Debugging engine knock on 325e - BMW Forum -

3-Series (E21, E30) Chat relating to the BMW 3-Series from 1975-1983 and 1984-1991 line. Specific models: BMW 315, BMW 316, BMW 318, BMW 318i, BMW 320/4, BMW 320i, BMW 320/6, BMW 323i, BMW 320i. E30 Family models include: BMW 325e, BMW 325i, BMW 325is, BMW 325ix.

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post #1 of 2 (permalink) Old 05-30-2006, 11:43 AM Thread Starter
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Debugging engine knock on 325e


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:

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:

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!

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post #2 of 2 (permalink) Old 05-31-2006, 08:06 PM
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Hmm, I tried the link but it didn't work for me, however from what you are saying it could also be a wrist pin.
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