1.8t - accelerator lags, keeps holding a second or two after I let go?

[volts]

Hi everyone,

When shifting in my 2000 1.8t (AWT), the rpms jump up higher than what I was currently driving at when I push in the clutch, even though my foot is completely off the accel pedal. Since I got this car, I adapted to this and usually wait a bit before pushing the clutch in, but now it's starting to annoy me.

I can hear the turbo flutter instantly when I put my foot off the accelerator (ik it's not supposed to flutter, I might have a bad DV), but even after hearing the flutter and pushing in the clutch instantly, the car still revs up a bit like I'm still holding the accel. pedal.

While cruising without accelerating and pushing the clutch in, the rpms stay still for a second or two, and then start dropping, it drops to 1.1k or 1.2k and stays there, it will go down to idle speed only if I completely stop. I also tried stalling the car and it just wouldn't happen because it was always at cca. 1.2k as long as I wasn't going uphill. It drove by itself in every gear, with my feet nowhere near the pedals, even fifth, not 'coughing' or anything. That made me think the ECU is adding this to prevent stalling? But would this be connected to the shifting situation I described earlier? And why would the ECU do this once I let go off the accel. pedal to shift?


I got it to do something werid before when testing: going down my driveway from 0 (it's downhill) I put it into third and slowly lifted the clutch, once I was rolling I completely let go of it, it coughed but kept going, I did about 10m then pushed in the clutch and brakes completely at once to come to a full stop, the result was the car running by itself at cca. 1.8k (can't remember exactly), holding it there perfectly and I was standing still with my clutch and brake pedals in, the gas pedal wasn't touched at all in this whole process, .... ??? I watched it run like this for like 20 seconds, thinking it will eventually stop, it kept running like this until i put it into neutral and let go of the clutch.

Anyone got an idea what to check? I got VCDS but I only have some HVAC codes and central electronics stuff (alarm triggers etc), nothing that I think would affect this.

[Cybersombosis]

Have you done a throttle body adaptation? Have you done a pressure test of the intake system for vacuum leaks?

Doug

[b5v6]

Vacuum leak? Does the engine speed (RPMs) jump around at idle?

Sent from my motorola one zoom using Tapatalk

[volts]

Haven't checked for leaks yet but that's my next step, the engine speed does not jump around at idle, it runs fine, it's problematic when driving
I did have a code once that was intermittent, saying something in the lines of higher idle speed than expected, but I've cleared it and haven't seen it since, need to check again today

[Cybersombosis]

Do a throttle body adaptation and report back. It’s free.

[volts]

Do a throttle body adaptation and report back. It’s free.

I'll try to do that today after my mechanic changes out some wheel bearings, and I'll report back.

Would it be better if I do the vacuum leak test first? (I'm thinking that if it's caused by a leak, the throttle adaptation might not be the best thing to do at the moment, am I wrong?)

[Cybersombosis]

Doing a TBA literally takes you 2 minutes to do. You have VCDS so all you have to go out to your car, plug it in, and run the adaptation. Pressurizing the system will take you time to gather parts and make the coupler depending on how you do it. One may or may not have something to do with the other.

[b5v6]

Clean the throttle body too, forced induction causes a lot of oil vapor to make it through the induction system. The throttle bodies get gummed up over time from this.

Instagram: @oliepyle

[volts]

Doing a TBA literally takes you 2 minutes to do. You have VCDS so all you have to go out to your car, plug it in, and run the adaptation. Pressurizing the system will take you time to gather parts and make the coupler depending on how you do it. One may or may not have something to do with the other.

Alright I did the TBA on VCDS and the problems persist,

(unrelated but) the engine module also showed three codes, 'powertrain data bus: missing message from ABS Controller/Instrument Cluster/TCU(?)' I fixed the Instrument Cluster one by setting some adaptation channel on the cluster module about immo3 (I've got IMMO OFF and a swapped cluster as of recently), I couldn't solve the ABS one, and I have no idea why TCU is even showing up, my car is manual.

Anyways, back to topic, my next step is doing the vacuum leak test, I'll try to do it over the weekend if I can buy the supplies tommorow and I'll also clean the throttle body, hopefully I can get to the bottom of this soon.
Thanks for the help so far guys:)

[b5v6]

Alright I did the TBA on VCDS and the problems persist,

(unrelated but) the engine module also showed three codes, 'powertrain data bus: missing message from ABS Controller/Instrument Cluster/TCU(?)' I fixed the Instrument Cluster one by setting some adaptation channel on the cluster module about immo3 (I've got IMMO OFF and a swapped cluster as of recently), I couldn't solve the ABS one, and I have no idea why TCU is even showing up, my car is manual.

Anyways, back to topic, my next step is doing the vacuum leak test, I'll try to do it over the weekend if I can buy the supplies tommorow and I'll also clean the throttle body, hopefully I can get to the bottom of this soon.
Thanks for the help so far guys:)

Is your ECU from an automatic?

Instagram: @oliepyle

[PZ.]

A bad clutch vent valve will cause the car to hold RPM between shifts too. Was there a manual swap on this car? Does the cruise control work?

[volts]

Is your ECU from an automatic?

Instagram: @oliepyle

No, the ECU is the original one for this car, it was just mapped for the k04 a while ago and recently got the immobilizer disabled.

[volts]

A bad clutch vent valve will cause the car to hold RPM between shifts too. Was there a manual swap on this car? Does the cruise control work?

The car was always manual and it doesn't have cruise control

[Ian Shire]

I've actually been having the same problem with my 2.8 12v from time to time. It only seems to do it when its cold, even then it's only sometimes. Haven't looked into it yet.

