Dirt

Hi all.

I had the oil burn piston and ring rebuild last year under the terms of the extended warranty. Prior to that rebuild I'd never received a LOW BOOST warning light. After the rebuild I've started seeing that light under certain conditions:

1. Low rpm
2. High throttle
3. Warm air temperatures

When all of these three conditions exist I frequently see the low boost light come on. If I am careful and avoid heavy throttle use at low rpm the light will clear itself automatically. My theory is that the old rings allowed enough blow by into the crank case that the peak cylinder pressure was reduced compared to the peak cylinder pressure the engine experiences now with the new and better sealing rings. The increased cylinder pressure coupled with the conditions listed above then makes it much more likely that the knock sensor will intervene. With the spark retarded to rein in the knock, the peak cylinder pressure is reduced due to the retarded spark and, subsequently, the flow to the turbocharger is reduced, leading the engine management system to interpret the conditions as low boost.

My first question is has anyone else experienced this problem after having the piston and ring rebuild? I know it can also be caused by a faulty diverter valve and/or a faulty turbo. However, the low boost light went away all winter with the cooler weather and just reappeared on our first 80+ degree day, which leads me to believe the turbo and/or the diverter valve are OK and the root problem is caused by engine knock. I'm reluctant to have the dealer investigate because everything runs fine apart from the light and I'd rather not spend a fortune on diagnosis.

My second question is what's the official name for the diverter valve? Is it the "Turbocharger Air Recirculation Valve" (see shop manual excerpt below)? I bought an Audi shop manual a while back and am not sure what specific part this refers to since there are no "diverter valve" entries. So, it must be called something else. I'm game to try the replacement myself as a trouble shooting measure if I need to, but want to be sure A) I'm looking at the right part and B) I'm not getting in over my head. I'd like to read up on it first before deciding whether I'll attempt the repair.