I have a very weird under boost issue on the 2016 Audi Q5 TDI. I got the error about a month ago and took it to the shop, they said that the turbo actuator was bad. So after a lot of searching I found a company that could repair the actuator, shipped it off and just got it back. But after plugging it all in I get the same glow plug flashing and the engine goes into limp mode. What is weird is that when I first start the engine and start driving there is no issue and you can feel the turbo boost. I took the car for a long drive because I thought maybe the actuator needed to "learn", but what I discovered was that if I am in manual "S" shift mode I can bang through the gears pretty fast and the turbo just rockets me over 120kph in no time and then as soon as I let off I get the under boost error again and the engine goes into limp mode. So it appears the turbo is actually working.

The only other things I can think of to mention is that just before the under boost issue we had the intakes and EGR removed and completely cleaned. We already replaced the turbo boost sensor. We also did a turbo boost leak test and did not find any issues.

Usually the under boost issue shows up in the first 30 seconds, but last night I put in manual mode and kept the RPM around 2500-3000 and drove it 40 minutes home through the city and down the HWY and the light didn't come on until I got almost home. So it seems if I can keep the RPM up the turbo and turbo boost are there. As soon as the light comes on the car goes into limp mode and I am done.

Any other ideas?

 

Thank you so much!

 

Following up on this when we tore the car apart we found that the VNT vanes on the turbo were missing. They had come off and where somewhere in the exhaust. So technically while the turbo was working the vanes to control boost pressure were not. We ended up having to replace the whole turbo.

 

Thank you so much!