[walky_talky20]

Whoa. There were A4's without Cruise Control? Crazy.

What you are describing is exactly what happens when the upper clutch switch on a Drive-By-Wire car is faulty or unplugged.

At a Basic level, the there are 2 different "throttle maps" on a Manual Transmission Drive-By-Wire B5 (2000+ model year):
Main Driving Map (SMOOTH MODE): Throttle Plate will not "snap shut" very quickly on throttle lift. This reduces momentary exhaust pipe emissions and also allows a smooth, luxury transition from Accel/Cruise into Decel. So your nose doesn't hit the steering wheel.
Shifting Map (SHARP MODE): When the clutch pedal is pressed (any amount), the throttle is now allowed to "snap shut" much more quickly. This helps with more accurate shifting and reduces "rev hang" during shifts.
*Obviously the E-Throttle maps are vastly more complex than this, but the above is suitable for this discussion.

Top Tip:
On a properly working manual-shift B5, you can switch to the "Sharp Mode" throttle map by resting your left foot on the clutch pedal softly. Just enough to close the upper clutch switch. Then experiment by taking your right foot on and off the gas. You will notice very harsh and immediate transitions from Accel to Decel. Remove your left foot from the clutch pedal. Test again - transitions are now SMOOTH.

Top Tip #2: You can determine the amount you need to depress the clutch switch by setting the cruise control and then pressing the clutch pedal softly until the cruise control cancels. The cruise control uses the same electrical switch.

Note 1: The above applies only to 2000+, Drive by Wire B5's.
Note 2: SMOOTH and SHARP Modes are not official terms. They are just names I used to aid in the description of the throttle behaviors.

[volts]

Whoa. There were A4's without Cruise Control? Crazy.

What you are describing is exactly what happens when the upper clutch switch on a Drive-By-Wire car is faulty or unplugged.

At a Basic level, the there are 2 different "throttle maps" on a Manual Transmission Drive-By-Wire B5 (2000+ model year):
Main Driving Map (SMOOTH MODE): Throttle Plate will not "snap shut" very quickly on throttle lift. This reduces momentary exhaust pipe emissions and also allows a smooth, luxury transition from Accel/Cruise into Decel. So your nose doesn't hit the steering wheel.
Shifting Map (SHARP MODE): When the clutch pedal is pressed (any amount), the throttle is now allowed to "snap shut" much more quickly. This helps with more accurate shifting and reduces "rev hang" during shifts.
*Obviously the E-Throttle maps are vastly more complex than this, but the above is suitable for this discussion.

Top Tip:
On a properly working manual-shift B5, you can switch to the "Sharp Mode" throttle map by resting your left foot on the clutch pedal softly. Just enough to close the upper clutch switch. Then experiment by taking your right foot on and off the gas. You will notice very harsh and immediate transitions from Accel to Decel. Remove your left foot from the clutch pedal. Test again - transitions are now SMOOTH.

Top Tip #2: You can determine the amount you need to depress the clutch switch by setting the cruise control and then pressing the clutch pedal softly until the cruise control cancels. The cruise control uses the same electrical switch.

Note 1: The above applies only to 2000+, Drive by Wire B5's.
Note 2: SMOOTH and SHARP Modes are not official terms. They are just names I used to aid in the description of the throttle behaviors.

Yes, there were A4's without cruise control in the EU. As far as I know, A4's sold in the US were full of extra options, including cruise control, all 4 motor windows, electrical seat moving, FIS displays etc...
I have the (almost) base EU model... with just a few extra options.

First of all, thank you for all the information, I really appreciate it.
My car is 2000 and it is DBW, so what you explained might be the problem. The only thing that I'm not sure about is, if non cruise-control b5's have this clutch switch, I'm having trouble finding this information online and I know you wouldn't know as well since all the cars there had cruise control... But you gave me another thing to look at so I'm less in the dark, I'll try to gather some info on this and test it out :)

[Blazius]

All manual cars have clutch switch. It is sampled by Motronic for a couple things, one is what walky explained basically, torque control changes when kuppl (clutch is on). If its broken/sticking it will cause some issues like RPM lag when clutch on shift etc.

[walky_talky20]

You should be able to see the status of the clutch switch using VCDS Measuring Blocks. Around abouts Group 060 or so (the Drive By Wire throttle / adaptation area), if I recall correctly.

[Ben830]

What's the part number for this switch? My car's throttle response isn't great when shifting, it sort of hangs (it's DBW) and improving it in anyway would be a benefit.

[walky_talky20]

Here's an old DIY I made on adding this particular switch to car that has been 5MT swapped. Lots of comments from other members about it fixing the throttle hang.
https://www.audizine.com/forum/showt...eed-Swap-(DBW)

I'm pretty certain this is the switch. It says AEB, but I think the later cars use the same one, too. They just leave the vacuum port disconnected.

https://www.ecstuning.com/b-original...11907343b~oes/

[volts]

All manual cars have clutch switch. It is sampled by Motronic for a couple things, one is what walky explained basically, torque control changes when kuppl (clutch is on). If its broken/sticking it will cause some issues like RPM lag when clutch on shift etc.

You should be able to see the status of the clutch switch using VCDS Measuring Blocks. Around abouts Group 060 or so (the Drive By Wire throttle / adaptation area), if I recall correctly.

Just yesterday I finally checked this out on vcds, turns out the switch is stuck at "1", so I'll try to see what's going on and replace it if needed. Thank you so much for the help!

[walky_talky20]

You are most welcome. Good luck with fixing.