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Discussion Starter · #1 ·
I heard a knocking when I got home from work a little over a week ago, that didn’t match the all the noise coming from my new power steering pump. The noise only started at around 2100 rpm and seemed to stop around 3000, wasn’t there at idle unless the car was at operating temp and then, it was sporadic. I stopped driving it and wasn’t able to look into it until last Saturday. I started with looking under the valve cover to see if I screwed something up when I replaced the exhaust cam and then pulled the drive belts to see if it was an accessory. No dice. I had my son hold it at 2100 and pulled the connector to cylinder one, the noise changed...
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Miraculously, the crank is ok(nothing that’ll catch a fingernail). I’m throwing new bearings, rod bolts, and an oil pan gasket in it and calling it a day. The car isn’t worth anything over that and would’ve gone to the scrap yard if the crank was toast.
 

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E30M3 Race F10 535 R1150Rt M Coupe
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So on an old school car, when trying to determine if the noise from the engine is a rod knock? You would pull off the spark plug boot from each cylinder one at a time. If the noise changed (compression with spark event, raising cylinder pressure) when removing the spark on a cylinder STOPS. It's likely a bad rod bearing.

Now that we have COP (coil on plug) you disconnect the connector from the coil pack on each cylinder.
The difference being:
Now that we have OBD 2 systems, if you remove the connector the DME see's your actions and will usually leave that cylinder turned off. We don't want that, so we must take an extra step:

Now we can do the above test with a scanner hooked up:
Start engine, remove connector from #1. Noise still present?
Key On Engine off (KOEO) clear faults. Reconnecting #1, Start car and remove #2.

Keep repeating the above until you find one cylinder that makes the engine quieter.
Bad rod bearing on THAT cylinder. He got to the offending one with #1.

Most usually, the rod bearings that go bad first, are the two farthest back in the engine. Owing to distance from the oil pump/supply.
 

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So on an old school car, when trying to determine if the noise from the engine is a rod knock? You would pull off the spark plug boot from each cylinder one at a time. If the noise changed (compression with spark event, raising cylinder pressure) when removing the spark on a cylinder STOPS. It's likely a bad rod bearing.

Now that we have COP (coil on plug) you disconnect the connector from the coil pack on each cylinder.
The difference being:
Now that we have OBD 2 systems, if you remove the connector the DME see's your actions and will usually leave that cylinder turned off. We don't want that, so we must take an extra step:

Now we can do the above test with a scanner hooked up:
Start engine, remove connector from #1. Noise still present?
Key On Engine off (KOEO) clear faults. Reconnecting #1, Start car and remove #2.

Keep repeating the above until you find one cylinder that makes the engine quieter.
Bad rod bearing on THAT cylinder. He got to the offending one with #1.

Most usually, the rod bearings that go bad first, are the two farthest back in the engine. Owing to distance from the oil pump/supply.
Thanks for the response! Two things led me to ask my earlier question; 1) what connector are we talking about (never heard of a 'cylinder connector' before) and 2) what could possibly quiet down a bad rod bearing?

Continuing on with the second question; why does that change the sound? I probably just don't know what I'm talking about but I assume the noise comes from the rod scraping/bouncing off of the rough bearing surface... and I don't see why that would change due to lack of spark. The rod is still moving on the bearing with the rotation of the engine, is it not?
 

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E30M3 Race F10 535 R1150Rt M Coupe
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The connector in question is the connector for each ignition coil.

The hardest thing a rod bearing has to do is sustain the pressure generated during an ignition event. Aided naturally with a thin coating of oil.

When one cranks an engine; lets assume the plugs are installed but the connectors are removed from the ignition coils. The cylinder pressure when the valves are closed is about 160-185 PSI. The standard compression an engine makes bereft of a ignition event. Very easy for a rod bearing to handle that small load.

Now lets have the ignition coils hooked up, shall we. As each cylinder fires, the compressed air & atomized fuel now receives a spark to light things off. The pressure in each cylinder can be about 800- 1,200 PSI. (depending on C/R, cams etc) If a rod bearing is bad, the much increased pressure will make the bad rod bearing make noise. Owing to the (now) increased clearance of the bad bearing.

The OP was astute and he stopped, diagnosed properly and caught in in time before it destroyed the large end of the connecting rod. (Usually gets oval shaped) and the hardened journal of the crankshaft hadn't run long enough to receive egregious pounding (clearance) and depositing of hot & melted bearing material.
 

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Discussion Starter · #13 ·
I've only had the car maybe 5-6 months, so not sure of it's history. I don't think it was maintained very well after the original owner(I bought it with a broken exhaust cam).

I can't see 700k(or over 300k) being very common, or is it that the rest of the car just falls apart around the engine and doesn't allow them to reach higher mileage...
 

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2001 330i (pre-facelift)
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That does seem like an untimely demise. Did the previous owner neglect it? I would scrutinize the oil pump pickup (and the pump itself), just to be safe.

Anyway, kudos to the OP for having the skills to diagnose the problem before it became a catastrophe.
 

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2005 330ci
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Any video of the sound?

Last time I came home from Roebling Road, I noticed a thumping sound between 2000 and 3000 RPM with light throttle. Compression is great and it runs and drives normally.

I’ve replaced the DISA, lifters and all of the timing components, including tensioners.

I still have the noise and it seems to be worse when the engine warms up. I assume that’s because expansion.

I’m headed to a friends house for Thanksgiving, and will have him help me pull the cop connections while raising the RPMs to see if it’s a bearing.
 

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2005 330ci
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Your right. I’ll see if he can come to my house to help.

When I did all of the timing chain work, I had to remove the oil pan. I should’ve inspected and replaced the bearings while it was open.
 
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