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BmurJoe
07-08-2010, 03:47 PM
I've had this problem for the last 2 summers. I get a hallow knocking noise from the engine compartment. It only happens when it's really hot outside and I've been driving the car for a while. It has to be really hot outside like over 30C. Stopped in Drive, a/c on. I turn the a/c off and let the rpm drop to 500. I turn the a/c on so that the engine comes under load. As the rpm rises slowly, you hear a hallow knocking noise. If you increase the rpm the noise starts to go away. You don't hear it when you drive, or if its cool outside. BMW first thought that it could be a rattling torque convertor, then they said it sounded like detonation. Problem is I first reported the problem when the car was under warranty, but it's hard to replicate the problem in the shop. Another mechanic seems to think it could be timing belt slapping. He thinks that when it's hot the oil becomes thin, oil pressure drops because of the low rpm, and it being under load is causing the problem. Nobody has ever heard of this, and BMW is perplexed. They've heard it in the parking lot, but never have the time to work on it at that time. Also, engine temp is 107C when this happens.

Any advice?

JOE

Keif
07-08-2010, 03:50 PM
Video w/ sound? Light/high tick or dull knocking? Any errors thrown?

kippk
07-08-2010, 06:22 PM
I hear a slight knocking at idle when warm. Was told when I had my 740iL that it is normal for this engine and is coming from ROLLER BEARINGS?? Not sure if I was fed a line or not but changing the blinker fluid didnt help any either:rofl::rofl:

I can tell you that I recently changed to 5W50 Mobil 1 and the ticking is GONE. I have 170,000 and the increased film strength seemed to stop it. I will switch back to a 0W40 Mobil 1 come winter. Still a bit heavier than 5W30 but man does is run smoother.

BmurJoe
07-08-2010, 08:11 PM
I almost sounds like a hammer hitting the bellhousing. It's sounds kinda hollow and you can hear it from 10 ft. It's not a ticking noise almost like piston slap. It doesn't happen in cold start. It's slow like a knock a second.

kippk
07-08-2010, 08:35 PM
I almost sounds like a hammer hitting the bellhousing. It's sounds kinda hollow and you can hear it from 10 ft. It's not a ticking noise almost like piston slap. It doesn't happen in cold start. It's slow like a knock a second.

Well then Dat Aint It:rofl::rofl::rofl: I hope you find an answer to the "ghost in the machine" and it's easy on the wallet.. :yikes:

1fatcat
07-17-2010, 06:14 PM
Could be a rod bearing? One thing to try, when the problem is happening, disable the cylinders one at a time and see if the knock goes away. If the knock goes away with one particular cylinder being disabled, then the problem will be either a bad rod bearing, a worn wrist pin or piston on THAT cylinder. So you can narrow it down to 1 of 8. The dealer might be able to disable cylinders through their scan tool, but Im not sure? Otherwise, you can do it by unplugging injectors, or spark plug wires...one at a time. I try not to recommend removing plug wires, because you can get shocked. It won't kill you, but you'll feel it. This test needs to be done with the engine running, so be careful.

Keif
07-18-2010, 10:59 PM
Look through the alternator threads to see if it's similar to that sound (there is one with a video). Mine sounded similar before seizure.

irokdastar383
07-20-2010, 12:56 PM
take it to a shop

BmurJoe
08-29-2010, 07:17 PM
I have the same knock as this engine. Listen carefully at about 36 seconds.

http://www.youtube.com/watch?v=K_HUxUyhvj0

JOE

Keif
08-29-2010, 11:24 PM
If it's a dull thud, then it's not the timing belt...btw, I wouldn't bring your car to the mechanic that said that, as these have timing chains, so he may not know this engine well. I'm still voting on it being a failing alternator, especially after hearing the knock you are comparing it to.