Sorry about the subject title, but I couldn't think of how to describe my problem. I have a 2002 Renault Megane, 1.6 petrol. The car always runs and never breaks down, but the running is simply not consistent. I've had it to all the dealers in my area countless times and all they do is check for error codes, of which none are recorded, and say therefore there is nothing wrong. Its got to the point now that if I book it in, later they say they cant fault it, even though it has only been driven round the back to hook it up to their computers. The car runs fine from cold, accelerates cleanly and quietly, and will stay like this for the length of my journey. However, if the car is started when warm, then I get random cases where the quiet smooth running is replaced by a deep gruff 'boom', most notable under acceleration and at higher revs. Sometimes, after a bad journey, the car can then run well again when restarted. The problem doesn't always arise, and the car can run for days without a problem, and then on one journey it is suddenly doing it again. Of course, it always seems to run well when at the dealers. I've had all the dealers check it over for exhaust problems and air leaks, but nothing is found, and they tell all me my car is fine. Different brands of petrol don't affect it, not even Optimax, and it is as if the car picks a random set of values each time it is started warm and runs with them. The car always idles quietly and smoothly, even when accelerating is really rough.
Would anyone know, what could be happening to the car to make it run like this? I thought maybe a sensor could be wrong, affecting the fuelling, but I suppose the computer check at the dealer would see this, which it doesn't. Would it be worth taking it to a local independent garage where they could check sensor values while the car is being driven, or is that not possible? The local dealers are unwilling to go into this kind of detail because they dont experience the problem. A local garage offers 'crypton tuning' services - would they have the kind of gear required to check these things on the move?
Sorry for so many questions. Im getting nowhere with local dealers, so I am looking at other avenues. The car is fine to drive when normal, but unbearable when its got this boominess to it.
Thanks for reading, any suggestions or help appreciated.
Ben