Recently, I encountered a weird issue with a B6 where it lacked power. If you drive the car around town, typically you won’t feel a lack of power. If you decide to do some spirited driving, around 80 mph, the car will fall flat on it’s face. Lights do not come on or faults stored for the lack of power.
I replaced mass airflow sensors, but they didn’t help the issue. Then I decided to check into the high pressure fuel pump. They all had correct pressure under high load. Next, I investigated the lower pressure fuel pump and it had the correct pressure under high load, (around 5.5bar of pressure). Finally, I checked all coolant pumps since the issue seemed to get worse the hotter the engine or air temperature got. It was discovered that the inter cooler pump was not pumping properly. What was needed was to remove and replaced the pump. Alpina their own proprietary pump. In the end, I replaced the pump and bleed the system. During the test drive, it was concluded that the power was functional.
The Alpina uses a modified N63 engine to create the extra power it is known for. This issue would transfer over to any N63 or S63 since they use the same system setup. Make sure if you’re having a lack of power with no faults or warning lights on that you check into the operation of the inter cooler pump.