sublm8

Longstanding issue with my 2000 E-gas C2 bucking or surging in stop/go traffic, just off idle at minimal throttle input. Coming off throttle or adding more fixes the bucking. No issues with more normal driving, it's just when crawling 4-5mph in first and only at the slightest of part throttle.

I ran some Durametric tests today and while the E-gas sender is reading consistently during these conditions, the throttle plate angle potentiometer is oscillating in time with the symptoms when the bucking is happening.

Cleaned throttle body, E-gas contact on the TB, and MAF without improvement.

Is there anything other than a faulty E-gas TB potentiometer or stepper motor that could be causing this? Again, the sender at the pedal seems to be reading fine.

hardtailer

If ignition on injection cuts out and back in on multiple cilinders you would experience the same.
However, based on your findings a worn carbon track inside the potentiometer sounds likely. OTOH it is generally a very reliable part.
Slack in the driveline from worn/cracked guibo coupling (if a c4 has that), worn drive shaft joints and/or severely worn torsion springs in the dual mass flywheel can cause this too. If slack is the cause it tends to increase in amplitude/severity until you step on the clutch, add or remove throttle.
What mileage has your car done?

ETA: you have a c2, so no drive shaft to the front wheels and no guibo disc either.

sublm8

Interesting thoughts. I have 200 cell cats with O2 sensor spacers on this car, also thought maybe it could be cycling enrichment at part throttle.

I do think it’s the electronic throttle body though. Here’s the Durametric for actual throttle value at steady part throttle with the engine off, which isolates it to only the throttle pedal, DME, and throttle body. It’s oscillating between 1.3 and 1.4%. This is what it looks like when bucking as well:




and here it is also engine off, with just a tad more throttle. Much more stable.



I visually verified that the E-gas throttle linkage is fine and the Durametric values at the pedal sender are stable and don’t fluctuate like this.

Porschetech3

The potentiometers at the pedal sensor and TB sensors are redundant...you have sensor A & B

at the pedal sensor A goes from low to high v and senor B goes from high to low crossing in the middle, this redundancy help the DME to ensure the Sensor function...

at the TB SEnsor A is half the value of sensor B .. this redundancy helps to ensure accruacy ...

If Sensor A&B do not coincide as expected a fault is stored ...

with this system it is very unlikely to have a faulty sensor/symptom with no fault code stored.

I would suspect another cause of the jerking like a MAF sensor that is unstable at low values...ot a purge valve that is flowing to much/little...

hatchetf15

OP - Along with sticking air system valves, as PT3 mentions, have you checked the butterfly on the throttle body? The plate potentiometer values may be indicating it’s the throttle body. If it sticks or ‘ratchets’ at all, with only a mm making a difference at such a small input value, the rpm’s will surge at lowest rpms if the operation isn’t smooth. Might also be the electrical connection internals to the tb. Have you checked continuity at the pins?

This small open value is why some tuners don’t like putting GT3 throttle bodies on C2 M96. GT3 tb is almost closed at low rpm on M96 motor, depending on how much air volume the system is using.

sublm8

Thanks - yeah I read that the voltage from the two pots in the TB will always add to 5V and I have no codes thus I agree it’s unlikely to be the pots themselves. The butterfly isn’t sticking and idle is fine.

Driving at 4mph around a parking lot, it will buck maybe once every 5 part-throttle attempts. It’s one exact spot on the pedal. When it happens the potentiometer voltage on the Durametric goes from relatively stable to oscillating, exactly in time with the bucking. I removed the e-gas cover on the TB, the gears look fine but there is a little play, unsure if that is normal though.

Regardless, my next step is to source and swap a known good TB before going further in troubleshooting. Will update.

theprf

Can't answer for the M96 cars, this is something that happens occasionally on the 996 Turbo. The carbon tracks on the throttle body wear out and that gives exactly this symptom. You thought about swapping the throttle body is quite reasonable.

Porschetech3

This picture is a graph of throttle % with engine off'?
Can you also graph the potentiometer V values of both Sensor A and sensor B ?

I highly doubt both pots will be failing at the same time but graphing them both side by side will confirm that ONE of them is unstable..

hardtailer

As above and get the Y-axis in the picture too this time, please.

sublm8

Update:

New TB. Durametric graphs were exactly the same as with the old TB but it did seem to drive a little smoother in stop-and-go. I think the butterfly pivot was little worn on the old unit.

The more likely culprit - discovered a bad vacuum leak at the brake booster elbow. Rescue taped it with further improvement.

It still surges once in a while off idle but it’s only in stop-and-go and not really a problem with current frequency. Sooner or later I’ll smoke test my intake and find the rest of my vacuum leaks.