C280 sport 1996 AIR MASS METER FIASCO

[PLOD412]

MY C280 HAS STARTED 'POPPING' DURING ACCELERATION BUT ONLY WHEN UP TO NORMAL TEMPERATURE. Thanks for everyones help so far. I am getting a bit confused now!! I have changed the mass air meter with a proven working one (tracked down an old mate with one fortunately) and the problem is still exactly the same. I am told by Mercspec our local Mercedes specialist that if the Oxygen Sensor or coil were faulty that the electrical diagnostic test would have thrown out different codes and I would probably have an engine warning light on? Secondly where exactly is the Oxygen sensor located? I thought the small plug further over the air intake housing was a tempeature sensor? I recontacted the garage who carried out the test and they were no help - they then dercided to tell me that their electrical test equipment only threw out the code for the Air mass meter - but I could put it on a disgnostic test at my local dealer which has a better diagnostic test testing all systems for about £70 but it would probably just churn our the same codes. (they didn't tell me they had a simple system when they charged me for it the first time! Have you any ideas of anything that might be causing this that would not be picked up on an electrical test? Or can you think of anything that might cause the Air Mass Meter to read a fault?