Two problems: M73 whining noise and misfires

[Aitch]

Recently I've had two new problems crop up on my 750. The first is that I can hear a slightly metallic whine from the front of the engine in the past week or so, generally after the car is warmed up and I've been driving for awhile. I'm guessing this is probably one of the bearings along the path of the serpentine belts?



Second, I've had a high-rpm misfire for a few months now. This would ONLY manifest if I was at or near WOT and above 5k RPMS, or if I go directly to WOT from a standstill. The codes were always "intermittent misfire", cylinder-specifc misfires (which move around on both banks, no consistency) and a few "spark duration too short". I've suspected the spark plugs (replaced by PO) and wires, but mainly think the distributors are likely getting worn. They are on my list to replace.

The past week, after the car warms up and has been driven for a bit, it occasionally goes into a rough idle and sounds like there is a misfire. Yesterday it did this and again threw out pending codes for a number of cylinders misfiring. I cleared those and drove home. This morning it started fine, drove to work, and by the time I got there (and recorded the video above) it was misfiring, but not codes thrown or CEL lit up (yet). I'm guessing this is likely just my previous misfire problem getting worse? Is it weird that it's idling rough but no codes are thrown right away?

[Timm]

Start with the plugs and distributors - the error 'spark duration too short' indicates that there is a long arc-path somewhere - either the plugs gapped too wide, or (more likely) worn carbon pieces in the distributors....

[Aitch]

Start with the plugs and distributors - the error 'spark duration too short' indicates that there is a long arc-path somewhere - either the plugs gapped too wide, or (more likely) worn carbon pieces in the distributors....

Cool, thanks Timm. Figured that's probably the best place to start, given the progression from intermittent/high RPM to more general idle issues.

[Aitch]

Follow-up question: Is the distributor cap one of those places where it should be Bosch or nothing? Or can you go with an off-brand item there? I'm wondering about the latter...

[Timm]

That would be a question better answered by members with more cylinders than I have! Us V8 owners don't have distributors at all - just eight coils....

[asianvenom]

Follow-up question: Is the distributor cap one of those places where it should be Bosch or nothing? Or can you go with an off-brand item there? I'm wondering about the latter...

Bosch, or bremi is fine. Bremi is actually came with the car (oe), bosch is an oem. That's the only 2 brand I'm awared of.

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[RIboater2]

I use a piece of clear plastic hose like from a fish tank to track down engine noises. Use it like a stethoscope.n stick one end in your ear and point the other end around. it really pinpoints sound pretty well. I had that sound and I think I recall it was an idler pully. sprayed some Wd40 and all was well.

If you don't get the misfires at normal throttle and rpm's, id suggest not going to wot and keep at normal rpm's. the car doesn't seem to like it.
i'm still on original wires caps and plugs at 165k. haven't had misfire or code or mileage problems. don't hate. these wires should never need replacing anyway. they don't break down.

[Aitch]

Bosch, or bremi is fine. Bremi is actually came with the car (oe), bosch is an oem. That's the only 2 brand I'm awared of.

Sent from my VS995 using Tapatalk

I use a piece of clear plastic hose like from a fish tank to track down engine noises. Use it like a stethoscope.n stick one end in your ear and point the other end around. it really pinpoints sound pretty well. I had that sound and I think I recall it was an idler pully. sprayed some Wd40 and all was well.

If you don't get the misfires at normal throttle and rpm's, id suggest not going to wot and keep at normal rpm's. the car doesn't seem to like it.
i'm still on original wires caps and plugs at 165k. haven't had misfire or code or mileage problems. don't hate. these wires should never need replacing anyway. they don't break down.

Thanks for your input guys. I'd be happy if the wires are fine, and hoping the spark plugs don't need replacing again. May have to check a couple of them though to see what their condition is. I'm hoping to have a chance to get one distro cap off this weekend and check it's condition finally.

[Aitch]

Long-time update. Whining has become more persistent, as have the misfires and idle issues. Went to the shop today for an oil change and they diagnosed the passenger CCV/OSV is leaking. Indeed, now I can hear that there is a vacuum leak there (I haven't been under the hood in a few weeks). Parts ordered! Will update later this summer when they have been replaced (traveling a bit first).

Edit: I should add, I'd already replaced the distributor caps which had no effect.

[RIboater2]

Another V12 guy had a leak recently. You can literally use a patch of duct tape to cover it and the leak will stop. It's a bandaid but will let you know if it's going to "fix" your problem with the new parts. Did you order just the valves or the grommets also? (which i have determined to be the most overpriced part on an entire E38). It's sometimes the rubber grommet that has cracked causing issues.

[Aitch]

Another V12 guy had a leak recently. You can literally use a patch of duct tape to cover it and the leak will stop. It's a bandaid but will let you know if it's going to "fix" your problem with the new parts. Did you order just the valves or the grommets also? (which i have determined to be the most overpriced part on an entire E38). It's sometimes the rubber grommet that has cracked causing issues.

I put some duct tape on the back for now which seems to have lessened the leaking sound. CEL came back on this afternoon though. I didn't order the grommets, I didn't realize they were a separate item. I'll try to tack them on to my order if that's the case.

[Zerocool818]

Sorry to resurrect a 5 year old post, but was this issue resolved? Just had a bout of the high rpm; WOT misfire happen. Plugs, cap and rotor are newish ~15k miles. The only issue I can think of is a couple of weeks ago I had a code for camshaft sensor, which was also replaced ~20k miles ago with a bmw part, since the car ran and drove fine I cleared it and it hasn't come back. I actually tried it multiple times in an effort to flag a code to help with later diagnostic but it never illuminated the CEL. Will check pending codes tomorrow.

Felt weird; above 4krpm and WOT it would just bog and lose all acceleration, almost like it hit the limiter, but low rpm WOT pulls just like a freight train. I don't know if maybe the issue was happening before, I don't usually get past 3k rpm anyway around town. Today I decided eh, what the heck do a pull on the freeway on ramp and felt it