VAG-COM throttle queries

[Smash Reckitt]

Hi all,

I’m new to the site so apologies for any obvious questions etc, but I’ll try to give a bit of background on this.

I’ve owned a ’99 A4 1.8 20V (APT engine code, DBW throttle) since October ’06. About a month after I got it, the EPC light started to come on very occasionally (once every 1-2 weeks) but this wasn’t a big problem as restarting the engine would clear the light. From December through to February however the problem got worse until EPC was lit almost constantly and restarting wasn’t clearing it. This was becoming a major driveability issue as it was limiting the engine’s power, particularly above 3000 rpm.

So I took the plunge and recently bought the licensed version of VAG-COM and did a scan, which showed the following 6 faults on the engine module:

6 Faults Found:
17579 - Angle Sensor 2 for Throttle Actuator (G188) Implausible Signal
P1171 - 35-00 - -
17972 - Throttle Actuator (J338): Under-Voltage during Basic Setting
P1564 - 35-10 - - - Intermittent
17536 - Fuel Trim; Bank 1 (Mult): System too Lean
P1128 - 35-00 - -
17765 - Cylinder 1 Ignition Circuit: Short to Ground
P1357 - 35-10 - - - Intermittent
17950 - Angle Sensor 1 for Throttle Actuator (G187): Implausible Signal
P1542 - 35-10 - - - Intermittent
17771 - Cylinder 3 Ignition Circuit: Short to Ground
P1363 - 35-10 - - - Intermittent

I didn’t know how long each fault had been there or if they were still current etc, but as 3 of the 6 mentioned the throttle I reckoned this was a good place to start. I removed the throttle body and cleaned out all the gum and crap (of which there was a fair bit) and re-fitted it. I cleared all the fault codes, did a throttle body alignment using VAG-COM, and set off for a good long test drive to check the fix.

I got about 50 miles without incident (can’t remember the last time I got this far without seeing the EPC light) and the car was idling smoother and picking up cleaner from small throttle openings. The light however reappeared but after a restart stayed off for the remainder of the journey. I got home and did another scan:

1 Fault Found:
17579 - Angle Sensor 2 for Throttle Actuator (G188) Implausible Signal
P1171 - 35-00 - - - Intermittent

Conclusion: Cleaning out the throttle body greatly improved the situation with the EPC light, but did not eliminate the problem.

Sorry if the above is a bit long-winded, but as a relative newbie to Audi I have a few questions:

• Is it likely that my DBW throttle body is failing? Is it common for these to wear out or fail in this way?
• In what way is the EPC light different from the general check engine light? (Haven’t seen the check engine light). Is one more serious than the other?
• How does EPC work? Does it switch the engine management onto a backup fuel/ignition map? Is this the same as a “limp mode”?

Any help on this would be much appreciated, as I’d like to get a better understanding of what’s going on with the car before I start replacing parts.

Cheers

[bearthebruce]

Don't know if this will help but you may want to try what this guy did: disconnect the connector on the Throttle Body, clean the contacts, re-connect. If the angle sensor is truly going, then you will need to replace the throttle body afik.

[Uwe@Ross-Tech]

In what way is the EPC light different from the general check engine light? (Haven’t seen the check engine light). Is one more serious than the other?

The "Check Engine" light generally indicates an fault that causes your car to exceed the allowable emissions of pollutants.

The EPC light indicates some other serious fault that may affect drivability but isn't causing an emissions problem.

In this case, because the ECU isn't 100% confident that it really knows the angle of the thottle, it will limit power.

If TBA ran sucessfully, your problem is obviously intermittent and likely due to a dodgy connection somewhere. Whether it's in the TB itself, in the wiring or connectors, or in the ECU is what you need to figure out. I'm with Bruce, the first thing I'd do is take apart and clean all connections between the TB and the ECU, and check the relevant ground points as well.

-Uwe-