Bucking 78 2.0L FI

[Willems]

Hello all,

The problem started around 2 weeks ago. The first time I noticed it was while accelerating in 3th gear. It was like the injectors didn't fire for a second.
I'll drive along and everything will be fine and then the bucking begins, I'll let off the accelerator and then press down again and it will go away. The second time the bucking lasted a little bit longer.

We where getting ready to make a trip (1000miles) and I replaced the fuel filter before we went. (last year I cleaned the fuel tank)

On or road trip the bucking was getting worse to the point that we had to gear down to third at times (no power). The next day the problem didn't returned. The 3th day after 30 miles the problem returned and remained and we coasted down to the town.

I shut down the engine in hope that it will solve te problem but it didn't. The engine was still hot and idles fine. Only when it needed some horsepowers it started bucking.
I checked the fuel pressure at idle and it was fine at 2 bar

After checking everything (points, filter, fuel pump, coil, spark plugs, spark plug wires, timing, valve lash,...) the engine had cooled down. The engine didn't idle anymore only with the throttle half down. The engine was smoking grey and running very rich (smell)


After towing the bus back home I checked the compression of the 4 cylinders and they all where the same at 120psi.

I checked all the wiring, everything OK
Temp sensor 2 is OK
I tried with an other ECU (don't know if its a good one), same problem.

I checked the AFM and measured the voltage between PIN 7 and 8. The signal isn't linear. Shouldn't it be??
Or should I connect the volt meter to PIN 6 and 7 like mentioned in this thread? https://www.thesamba.com/vw/forum/viewtopic.php?p=4667033 t



Link

[furgo]

You should be measuring the AFM voltage signal to the ECU between:

- Pin 7: air flow signal
- Pin 6: ground

That should give you a curve of voltage vs. angle of deflection of flap that is not linear, but rather logarithmic(-ish), as is the relationship between air flow and applied force to deflect the vane. I did this measurement a while ago on a non-bus AFM, but if I'm not mistaken, the voltage range and variation is similar, if not the same. The power supply voltage was 12.4 V.

Notice the dead zone at the start of the track, where the voltage does not change until the fuel pump contact has been opened.

Here's the curve:

Same, with logarithmic axis for the vane deflection angle (i.e. force exerted by air flowing):

_________________
'79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)

Decode your M-Plate

[Willems]

I've measured the voltage between PIN 6 and 7

It seems that the AFM is OK.

Any other thoughts?? Thanks


Link

[furgo]

Yeah, looks ok rangewise. The only thing I noticed in the video is when the multimeter goes to "OL", but I think it's just it switching ranges in the auto mode. What do the resistor track look like? Is it particularly worn on a given spot?

Does the distributor advance (both centrifugal and vacuum) work?
_________________
'79 Westy, P22 interior, FI 2.0 l Federal, GE engine (hydraulic lifters)

Decode your M-Plate

[aerosurfer]

[Willems]

The resistor track looks OK. Vacuum and centrifugal advance both work.

Changed the points (only 1000 miles old) and it fired right up. I went for a test drive but had to return immediately, no power and bucking, idle was bad also but it kept turning.

Put some sand paper between the new point and the idle was back OK. Went for another test drive and sometimes it went well through all the gears and sometimes it started bucking again.

Put the old points back in and the engine won't start. So I think we have found a part of the problem.

I also checked the condensor with my multimeter and it gave 0.200 micro F

[Willems]

I went for a longer test drive and at some point the engine started bucking badly again, no power, had to drive back home with 15 mph.

So after cleaning the points with sandpaper I can drive for about 1 minute before the engine starts bucking again, after 5 minutes its bucking so hard its not going faster then 15mph on a flat road.

[Randy in Maine]

I would first be looking at the electrical connection for the TSII as it fits into the engine wiring harness.

After a bucking event, see if ground out the connection at the wiring harness to a known good ground makes it go away......since it is free to do.

[Tcash]

Bucking is usually electrical. Clean all your ground especially the two under the plenum.

I also checked the condenser with my multimeter and it gave 0.200 micro F
Change the condenser, they can fail intermittently.

Ignition High Tension Leaks

Good luck
Tcash

[Randy in Maine]

And make sure the condensor is securely attached to the distributor. When they get loose they can do that somethimes.

A broken wire in the engine wiring harness can also do that sort of thing. Your VOM set on continuity can really help here.

[tootype2crazy]

Run your hands along the spark plug wires while it's running (be prepared, no long clothes/hair) if you get shocked you have a spark grounding out and will cause bucking. Broken insulator ends where the wire hooks up the the spark plug cause bucking as well, ask me how I know.
_________________
air-cooled or nothing for me
1978 Sunroof Deluxe Bus (daily driver)
1978 Transporter (mom's, making into a camper)
1970 Single Cab 2.1 turbo/EFI 6 Rib, 78 front beam, vanagon backing plates on rear (project)
2001 GTI VR6 (wife's)

[timvw7476]

the condensor can ground out when heated up.
Pulling the engine hatch & looking for the jump to ground is easy,
keep the engine lid closed to keep it dark in there so you see it rather
than feel it. You may even see the discharge point on the can when you
remove it.

[Tcash]

[Tcash]

Condenser Failure in Conventional Ignition Systems
Steve Maas
http://www.nonlintec.com/sprite/cap_failure/

[tootype2crazy]

[Tcash]

Ya I've looked down the throat of a carb, a 1000 times trying to fire an engine , until I got my eyebrows burnt off.

[Willems]

Replaced the condensor today and cleaned the points again. Went for a testdrive and no bucking anymore!

I wish I hadn't thrown the old condenser away. It was 40 years old and still working. The new one only lasted 1400 miles

[Abscate]

[Willems]

I'm trying to fix the next 'problem'. When I'm in neutral and rev the engine to 3000 - 3500 rpm it is slow to return to 950rpm.

I noticed when I disconnect the decel valve from the break booster T (and plug the T of course) it drops to 950 immediately.

But the whole idea behind te decel valve is to let air in te system if you decelerate?? So maybe its normal...??

[SGKent]