I have a 2.5tdi AFB engine for over a year now, but I have some problems I can't solve for weeks now. One of the older problem is that I always have the 00550 fault code. I have checked the timing, air in the fuel but everything seems fine. It doesn't bother me much since the engine performs great. Until few weeks ago. Some weeks ago I filled my tank with some diesel from a dubious fuel station. From that on, my problems started. Or so I think. The next day was a colder day(around 10 degrees Celsius) and the engine started with a lot of white smoke and wouldn't rev up over 2000 rpm. As the engine warms up, it would rev higher and higher: 2400, 2600, 3200 rpm... I have drained the tank and filled with winter diesel from a trusted source, but I couldn't get ALL the diesel out, so with the new "mixed" diesel, the engine was performing sometimes bad, sometimes very good. I couldn't find any pattern, didn't matter if the engine was warm or not. When it was performing bad, under heavy acceleration was barely moving and by 3200 rpm it would put a big white smoke after me. When it was performing very good, it can rev up to 4000 and everything was fine. I have filled with diesel just from that gas station and now 90% of the time it performs well. Now it can't be because of the diesel, since that is burned already. What bothers me is that I can't find a pattern when it goes bad. I can start off a red light and can't rev up over 3200 plus a big white smoke, and at the next light I can start with the engine performing great. Since I have had that dubious diesel, I have noticed a whistle at the back of the exhaust past 2500rpm, which is proportionate with the power needed/delivered. But again, with no known pattern. I can drive 5 miles without the whistle and then the next 20 miles with the whistle. The whistle doesn't seems to affect the performance at all! And it doesn't matter if the engine can rev or not over 3200 with or without the big white smoke. For the last year I have ran with the EGR off. My first thought is that the catalyst was clogged so I replugged the EGR. It didn't seem to affect any performance at all. Again, the whistle is intermittent. To be more precise, the whistle sounds like something is clogged and it is forced out under heavy loads. But if it was the catalyst clogged, the whistle would be permanent, right? Sorry for the long post, but I have a feeling that the experience with bad diesel has something to do with it. Also, I thank everyone who had the patience to read the entire post.