Scratching/Grinding Sound below ~3k RPM - 1991 318i - VIDEO

[91FourDr5spd318i]

Hi all, first post here. Sorry it's a negative one, but I suppose that's what this forum is for! I was so excited to discover a site dedicated to my favorite engine. I'm hoping that I won't have to take it to a shop and pay them to do something I can do myself.


https://m.youtube.com/watch?v=jkdeTUUuh3Y


This grinding sound has been happening since I bought the car three months ago. As you can see, it basically stops at 3K rpm, then as the revs slow, you can hear it come back on strong. PO says the PO before him (a family friend he knows well, both of whom know and love these cars greatly, had the engine rebuilt ~75,000 miles ago.

Previous owner said it was there when he had it too. He also says it's had the timing chain been checked out as an attempt to diagnose the sound, the shop said it was the AC compressor clutch bearing. Shop also claimed to have used a stethoscope to confirm this. After removing the AC belt today, the sound is obviously still there. I'm doubting the legitimacy of whoever inspected the vehicle.

Listen for the higher pitched, "oscillating", repeating pattern of scratching, scraping, grinding at idle. It becomes harder to hear in the video as the revs increase. But see as RPMs dip back below 3k or so, the sound returns, very loudly.


I notice an increase in torque around 3K rpm, then an even greater increase in torque at 4.5K rpm. Occasional when cruising at low RPM, low throttle, whatever the source of the sound causes slight acceleration/deceleration in sync with the noise's modulations.

I've considered timing chain guides, tensioners, or the cover. I also read somebody claiming that when changing their water pump, a sound described similarly to this one disappeared. My father speculates oil pump, but he's no car guy.


As a side note, there's occasionally a rough idle at start up, and the intensity of the sound changes every time I drive it, and after warming up. Also, a very recent development, there is sometimes a faint fuel odor from the engine bay, and/or a stronger smell from the gas cap area. Fuel level gauge sometimes reads incorrectly as well. Smell occasionally (quite rarely) becomes unbearable.


Thanks in advance for your time and suggestions!

[locknload]

First thing I would do is, remove all the belts and run it briefly to see if anything changed.  If the noise goes away, it's either the alternator, PS pump, water pump or viscous fan clutch.  If you still have the noise with no belts on, the next thing to check is the timing chain and tensioner.  I would drop the front oil pan and see if there are any chunks of plastic or metal.  If so, chain guides are bad.  If not, I would suspect a bad chain tensioner.  They are cheap enough ($30 ish) and an easy task.  It is located near the A/C compressor.  This might also explain the failed A/C clutch diagnosis.  Compressor is hard bolted to the block and may be telegraphing chain noise.

The "torque improvement" is probably just the second stage of the throttle body opening coincidentally with the decrease in grinding noise.  Don't let it distract you, most likely not related to your problem.  Sound change of the noise is probably due to oil thinning as it heats up.

[91FourDr5spd318i]

First thing I would do is, remove all the belts and run it briefly to see if anything changed.  If the noise goes away, it's either the alternator, PS pump, water pump or viscous fan clutch.  If you still have the noise with no belts on, the next thing to check is the timing chain and tensioner.  I would drop the front oil pan and see if there are any chunks of plastic or metal.  If so, chain guides are bad.  If not, I would suspect a bad chain tensioner.  They are cheap enough ($30 ish) and an easy task.  It is located near the A/C compressor.  This might also explain the failed A/C clutch diagnosis.  Compressor is hard bolted to the block and may be telegraphing chain noise.

The "torque improvement" is probably just the second stage of the throttle body opening coincidentally with the decrease in grinding noise.  Don't let it distract you, most likely not related to your problem.  Sound change of the noise is probably due to oil thinning as it heats up.

Thank you so much! I bet it is a tensioner, if my understanding of what that does is correct.


I must ask though, regarding the dual stage throttle body, I already notice a change in torque, and a bit of a rougher/stronger pull before the sound goes away, if I depress the accelerator further than the first 1/8-1/4 of its range of motion. Even after that increase in torque, the feeling is still there. Is this initial torque increase not the second stage throttle body?

[locknload]

I strongly encourage you to sort out the grinding noises before you start goosing the throttle up & down.  If your timing chain is slapping around in there, you are heading for a very expensive lesson in how to prioritize engine repairs.  It's very easy to skip a tooth if your chain is not tensioned properly.  If that happens and it skips the wrong way, start shopping for another engine.  Do what I suggested first, then figure out the throttle.  At this point, you are rearranging the deck chairs on the titanic.