weird engine noise! [w/ sample] [FIXED!]

[the_ps]

'91 325i with a stock m20b25 engine.

noise: http://junkyman.org/tmp/e30clatter.mp3
or http://junkyman.org/tmp/e30clatter.wav

This is loudest if I put a stethoscope around the crankshaft end behind the crank pulley under the car. If you analyze the waveform, it happens every 103 ms, or 580 times a minute. The tach shows 700 rpm. There is also a sequence of 3 knocks, where the first is loudest, the second is in the middle, and the last knock is quietest.



You can hear this.

I tried a few things and nothing made the noise go away:
We tried disconnecting spark plugs one at a time. We removed the fan and fan clutch. I verified the valve adjustments. The speed goes up with the revs, but at 2k+ it is muffled by normal engine noise. If I slip the clutch in 5th gear to put some load on the engine, there is no change. A/C on or off, no change.

I am absolutely stumped.

This doesn't happen when the car is cold. It is unsteady at first, then becomes very solid when the car gets to operating temp. It can't be heard inside the car with the hood down unless the windows are lowered and you are listening for it.

History:
100 miles / 3 weeks ago, I replaced the oil pan gasket, timing belt, timing belt tensioner, water pump, clutch, pilot bearing, oil, oil filter, coolant, all external seals and gaskets, giubo, csb, had injectors rebuilt and flow-tested, some other minor stuff I can't remember. Did not touch any crank or rod bearings.

70 miles / ~10 days ago, I got gas.

10 miles ago / 5 days ago it started overheating at traffic lights (not into the red, but 3/4 there), and I replaced the aux fan switch (which wasn't the problem), and discovered the thermostat was in upside down. It didn't seem like it should matter, but somehow it did. There could have just been air that wasn't bled. Anyways, once this was done, the gauge was in the middle and all was well.

5 miles ago / 4 days ago, I drive a few miles, no noise (was at operating temp), drive home, heard the noise for the first time.

The clattering doesn't happen until the engine is at normal temperature. It doesn't seem to be any accessories.

[jlevie]

The noise sounds to be about the right frequency to be from the valve train, but it could be a rod bearing. A bit of work with a stethoscope will tell if it from the top or bottom of the engine and thus tell if it is from a rod bearing or the valves.

[FLG]

Double check the exhaust manifold area for leaks...they can sometimes sound like bad valve/engine noise.

Sent from my Galaxy Nexus using Tapatalk 2

[Exodus_2pt0]

Have you checked your timing belt tension?

[the_ps]

Have you checked your timing belt tension?

I took off the distributor cap and rotor and I could easily deflect it with a screwdriver with little force near the camshaft sprocket. I'll take the upper timing cover off tomorrow. I'm not sure how tight it is supposed to be since it's a toothed belt. I did follow the manual on tensioning it.

I'll spend more time with a stethoscope tomorrow night. :(

[Exodus_2pt0]

Honestly I listened to that file for a good 5 minutes, until I felt like I was being hypnotized by it.

Belt is the only thing I could think of, I have heard them make some strange sounds when too loose. I'll keep it on my mind and if I come up with any other idea's i'll let you know.

Good luck.

[the_ps]

Honestly I listened to that file for a good 5 minutes, until I felt like I was being hypnotized by it.

:)
It was the intermediate shaft bolt. It was a little loose. There were metal shavings. No serious damage it seems. I can't get the intermediate shaft sprocket off without removing the big crank bolt, which I torqued sufficiently well a couple of weeks ago that I now have to buy a holder if I want to get it off. I guess I'll buy the tool and redo the job if the problem comes back.

The intermediate shaft sprocket has more teeth than the crank sprocket, hence, 580 rpm. The pattern of three ... no one will ever know!

I broke the stupid plastic clip that keeps the sensor wires out of harm's way, so I have a few days to count the teeth on each sprocket and come up with a complex explanation of how the camshaft profile lines up with the intermediate shaft rotation.

The water pump had a little side-to-side play (w/ 100 miles on it). Anything to worry about?

I will post towards the end of the week when I get the plastic clip and I can run the engine again.

[the_ps]

Ah. So the big pulley does come off. Inspected intermediate shaft. No real damage. :up:

[Exodus_2pt0]

Good to hear. As for the water pump, since it is not driven by the timing belt I wouldn't worry about it until it makes noise or leaks.

[the_ps]

FIXED! Finally got the plastic wire guide today from the dealer. And 10 brackets because that's what the warehouse sent them.

I know many of you were mesmerized by the 3/4 time signature of my intermediate shaft. The reason is that the intermediate-shaft sprocket has 32 teeth. The camshaft sprocket has 48 teeth. So every time the intermediate sprocket rotates into the angle where it knocks, the camshaft will be in one of three angles putting a slightly different load on the timing belt.

[paperplane94]

Dang, That's technical.
:)

Waterpump should have a little bit of play.

Less than 1 mm to be technical (not really).