Audi C4 AAT: No Power After 3 Months Idle

"I bought an Audi C4 station wagon with an AAT engine 3 months ago and only started using it recently. During the drive home after the purchase (approximately 200 km), the Audi had good performance (around 200 km/h) with some smoke development. The car has only 180,000 km on the odometer (which is verifiable)."

After registering the Audi three months later, the engine now has very little power and struggles to reach 140 km/h. I checked the entire pressure line from the turbocharger to the intake manifold for leaks, but found none. I disabled the EGR valve by covering it with a metal plate. During a test drive with VAGCOM, the boost pressure at idle was just below atmospheric pressure, at approximately 900 mbar, and reached its maximum of about 1250 mbar at around 2000 rpm. After that, it dropped significantly and remained at around 1000 mbar until approximately 3000 rpm. Currently, there is also a noticeable amount of soot buildup visible, even in daylight.

"The air filter, oil filter, diesel filter, and engine oil are all new. Since the turbocharger isn't building proper boost pressure, I wanted to check the pressure sensor for the wastegate. Now I've noticed that on the AAT engine, the linkage from the pressure sensor to the wastegate is located inside the turbocharger housing, making it difficult to access (unlike my Audi 80 TDI, where it was easily accessible)."

I'd like to ask if anyone has had experience with a stuck wastegate on an AAT (Audi Adaptive Transmission) system. Specifically, how can you check the wastegate in this situation, especially when you can't access the linkage?

I browsed the forum this afternoon and couldn't find a comparable case specifically for the AAT.

Does anyone have a diagram or overview of the boost pressure system, including the actuators? Workshop manuals would also be very helpful.

I hope you can help me.

1250 mbar would be a bit low; that would only result in approximately 0.250 bar of boost pressure.
Take a look and see if maybe a badger has been visiting .
How did you check the charging cable length?

The WG is pressure-controlled.
If there was a weasel there, it would NEVER open [something], and there would initially be a lot of excessive pressure.
Exactly the opposite of VTG.

m;

It wasn't meant specifically with regard to the shared apartment, but in general, especially since the weasel prefers cables.

Today, I drove a longer distance on the highway again, and I kept accelerating. And sure enough, it started pulling better and better. I eventually reached at least 180 km/h. Tomorrow, I'm going to connect an analog pressure gauge to the boost line and report what the actual boost pressure is. Does anyone have workshop manuals for the AAT engine? I'd be interested in seeing them.

Hello.

Take a look at the double connector on the turbocharger control valve (the round metal box with the Pierburg label). I've seen this happen on three different vehicles where oxidation has formed there. When the valve is not properly energized, it allows a direct path to the wastegate, which opens quickly -> the maximum pressure of 1250mbar is still reached. There's a lot of black smoke, and the maximum speed is around 100-120 km/h.

The connector often gets noticeably hot during operation (due to high resistance). So far, I've only seen this type of valve on the 5-cylinder TDI engines in the C4 models. There are two round connector pins in the connector, which are not sealed, located in the center of the firewall. To fix it, disconnect the connector, clean the "male" contacts on the valve with a brass brush, apply some contact cleaner to the vehicle-side connector sockets, and plug and unplug the connector a few times.

So far, I have been able to fix this exact error three times.
The intake manifold pressure should then be back within the range of 1700-1950 millibars.


Best regards,
Mfg Mpire

"Thank you for the tip, I will check it out tomorrow and let you know what I find. Is it definitely the case with the C4 that, if the control valve is not activated, all the boost pressure is directed to the wastegate actuator? The wastegate actuator has two connections. One comes directly from the turbocharger (a short metal pipe on the turbo), and the other comes from the intercooler, via the control valve, to the wastegate actuator. Do you know exactly how the boost pressure regulation works with the AAT? I haven't quite understood it yet."

I previously owned an Audi 80 TDI with a 1Z engine. In that car, the turbocharger's boost pressure went directly to a switching valve. When this valve was disconnected, the entire boost pressure went to the pressure sensor, and the wastegate would open at approximately 0.7 bar of overpressure. When the switching valve was activated, only a portion of the boost pressure reached the pressure sensor, and the engine control unit regulated the boost pressure to a maximum of 0.9-1 bar.

Do you perhaps have a diagram or explanation of how the boost pressure control system works in the AAT (Audi Adaptive Transmission)? I would be very grateful if you could help me with that.

audiB4avandt wrote:

Do you perhaps have a diagram or explanation of how the boost pressure regulation works in the AAT? I would be very grateful if you could help me with that.

Hello, no, unfortunately, I don't have a wiring diagram or pinout for the C4.
The metal pipe intercepts the pressure from the compressor at the point with the lowest pressure and then unregulatedly transfers this pressure to the membrane of the wastegate, which opens at 1250 mbar. This serves as a protective function. Only when the boost pressure is higher or increasing is the membrane pressed against from above (regulated by the control valve) with intake manifold pressure (higher) in order to counteract the pressure -> 1950 mbar, or to release some pressure, thus regulating. This regulation system has the advantage that overshoots can be almost completely excluded, the turbocharger is protected from pressure spikes (extending its lifespan), and a more uniform/gentle regulation of the boost pressure is achieved. The membrane is essentially held "floating." During a logging run and with a functioning system, it is quite possible to observe the gentle attainment of the maximum pressure (Pmax).


Best regards,
Mfg Mpire