My 1996 GLE (42K original miles) occassionally knocks upon acceleration. Took it to the Nissan dealer in 2001 and they diagnosed the problem as carbon build up and "cleaned" my engine. Didn't solve problem and I left it go. Still doing it. I use only 93 octane gas and switch stations to insure it's not caused by bad gas. A TSB talks about a bad ECM as a possible cause ($750 just for part remanufactured). Any experience out there? Car runs great otherwise. Got 31 mpg on recent long trip.