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

 

[jrt67ss350]

My '87 325e makes the same noise when cold, except mine isn't nearly as loud as yours. I'd be interested in knowing what it is if you figure it out. I always have just assumed that my noise was some slop in one of the rods since my car has 300k miles on it, but piston slap does seem like a possibility.

 

[Big Daddy]

Could it be timing belt/chain related? I know that the M42, a chin driven motor can make a lot of noise when cold due to the chain tensioners being worn out. Just some out loud thinking.

 

[jrt67ss350]

Could it be timing belt/chain related? I know that the M42, a chin driven motor can make a lot of noise when cold due to the chain tensioners being worn out. Just some out loud thinking.

Good thought, Dan, but it shouldn't be timing belt related. The M42's chain tensioners dynamically tension the chain which is why they make lots of noise when they wear out - they slap around in there when worn. The M20's timing belt tensioner does NOT dynamically tension the belt - you set the tension when you install the belt by rigidly bolting down the tensioner at the appropriate position. It's a static system. A worn tensioner would basically make the same noise as a worn idler pulley on an accessory belt system since it's essentially the same thing - usually wouldn't be a knocking sound.

Good thought, though.

 

[vw_michael]

The timing belt has been recently changed (I'm not sure about the tensioner though, as I didn't own the car at the time). I believe the knocking may be coming from the top/middle of the engine around cyl 5, but I couldn't pin-point it with a stetoscope. The tensioner seems not to be the issue, but I will double-check that. thanks.

Could it be timing belt/chain related? I know that the M42, a chin driven motor can make a lot of noise when cold due to the chain tensioners being worn out. Just some out loud thinking.

 

[epj3]

you try changing oil weights? I think it's probably some out of spec rocker arms. If you pull the valve cover off, I'm willing to bet you can slide those rockers pretty far... probably way beyond specification.

 

[vw_michael]

Could you explain what you mean by 'slide'?
Do you mean try to 'twist' them to check the clearance to the shaft, or actually move them sideways? I have Castrol 20w50 in there now, and had mobil 1 5w40 before.

Thanks,
Mike

you try changing oil weights? I think it's probably some out of spec rocker arms. If you pull the valve cover off, I'm willing to bet you can slide those rockers pretty far... probably way beyond specification.

 

[E30m42cab]

I know the valves on the 325e's need adjusting every 40,000kms or so (I don't remember the exact intervals). Kinda makes them sound like a diesel. I've never hear one that sounds that bad though.

 

[epj3]

E30M42 - for those valves to be out of spec enough to sound like that - the motor wouldn't run.

vw_michael - I mean sliding them on the shaft.

 

[vw_michael]

E30M42 - for those valves to be out of spec enough to sound like that - the motor wouldn't run.

vw_michael - I mean sliding them on the shaft.

I will try that the next time the cover is off .... Can I ask what the logic behind this is? I thought that the rockers were free-sliding anyway, and the only thing keeping them in place was the spring-like piece made of sheetmetal around the rocker on the rocker shaft.

Mike

 

[epj3]

I will try that the next time the cover is off .... Can I ask what the logic behind this is? I thought that the rockers were free-sliding anyway, and the only thing keeping them in place was the spring-like piece made of sheetmetal around the rocker on the rocker shaft.

Mike

Yes but I believe there is supposed to be only a certain amount of play - I know mine had a bit of play in two of the rockers and it made it quite loud. The loudest thing in these old valvetrains (on the SOHC motors) are the rockers.