Accelerator Position, limp mode (P1630, P1388)

[welljim]

Hi all, I have a tuned AUDI S3 8L 1999 (APY engine), which has been driving me crazy with random / intermittent EPC light coming on and the ECU getting into limp mode.

I had it tuned right after I bought it, so I'm not sure if the problem was there before the tune, or as a result of the tune. But it's been going on for years. It wasn't happening that regularly before, but as of recently it has started to occur very regularly and these days I'm pretty much driving in limp mode most of the time.

The issue seems to relate to sensing the correct throttle pedal position. When EPC comes on, I need to depress the throttle pedal 80% of the way just to get going. Then all of a sudden the pedal travel might start working correctly again, and the car seems to enter a semi-limp mode: it will respond lively, but cap boost to 0.5 bar instead of the tune's 1.5 bar. Then after a while it might drop back to full limp mode and require almost full throttle just to get going. One morning it was so bad that full throttle was not increasing the RPM at all. I switched off, waited for an hour, then it was good again. The problem may occur in both open and closed-loop conditions, I haven't noticed any particular pattern that may trigger it. WOT does not make it go away.

When the EPC light does not come on, the car works really well and the ECU program is super smooth and responsive across the entire RPM range. Arguably one of the best tunes I've had in any of the cars I've owned before. The output is 350HP @ 1.5bar. The tune consists of ECU reprogramming, upgraded turbo, higher flow fuel pump, performance catalytic converter, and a few other things that are irrelevant to this discussion (clutch, suspension, etc).

I mention the upgraded cat because I can't remember if it retained the O2 sensor, and if it did, I don't know if it reads the same as in the original cat.

The Secondary Air Injection Solenoid Valve that comes up as "open circuit" in the ECU scan below, has been removed completely and it's not in use. I believe its purpose was to preheat the cat faster in order to reduce emissions.

This is what I get from VCDS:

Friday,15,December,2023,14:24:47:62795
VCDS -- Windows Based VAG/VAS Emulator Running on Windows 10 x64(Parallels)
VCDS Version 23.11.0.0 (x64)  HEX-V2 CB: 0.4643.4
Data version: 20231027 DS351.0
www.Ross-Tech.com

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Address 01: Engine       Labels: 06A-906-032-APX.clb
Control Module Part Number: 8L0 906 018 K
Component and/or Version: 1.8L R4/5VT         0005
Software Coding: 05610
Work Shop Code: WSC 66565
VCID: 372A2C214652D002DE9-5161

5 Faults Found:

17840 - Secondary Air Injection Solenoid Valve (N112)
            P1432 - 35-00 - Open Circuit
18038 - Accelerator Position Sensor (G79)
            P1630 - 35-10 - Signal too Low - Intermittent
17978 - Engine Start Blocked by Immobilizer
            P1570 - 35-10 -  - Intermittent
17796 - Control Module Malfunction - DBW Throttle Monitoring
            P1388 - 35-10 -  - Intermittent

Readiness: 0000 0000

"Engine Start Blocked by Immobilizer" happened a few times over the summer. The immo light was off as normal on the dash and no blinking, yet the car refused to start. Engine cranked but it wasn't firing. It started normally after a few tries. After the summer when the weather cooled off a bit, the problem disappeared. That's another issue that I'd like to resolve.

Anyway, back to the more important problem, I remember a few years back right after getting the car from the tuner and telling them about the intermittent EPC issue, they took it back for inspection. They told me that they changed the Accelerator Position Sensor. I hope they did, but I can't be sure: as I was driving out of their shop, the EPC lit up again. They were pulling their hair out. However they couldn't think what else to check so they made me take the car away anyway.

This past weekend I fired up VCDS to check what throttle pedal angle the ECU is seeing (Function 08, Display Group 062). I ran the test when the car was in the severe limp mode condition, requiring almost full throttle to get going. This is a short clip of the result:



The manual says values in Zone 3 should always be about twice as high as those in Zone 4. In other words, it should look like the chart on the left. However if I plot the values from my video above (see chart on the right), the chart looks all wrong. The values do increase gradually, but at a different rate. In my case Zone 4 starts higher, and around half the pedal travel they switch and Zone 3 becomes higher!? Also at fully depressed pedal, the ECU thinks I'm 49% there.



After seeing this I thought to do a Throttle Adaptation, however sometimes the car gets into a less severe limp mode (see above) or even at no limp mode at all, in which case the throttle responds correctly. So it seems the problem is elsewhere, something is intermittent. Also adapting the throttle to the condition above will probably be dangerous when out of limp mode, as in that case the adapted settings will result in WOT at half the pedal travel.

Checking the voltage at the throttle pedal connector gives me the expected values as per manual:

Terminal 1 to ground = ~ 5v
Terminal 1 to terminal 5 = ~ 5v
Terminal 2 to ground = ~ 5v
Terminal 2 to terminal 3 = ~ 5v

Then the manual suggests to check the signal wires using a VAG specific test box:

Connect test box V.A.G 1598/31 to wiring harness for engine control unit. Do not connect to the engine control unit itself. => Page 24-16

I don't have the VAG box. Is there a method to perform this test without it?


