I have a 1989 (m20 engine etc) 525i. About a month ago, it started developing a loud rhythmic click/rattle noise from the engine bay. It makes this noise loudest when the throttle is open engine is under load, like going up a hill in a high gear, etc. IT doesn't seem to occur (or at least is too quiet to hear) at low throttle openings. The noise gets faster with rpm but occurs only when the throttle is open past a certain point (ie if i stamp on the gas at 1500 or 3000rpm it makes a very similar sound). The normal note of the engine is there, just with this extra rattle. Idle seems to have also developed a slight clatter. This noise sounds very much like descriptions I've read of pinking and is fairly loud - i can hear it in the cabin etc!
After a bit of investigation, it seems cylinder 5 is responsible. Disconnecting this plug at idle removes the clattering note. The idle is rock steady with/without cylinder 5 connected. A short drive with plug 5 disconnected sees the rattle completely removed. So I'm assuming that something is going wrong with the combustion process in cyl 5. The injector seems fine (impedance is 15.5ohm and i can hear it clicking normally), the ignition system is in fair shape. Engine temperature is always at the normal mark, ie. no overheating. When examined, plug 5 was in good shape if a little fouled, replacing it with a known good one had no effect.
So. Has anybody else had this problem, or can anyone give me some pointers on what else to check, and what could be causing this noise? Any ideas appreciated!
Gone but not forgotten: '87 UK 318... Carb madness!
Currently: '90 UK 525i completely stock
"Heaven for the climate, Hell for the company."