View Full Version : Underboost / P0299 / Adaptation of new turbocharger

12-05-2017, 09:15 PM
I've just had a new turbocharger installed into my 2008 Touareg R5 TDI. The old one was replaced, because it triggered a CEL and went into limp mode. The dealership told me the turbo needed to be replaced - but I'm not sure any more whether that was really the case. The fault code was "P0299 - 000 - Control Range Not Reached - Intermittent".

Anyway, the new turbo has the same part number (070145701Q) as the old, stock one. Even though it was sold as brand new to me, it think it was actually a remanufactured one, because it had a custom "G-FORCE TURBO" label on it. Other than that it seemed like new, because it had no discoloring or dirt on the housing or the fan, but was perfectly clean.

Now, my problem is that when I check the turbo performance with VCDS, my actual turbo boost pressure trails the requested boost by about 80-100 mbars. The actual pressure seems to follow the curvature of the requested boost pretty good under real world driving conditions, but there's almost always that 80-100 mbars difference remaining.

Casually - while driving - the turbo develops and even larger, ~400 mbars gap between the requested and actual boost pressure for a period of time. Just like with the "normal" gap, this also seems pretty constant, ie. the actual pressure curvature follows the requested pressure curvature very good, but stays 400 mbars short. This usually happens when the requested pressure drops heavily and starts to decline even further (like when reaching a cruising speed at highways, or the top of a hill). Then all of sudden, when the ECU begins to request a larger boost again (for acceleration or going up hill), this larger gap disappears and we have the ~100 mbars gap again.

Please see the attached VCDS graph! I've recorded this under real world conditions on real world trip. The graph depicts a trip of about ~10 minutes and is horizontally compressed (compared to default VC-Scope graphing) because it uses a "PixelShift 1" scaling.

Now, my questions are:
- How serious is the 80-100 mbar underboost condition? I've been told it's a "feature" of the turbo, but I don't trust the dealership anymore.
- Is there a way to somehow adjust the turbo (or more the ECU to the turbo) to make it match the requested boost pressure more closely?
- And what about the casual larger, 400 mbar gaps? Are those normal (as in no serious problem) or do they mean there's something wrong?

The intake hoses and the vacuum lines have been checked for leaks, and they found none. The turbo can't be adjusted manually as this model (070145701Q) has a fixed VNT rod. The car drives otherwise fine, has a nice pull. However, it already triggered the same P0299 underboost code once, with the new turbo installed, a few hundred miles ago, but not again since that though.

Any tips or information welcome. Thank you.