Yesterday I tried another mechanic. They changed the battery to a new one, cleaned the throttle body and reset it, cleaned the MAF sensor. Obviously that didn't help. I performed another VCDS AutoScan after I got the car back and noticed some new weird codes, including a P1609 Crash Shut-Down Activated, even though no crash took place.

Address 01: Engine        Labels: 06A-906-032-APX.clb
   Part No: 8L0 906 018 K
   Component: 1.8L R4/5VT         0005
   Coding: 05610
   Shop #: WSC 66565
   VCID: 372A2C214652D002DE9-5161

6 Faults Found:
17840 - Secondary Air Injection Solenoid Valve (N112)
            P1432 - 35-00 - Open Circuit
18017 - Crash Shut-Down Activated
            P1609 - 35-00 -
18010 - Power Supply Terminal 30
            P1602 - 35-10 - Voltage too Low - Intermittent
16885 - Vehicle Speed Sensor
            P0501 - 35-10 - Implausible Signal - Intermittent
18038 - Accelerator Position Sensor (G79)
            P1630 - 35-00 - Signal too Low
16486 - Mass Air Flow Sensor (G70)
            P0102 - 35-10 - Signal too Low - Intermittent
Readiness: 0000 0000

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Address 03: ABS Brakes        Labels: 8N0-907-379-MK20.lbl
   Part No: 8N0 907 379 C
   Component: ABS/EDS 20 IE CAN  V003
   Coding: 13504
   Shop #: WSC 06435
   VCID: 3C20330D7F74F35A33B-4B1F

No fault code found.

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Address 08: Auto HVAC        Labels: 8L0-820-043-2D.lbl
   Part No: 8L0 820 043 M
   Component: A3 KLIMAVOLLAUTOMAT_D77
   Coding: 03141
   Shop #: WSC 06313
   VCID: 323C1D35AD30352AE97-25A7

No fault code found.

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Address 15: Airbags        Labels: 8L0-959-655-AI8.lbl
   Part No: 8L0 959 655 G
   Component: Airbag Front+Seite 1007
   Coding: 01104
   Shop #: WSC 00001
   VCID: 42DC4DF51D9085AA797-5161

1 Fault Found:
65535 - Internal Control Module Memory Error
            00-10 - - - Intermittent

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Address 17: Instruments        Labels: 8L0-920-xxx-17.lbl
   Part No: 8L0 920 980 B
   Component: AB-KOMBIINSTR. VDO D05
   Coding: 01444
   Shop #: WSC 00428
   VCID: 323C1D35AD30352AE97-5161

   Component: AUZ7Z0X1122827
   ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿ     AUZ7Z0X1122827

1 Fault Found:
01176 - Key
            07-10 - Signal too Low - Intermittent

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Address 22: AWD        Labels: 02D-900-554.lbl
   Part No: 02D 900 554 B
   Component: HALDEX LSC  ECC     0006
   VCID: 1F1AE481CEA28842369-4B1F

1 Fault Found:
65535 - Internal Control Module Memory Error
            00-00 - -

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Address 35: Centr. Locks        Labels:. 8D0-862-257.clb
   Part No: 8L0 862 257 P
   Component: CV-Pump, Alarm, RC  D13
   Coding: 13130
   Shop #: WSC 00000
   VCID: 42DC4DF51D9085AA797-4F0D

2 Faults Found:
01368 - Alarm triggered by Luggage Compartment Switch
            35-00 - -
01134 - Alarm Horn (H12)
            49-00 - No Communications

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Address 45: Inter. Monitor        Labels: 4B0-951-173.lbl
   Part No: 4B0 951 173
   Component: Innenraumueberw.    D03
   Coding: 00001
   Shop #: WSC 00000
   VCID: 6098A77DB35CF7BA4F3-4AC3

No fault code found.

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Address 55: Headlight Range        Labels: 4B0-907-357-AUT.lbl
   Part No: 4B0 907 357
   Component: LEUCHTWEITEREGLER  D003
   Coding: 00002
   Shop #: WSC 06313
   VCID: 6596B669AC7E9292BC5-5185

No fault code found.

End----------(Elapsed Time: 03:10, VBatt start/end: 12.0V/12.2V)-----------

My suspicion is bad wiring or ECU. I've noticed that sometimes, but not always, the problem is corrected the moment I switch on the headlights!!? Looking at the Autoscan, there are a few seemingly random faults like the immo, the vehicle speed sensor, MAF, Crash shut-down, all of which are intermittent. I've looked at the wiring diagram but I haven't been able to locate a junction that would connect all these faults.

I'm pretty stuck, any help will be greatly appreciated.

[welljim]

bump

[Silicon Voodoo]

you got a lot of electronical problems. maybe you threw the ECU off during tune, maybe reflash it to factory

[welljim]

Here's something crazy I discovered last week:

If I switch on the head lights before pressing the throttle, the ECU never goes into limp mode. And if it's already on limp mode and then I switch on the head lights, then it goes out of limp mode!

I've checked for corrosion and loose ground connections on the chassis wherever I can find see them, but they all look fine. Any tips on how I can trace the problem?