I have a BMW 3 series convertible, registered 2001, and have recently had a problem that turned out to be a faulty engine breather. This caused chronic but intermittent backfiring over a period of some weeks that the garage couldn’t isolate the cause of. During one of those episodes something else went wrong within the engine that despite the breather problem now having been cured, by way of its replacement, the engine continues to run out of balance. The whole of the ignition side has been reviewed, repaired and or replaced and the tests show that is all is running fine yet the problem continues to persist. The head gasket is fine and there is no water in the engine oil or water or oil loss so the block and the head seem to be fine. I have now had three different mechanics look at the problem but none has cured it. Do you have any advice on what it could be? Could it be the petrol injection jets, or anything to do with the fuel injection system, given that from the regularity of the ‘roughness’ of the engine running that it appears to be affecting only one cylinder?