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

[wswilburn]

I'm certainly no expert, but since none have replied, what the heck. The 17Hz peak in the spectrum implies something that happens once per crank revolution alright. Could be a piston, but have you considered other possibilities? Before tearing into the engine, I would certainly check everything I could. Exhaust leak? Valve train problem (this would probably be at half the frequency though, unless it is a 2nd harmonic)? Taking out sparkplugs is a good idea. Sorry I couldn't listen to your mp3. I run Linux which doesn't support that format.

[vw_michael]

Originally posted by wswilburn
I'm certainly no expert, but since none have replied, what the heck. The 17Hz peak in the spectrum implies something that happens once per crank revolution alright. Could be a piston, but have you considered other possibilities? Before tearing into the engine, I would certainly check everything I could. Exhaust leak? Valve train problem (this would probably be at half the frequency though, unless it is a 2nd harmonic)? Taking out sparkplugs is a good idea. Sorry I couldn't listen to your mp3. I run Linux which doesn't support that format.

FYI, I made the mp3 on a Linux box ;-)
If you care to try, I have the original .wav file at
http://snooper.homeunix.org/pub/bmw_noise.wav

I'm currently trying to find out the recommended valve clearance for the 325e - I had set the valves as per 325i spec to 0.010in cold.

[Jared at Pelican Parts]

sounds more like an exhaust leak to me...