Debugging engine knock on 325e

[vw_michael]

Hello,

I'm trying to figure out the 'new' old car engine noise. I was hoping someone here can shed some light. My current educated guess is that it's piston slap. The noise is only present for ten minutes after cold morning start. The noise is most apparent under load below 2500rpm, and when blipping the throttle.

I made a recording available at:

http://snooper.homeunix.org/pub/bmw_noise.mp3

The recording was made with a pro microphone in the engine bay by the firewall, connected to a laptop. I drove at around 10-20km/h, at 1000rpm in 2nd gear with the breaks lightly on (this seems to make the noise louder).

I analyzed the recording with spectrum analyzer software, and it seems that the frequency of the noise is the same as the rpm (i.e. at 1000rpm, i get 17 'knocks' per second) with a very regular interval, which points at one of the cylinders.

Pic at:
http://snooper.homeunix.org/pub/bmw_noise.jpg

The car is an 1985 325e, 2.7l, 75k miles. Other than the d**n noise it's running very well, so I'm determined to figure this out. I'm currently looking at a new piston, rings, and possibly a rod. I'm going to find out which cylinder it is by taking out the respective sparkplugs and listening for the noise while driving.

Any help/comments appreciated!

Thx,
Mike

[Espen]

I dont know the english word for it, but that sounds like an good example of "rod-knocking" the bearings that are between the pistonrods and cranckshaft make those noises if they are worn out.

[vw_michael]

I dont know the english word for it, but that sounds like an good example of "rod-knocking" the bearings that are between the pistonrods and cranckshaft make those noises if they are worn out.

Thanks - I hope not though ;-) Wouldn't rod knock occur when the engine is warm and the oil thins out? I'm going to install an oil pressure gauge to keep an eye on oil pressures.

Mike