when revving it when parked.
I have checked everything that I could come up with.
I have change spark-plugs and wires, I have changed the electronic module (i have multiple, one compufire, one pertronix) in the distributor. I have tested the mechanical and the vacuum advance for proper function. I have tested for vacuum leaks with a smoke generator. I have switch the AFM with a backup that I have. I have tested for fuel pressure at the fuel rail and just because I could changed the fuel filter and the fuel pump.
I have checked all fuel and ignition related wiring. I have lubed the distributor. I have checked timing and advance. I have done a valve adjustment. The needle of my tachometer is not affected by the bucking which leads me to believe that it is not ignition related.
It only does it in the above mentioned RPM range. Could it be the mechanical advance?

I have already read all topics here on thesamba and any other webpage about related topics but so far I haven't found the smoking gun
If anyone has suggestion I would appreciate your help.

thanks,
Heiko

[udidwht]

Sounds like it's a lean misfire (surging).

What did the fuel pressure check out at?

How accurate/reliable is your fuel pressure gauge?
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[SGKent]

quick smoke test to be sure there are no vacuum leaks

booster test to be sure it isn't leaking (2 minute test is fine)

Check timing with timing light to see if it flickers or bounces at 3000 - 3800 RPM. If you have a dwell meter and are using points put the dwell meter on it and see what the dwell is. If you have an electronic ignition or different coil other than stock or the good blue one it can be the cause because the ECU relies on the coil primary pulses to set up the injector pulses.

Anything else would be an outlier like loose wire that vibrates at that speed, timing too far advanced causing pinging / detonation etc., loose connector on ECU/AFM/injector that vibrates at the RPM etc. Don't think it would be in the mechanicals because valve / lifter float etc gets worse with higher RPM. It wouldn't be a come and go thing. Having an issue like that at peak torque could point to an ignition or timing issue.
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[timvw7476]

the short hoses (4) where the intake tubes join the intake manifold?
check there for vacuum leak(s).
other than swapping points & condenser for compufire, can't think of any else.

[Wildthings]

I would begin by dumping a bottle of FI cleaner into the gas tank and see what they does for you.

A clogged fuel filter may not let enough fuel reach the engine under high load, high rpm operation.

Yes as others said, check for vacuum leaks.

[heiko910]

[heiko910]

[heiko910]

[heiko910]

[Tram]

Heiko- You say it's all stock. If "all stock" also means you still have the stock spark limiting rotor I'll bet that's your issue. We saw these damn things cause runability complaints constantly.

The other close second would be a wobbly points breaker plate. This was very common also and you'll find it with your timing light. Either the mark will dance around in the light at idle or it'll start dancing at about 3000 RPM and get worse. It'll advance OK but be all over the lot at steady speed.
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[heiko910]

[busdaddy]

Strange as it sounds I've seen this resolved a few times by replacing the TS2, couldn't hurt to try.
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[Tram]

[SGKent]

1977 has the electronic limiter in the ECU.

The way the fuel system works, the excess is always circulating so the demand is the same on the circuit.

I would be running points and the original coil to troubleshoot this. The ECU totally relies on the shape of the coil wave to establish the baseline injector pulse. Change the shape of that wave and the ECU doesn't know how to deal with it. You can also change the condenser on the distributor is you have one.

We had a 1967 Chrysler 383 when I was in my teens. It would buck like a mule once in a while headed up one grade in San Diego. Chrysler did about everything they could trying to find the cause. Like yours it was RPM dependent. Finally after about the 10th trip they replaced the plug wires although the car only had about 15,000 miles on it. That was the problem. Had it not been a car that I drove sometimes, and had I not seen the issue and the time put into it I would not believe someone if they told me wires could fail that way. Likewise I replaced plugs on an Opel GT (NGK) and took a trip to San Francisco from LA about 2 weeks later. It started cutting out and dropping cylinders on the trip under the heaviest load. After looking at everything else I put the old plugs back in which I was lucky to have with me. Problem went away. Put the new plugs back in when we got home and the problem came back. Same plug number, just a bad batch.
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[wcfvw69]

Another vote to put points and condenser back in it and try again. I know I've read before that certain FI's don't like those electronic modules.

The other thought was mentioned already. A loose electrical connection in the FI harness plugs.
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[TomWesty]

I’ve had the rotor cause similar symptoms, but on a dual carb 1776 type one motor.
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[heiko910]

[Tram]

[aeromech]

So you checked the fuel pressure while under heavy load between 3000 and 3800 rpm and it was good?
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[heiko910]