Engine rattle

[broketiltuesday]

Hi,

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!

Cheers,
Tom

[Torque]

Tom,

I'd start by having a compression test done to make sure that cylinder 5 is not indeed completely gone if that's what you suspect is. Other than that, only source of rattle I can possibly think of is one of the 2 valve lifters for cylinder 5 -- but then again, that wouldn't go away if you took out the cyl. 5 plug as the camshaft still hits them.

Anyway, at the top of the 5 series forum is a 'Bentley Service manual for e34'. If you don't have it, download it and look through there as it has much more information than I can come up with.

Good luck, and please post back when you figure it out, as I am very interested in this.

[broketiltuesday]

Torque,

Many thanks for the reply. I'll get a compression test (a shop diagnosis as well unless I have any more ideas) when I go back home in 3-4 weeks. I don't know any good shops around here and I don't drive much atm so it can just wait.

A post on roadfly is suggesting wrist pins/rod bearings for that cylinder, ie the force under combustion is placing a force on the rods/pins that's making the rod knock against some other component on the downward stroke hence the noise. This doesn't occur when the piston is 'freeloading' when not under combustion. It sounds a little lighter than I'd associate that kind of knocking with but I can't think of much else atm (unless there's something wrong with the mixture and the fuel isn't detonating correctly, hence causing some kind of noise).

In the meantime I'm pretty puzzled but will post if anything else occurs.


Cheers,
Tom


[Edit: good job on the bentley's by the way. it's a lot better than my haynes book...]

[billB]

Tom

When you get back home try to start the engine for 20 seconds only; listen to the sound and then Shut it down , pull the spark plug from # 5 and pour little bit of motor oil into its cylinder . Reinstall the plug, start the engine up again and listen. If you have piston slap the noise will be reduced - possibly eliminated…..BUT for a very short period of time.....Then the noise will come back.

Regards
billb