I ask forgiveness in my low level of knowledge and apologise in advance for poor descriptions.
I have a 2007 a3 TDI 170ps sportback 90,000 miles that I have owned for 2 years now. I will give some history with the vehicle to maybe make give some clues.
A few months after purchase I started to have the glow plug light come on and flash until turning off and turning on the engine. I attempted long high-rev journies to maybe clear the DPF but it didn't seem to make a difference. Eventually the check engine light came on as well so I called the dealer and, as the car was under warranty it was put into a local Audi specialist and they determined the EGR valve was 'blanked off' and they replaced it.
The light stopped coming up, which was good, however, after this I noticed that at idle the rev needle would bounce around 800-900 revs and the car was producing quite a lot of white smoke. After trying redex and other treatments I mostly just put up with it as nothing else seemed to be going wrong and it was due a service in the not too distant future. Also found that the injectors needed replacing due to recall.
Booked the car in for service and MOT at Audi dealer. They replaced the injectors and gave the car a clean bill of health. I queried the exhaust and they said it was fine. This did not fill me with confidence.
The car did seem to smoke less and the bouncing at idle had stopped so I was mostly happy at this point.
Jump ahead about 4 months and one day the car just wouldn't start - engine would turn over but just not ignite. Had a friend read the car with vag com, determined there were a number of errors pointing to MAF. Replaced the MAF to little effect. The car's ECU was then reset and the keys re-assigned to the car - all was well. Smoother accellaration, little smoke. Seemed really good.
A couple of months later and the car started behaving oddly. At around 1800 to 2000 revs when driving there would appear to be a loss of power, I can only describe it as choking or stalling, rev counter bounces, car shudders, blows out a fair bit of white smoke, but eventually continue going. The car would do this only if I have been driving at a steady pace/revs and then put a little gas in. This would happen anywhere between 1800 and 2200 revs. However, accellarating through this range showed no trouble at all. Sitting in neutral and pushing through the revs also showed no sign of the issue. Friend with vag com has moved away so I was unable to get another reading.
This has continued for a couple of months until yesterday when the glow plug light came on again for a couple of journies, then the engine light came on with it, now it is just the engine light which is ignited. The issue with shuddering at 2000 revs has apparreently disappeared, but now the car struggles to pull away initially unless I give it quite a lot of revs, there doesn't seem to be a whole lot of power up higher in the rev range like I used to get and accellaration doesn't seem as smooth.
On occasion the power issues will seem to disappear but then come back.
My question is does anyone have any idea what could be causing the most recent problems, as I am obviously going to put the car in to a garage to get looked at, but I would like some idea of what could be the causes so I do not get fobbed off by the garage with some part replacement due to my apparent ignorance, yet ultimately leaving me with no resolution to the issues.
I'm more than a little frustrated as I went with an Audi from a dealer and with a warranty precisely because I wanted something reliable and well built - so far I have not been impressed!