hoeser

Hi Guys,

I've recently picked up a 2001 S4 as a project car. Pretty clean, 195,000km. Runs perfect aside from this very bizarre issue. The car is stock.

Under heavy acceleration in any gear (from about 40% pedal to WOT) it will surge and buck back and forth violently, it feels as if the car is either not getting any fuel or totally shutting down the ignition. Boost seems steady the whole time - here's where it gets weird - if you hold it through this surging and bucking for about 10 to 15 seconds, it will all of a sudden pull hard and run totally normally until you shutdown the car, and then the whole scenario starts over.

If you don't hold it through the surge the problem will persist, if you hold it through the surge and "clear" it - it runs fine for hours (until you shut off the ignition)... this seems so weird to me. It only throws one code and its 17964 Negative Pressure Deviation.

I've replaced the following:

* Diverters
* N75
* Coils
* Plugs
* Bad B1 S1 O2 Sensor

None of the above have resolved the issue. I'm going to be building a boost leak tester tonight, but I really have a hard time believing its a boost leak. It seems like an electrical gremlin to me but, I'm no expert on these cars as this my first B5.

Any insight into this issue would be appreciated.

Prospeeder

I would suspect its going lean. Mass air flow sensor could be reading erratic. So you have a scan tool? Can you watch what fuel trim is doing when this is happening? As well as MAF reading and misfire moniter? VAGCOM would be best.

hoeser

I did some more debugging and found my fuel trims at -22%. So it's running super rich and pulling fuel, indicating that indeed it is a boost leak. Pressure tested the system and found a significant tear in the throttle body boot. Replacement ordered. Will report back on Monday.

Prospeeder

Sweet that is probably the issue then! I had to replace that boot also when I got my S4, It wasn't obvious till I dismantled the engine to do headgaskets. But it was pretty destroyed

hoeser

I got the replacement boot today and it definitely was the issue. Car pulls hard now, actually a little surprising how hard it pulls... I thought these cars were slower (stock).

Prospeeder

For 250 hp they are surprisingly quick. Mine is chipped so its probably 300hp and very fun!

gt35r7

I had this issue with a bad wastegate line as well. The little runner connector sections like to fail.