2006 Audi A4 Quattro 2.0 Turbo FSI P0299 Underboost

Year/Model
2006 Audi A4 Quattro 2.0 37,000 miles

Problem
P0299 Under boost condition

History
Owner tried to smog vehicle and when tech connected obd2 found code for P0299.

Diagnostic
Customer brought vehicle to our shop and noted no drive ability issues. We connected our VAG-COM to the car and found this code stored in the vehicle. The instant data showed that this happened at 2800 rpm and the frequency was only once. Upon our test drive, boost was fairly normal and was kicking in around the 3000-3200 rpm range. We then used the graph feature of the VAG-COM and checked measuring block 115 which had actual and specified boost conditions in mBar. Upon driving, we found that the car was providing boost but was 500 mBar under the specified value.

The next step was to smoke the vehicle and check for any vacuum hose leaks.  The big difference was that from the 1.8t engine to the 2.0t engine is that they got rid of many vacuum hoses and updated to electrical controlled valves. This is a good and bad as its much easier to replace and cracked hoses are less frequent. We found no leaks at this time which showed us that the leak was not venting to the atmosphere. On the older 1.8 the diverter valve would either go bad or the vacuum hose going to the valve would crack. We decided to pull the valve and check.

Results shown below.

Repair
The diverter valve is located in the front of the turbo manifold on the passenger side right behind the radiator. The best way to access was from below after the splash shield is removed.  There is one electrical connector attached. Sorry but i have no picture of the actual location. Here is the Audi print. It is part number 15 in the picture and is held in via 3 allen screws.

Upon removal of the part we found that it was the older style valve and was never updated from Audi. The valve moves in and out as boost increase or decreases to adjust for any variance. As you can see in the picture the main diaphram is made of a rubber material and has torn. This tear causes a minor leak in the boost causing the vehicle to not reach specified boost. The picture below shows the wear on the rubber as well as the old original part.

As you can see Audi recognized this problem and has updated the part and superseded the part # 06H 145 710D. By the time you read this, they might superseded it again. As you can see the valve now offers a shroud around the moving diaphragm in case there is any debris that may be inside.

Upon replacement, we cleared the vehicle check engine light codes and graphed our data again. On our test drive from partial throttle to WOT we found the difference between actual and specified values to be only 5-30 mBar which is acceptable. The customer was instructed to drive the vehicle for a few days and return if the light returns. If the light does not return, the customer was instructed to return to the smog station for a retest.

Disclaimer
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