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Discussion Starter · #1 · (Edited)
Hey guys, need some professional help.

Issue: car out of nowhere started bucking/jerking (you can see the tach do very slight and quick bounces) when decelerating.
Pressing the throttle (even slight acceleration), pushing in the clutch pedal or going into neutral -- all alleviate the issue. No issues with idle.

There are ZERO codes other than a bank 1 S2 (but that doesn't control any fuel regulation and has been like that for ages).

Well before any issues occurred, I had done a bunch of fairly regular maintenance: oil pressure switch, fuel filter, oil change+filter, spark plugs (BKR7EIX -- I'm on a stage 1 unitronic and have been for years), snub and both other engine mounts, transmission mounts, puck valve, brakes all around with new fluid, coolant flush and new thermostat, OE axles. Car was driving with zero issues for 5K at least.

I've cleaned the MAF sensor (with proper cleaner), cleaned the TB and adapted with VCDS, checked pedal sensors on VCDS and doesn't look like it "hangs," swapped axles to test they're not binding, tried to do a leak test with propane as best I could, swapped in NGK BKR6E copper spark plugs to test, swapped coils.

Only other thing I've been meaning to try is to check each of the one-way valves. I think this car may have 4 of them.

Any thoughts or opinions where to go from here? I'm stumped!

Car is a 2000 Passat ATW Wagon 1.8T FWD with Manual Transmission
 

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Between the 'bounces' on the tach and the fact that there are not codes, I'd suspect the ESS (engine speed sensor). I think that supplies the input to the tach and almost certainly also the ECM. It is also notorious for throwing no codes when it starts to fail and then completely failing. Usually that results in a no-start condition and I don't know if that could cause bucking, etc but if the ECM thought the engine wasn't turning it might shut off fuel. Or not. But it's a cheap part and easy to access so it might be worth a shot. I don't know how to test one without hooking up an oscilloscope though.
 

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Discussion Starter · #3 ·
Between the 'bounces' on the tach and the fact that there are not codes, I'd suspect the ESS (engine speed sensor). I think that supplies the input to the tach and almost certainly also the ECM. It is also notorious for throwing no codes when it starts to fail and then completely failing. Usually that results in a no-start condition and I don't know if that could cause bucking, etc but if the ECM thought the engine wasn't turning it might shut off fuel. Or not. But it's a cheap part and easy to access so it might be worth a shot. I don't know how to test one without hooking up an oscilloscope though.
The part in question wasn't replaced long ago... you may be onto something?

That being said, whenever I've had an issue with the ESS (crank position sensor) on multiple other cars, it has been a no-start after car has warmed up or cutting out while driving. The tach bouncing is a check when starting the car to check if this part is an issue or not.
 

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Discussion Starter · #4 ·
SOLVED:

SOMEHOW, a cupped rear tire was throwing this all off. I can only assume that somehow a rear speed sensor was triggered and maybe applying the brakes?? I have no idea how or why, given that it isn't a 4motion but a FWD car. I had tried turning off the ASR button, but that made no difference early in my diagnosis.

Food for thought for someone if they come across this issue.
 
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