I have a problem (actually, if you read the forum, I have quite a few :| ) with the engine - or the injection pump. I have had the 00550 - Start of Injection Regulation; Control Difference - Intermittent fault code for over a year now but I didn't paid attention since the car performed very well (in the last year I can remember a maximum 10 times when the engine would only start after a long cranking, but that did not happen for at least 6 months now - the cause of that is still unknown). Now it's starting to bother me because when I start the engine when cold, it makes a big white/blue smoke until it warms up. Also, the engine won't rev over 2000 rpm when I just start the engine ( ~10 degrees celsius), BUT it can rev higher as the engine warms up and around 80 degrees, I can rev up to 3500-4000 with no problem. According to Ross Tech, the possible solution to this error are: 1.Fuel Tank incorrectly filled with Petrol or Biodiesel 2.Air/Water in Fuel System 3.Injection Timing incorrect 4.Electrical Fuel Pump faulty 1. This can't be. One can't fill the tank incorrectly for over a year, right? 2. Water -I have replaced the fuel filter twice in the last year (I made 20 000 km in the past year and I change it every 10 000 km) so this is kind of ruled out. Air - at idle, there are little bubbles of air but sometimes when I rev the engine while standing still, more bubbles comes from the filter) that is the only transparent hose I have before the injection pump. I tried changing the valve on top of the fuel but to no avail. Possible solution (for which I need opinions): should I replace the fuel filter with just 2 lines, in and out, and bypass that valve on top of the filter so the return line goes directly into the tank? My theory is that the fuel/valve is letting some air on the feeding line, despite changing both of them. 3. I have hooked the VCDS and checked the timing. It's like a pixel over the green line enough to say at the bottom of the window: timing out of spec (too advanced). Just every few seconds, the timing gets below the line and then it says timing within spec. Possible solution (for which I need opinions): Can that fraction of the timing out of spec cause the error? But this does not explain the smoke at start up nor why it won't rev when cold. I also noticed that sometimes when I check the timing, the periods when it should be full retard to full advance, the engine doesn't change it's noise. It's exactly the same! Does this mean it's something faulty with the electrical part of the pump? When this occurs, the fault always comes on. On a side note, I remember a few months ago, when I checked the timing, it would not show the yellow lines at all, but I didn't gave any importance, again, because the engine was running beautiful. 4. I have checked the pump in the tank but the pump is almost useless in my opinion (at least at my model). I have the model in which my fuel pump just moves the diesel from the tank a few centimetes away from the pump into a very small reservoir (which should be heated). The pump does not force diesel into the filter nor into the injection pump! That's why changing my fuel filter is a pain in the a**, compared to other 2.5tdis newer than mine in which the pump actually delivers fuel to the pump, thus priming the system is very easy! The car is a 1998 AFB engine with ~300500 km on board, with the cams replaced at 227000 km but I have no idea if the injection pump is replaced or not. Thank you for reading this extremely long post and hopefully, together, we can find a solution.