Stock 2001 S4 - Unusual Limping Problem

[hoeser]

Hi guys,

I've got a bit of what I think is a rather unusual problem with this 2001 S4 I recently picked up cheap as a project car. 196,000km on the car, 6MT.

Under heavy acceleration in 2nd or 3rd gear the car will pull normally for 2-3 seconds, and then start lugging as if its either not getting fuel or ignition is totally cutting out - seems to be making boost the whole time. If you stay on it at WOT and let it do this 'bucking' for 5-10 seconds - it seems to clear whatever problem its having and will run totally normally until you shut the car off. It'll pull 0-100mph runs all day until you shutdown and restart, then it starts all over until you let it "buck through the issue".

I've been able to duplicate this behavior consistently. I have no idea what's going on as it doesn't throw consistent codes for this, though occasionally maybe 1 in 3 times it will throw a 17964 - negative pressure deviation. It really doesn't seem like a boost leak since once it initially "works through" the hesitation it will run flawlessly until you shut the ignition off and restart, then it all starts over. It seems electrical or fuel delivery, but the fact that it all of a sudden snaps out of it and just works fine is really confusing me.

Known issues at present that I will be resolving (parts on order)

* Leaking inner valve covers (oil soaked spark plug)
* B1 S1 O2 Sensor code 17521

Hopefully one of you have seen this - any help is much appreciated.

[hoeser]

Did a bunch more digging, I guess it's possible that its the N75. I have a replacement coming but nothing seems to match up exactly to this issue. A lot of my coils were in pretty rough shape and a couple had cracks in the plastic, so I'm replacing those also.

[redline380]

Unusual limping problem? Has your car inadvertently turned into a pimp? You could try a pimp cane. It will not eliminate the limp, but it will help a little.

[hoeser]

Unusual limping problem? Has your car inadvertently turned into a pimp? You could try a pimp cane. It will not eliminate the limp, but it will help a little.

I'll try that, but I want an OE cane, not some aftermarket knockoff.

[hoeser]

Well this has been a frustrating night.

Replaced the valve cover gaskets, installed new plugs and 6 new coils. Swapped the N75 for a new one. Problem persists. Also replaced the faulty B1 S1 O2 sensor.

Car pulls for a second or two, then completely gives up and jerks harshly back and forth - again it seems like fuel or ignition. All of a sudden if you hold it there long enough it clears whatever fault it has and continues along perfectly until you shutdown and restart, then it all starts over again. The only code it throws is 17964 negative pressure deviation.

Someone has to know what's going on here... anything else I can look for? I can't imagine its a boost leak because a boost leak doesn't come and go with a restart of the engine.

[slowSfaux]

I think it's a no brainer to perform a boost leak test at this point. Only once you've performed that should you move on to the next step with these cars.

[hoeser]

I think it's a no brainer to perform a boost leak test at this point. Only once you've performed that should you move on to the next step with these cars.

Fully agree, I'll be building a boost tester tonight and report back with my findings.

[Perth_RS4]

Is your S4 tuned with aftermarket wastegate springs?

[Perth_RS4]

Extract from tuning wiki
Negative deviation (overboost)
If you don't get all of this just right, and your actual boost goes too far above requested boost (by ~200mBar), you may experience overboost throttle cut due to negative deviation, which is ME attempting to get boost back under control by temporarily closing the throttle plate. If it happens enough, and an I-Regulation adaptation value reaches its negative limit, you may get a P1555 - Negative Deviation DTC, and the car will go in limp mode (permanent 0% WGDC).

With aftermarket and/or tighter wastegates, you may experience bucking and choppy throttle response in part throttle near waste gate cracking pressure[26]. The throttle angle desired is dependent on the boost pressure in a turbo car since after a certain angle, throttling losses are negligible and it is better to hold the throttle wide open. To correctly determine how much throttle opening is required to produce the amount of torque requested and prevent the ECU from going WOT unnecessarily, a few maps have to be tweaked in the module LDRPLS to set the base boost pressure in the ECU. The maps that set base boost are:

[hoeser]

When driving to pick up the boost leak tester build supplies I noticed two things - 1) the car felt fine, no sign of the issue - 2) a new CEL.

Pulled the codes and got fuel system too rich on both banks. Checked adaptation and I'm sitting around -20%. When I cleared the codes the problem returned. So, something is clearly causing it to pull 20% fuel across the board. Boost leak likely? Will be pressure testing after work.

[hoeser]

Is your S4 tuned with aftermarket wastegate springs?

The car is stock in all regards, except new 710P's. I just did a real quick leak test and found at least one leak at the throttle body boot. I'm going to remove it, plug the uplines from the intercoolers and see if I can find any more.

[hoeser]

Update: Got replacement throttle boot today, car pulls hard - no codes - no issues.

[M551_Sheridan]

I'm glad you got this sorted out! TBB was the first thing I did to my S4 just as insurance. In the end at least you have new coil packs/N75 etc :)