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Discussion Starter #1
I've noticed a "rev hang" issue for a long time now, but my wife finally suggested it might have something to do with whether or not a gear was selected or the gearshift was in neutral. I tried this today, and sure enough, she was right. My old school thinking was not taking into account how much things like gear selected play in the grand scheme of electronic engine controls. Good job honey!

If I have the gearshift in neutral and pop the throttle (say enough to pop engine revs up to ~2000 rpm), they fall right back to idle. However, if I select a gear (any gear it seems), the ECU knows the state of the shift lever since now the revs hang (it appears to lag the closure of the throttle plates) and it feels more lethargic to respond to dabs in the throttle pedal. I had the clutch depressed for both tries (obviously you need to when it's in gear, but I was thinking maybe it had something to do with knowing the clutch pedal was depressed which it doesn't).

Can anyone check this behavior on their M5?

1) Have gearshift in neutral, clutch depressed, dab and instantly release throttle enough to get revs to ~2000rpm or so. Notice throttle response and how quickly it comes back to idle.

2) Now do exactly the same thing but with a gear selected. Notice any change in throttle response? Revs hanging longer? Does it feel more lethargic to respond to the throttle?

One other thing is to hold revs at 1500 to 2000rpms and then instantly release the throttle and notice how quickly it falls to idle. With a gear selected as the test above, it is much slower to fall back in my car.

Thanks,
Chuck
 

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Discussion Starter #3 (Edited)
Yes.

It's so that you don't shock the engine/transmission in a mis-match on rev's.
Hmmm...so you've noticed this/tested this on your car? The reason doesn't make any mechanical engineering sense because it doesn't contribute to less shock on gear shifting -- especially at higher rpms where the issue isn't evident and where shock loads would be dramatically higher. Now if it was to lessen the off throttle shock up the drivetrain (i.e. to lower the reverse torque coming through the clutch for example), I might be able to buy that, but it doesn't even come close to the shock load the drivetrain sees when you nail open all 8 throttle bodies...not even close. Who told you that was the reason and what was their logic, if any, behind it? More importantly, have you tested this on your car, and it responds just like I said above?

Thanks,
Chuck
 

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Chuck,

I have just checked this for you. The response of the engine revs to throttle position with the clutch in is identical on my car whether I have a gear selected or not. I tried holding the revs at 1500, 2k, 3k and 4k rpm and then releasing the throttle with the clutch in with the gear stick at neutral, 1st gear and 2nd gear position; no difference in the rate of revs falling at all.

My car is a stock 2001 model with 131k miles and was not completely up to temperature; yellow tachs starting at 5500rpm.

Hope that helps my friend:)


Edited to say that I do live across the pond from you so could it be possible that the ecu is set up slightly differently? Also, sport mode was off.
 

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Discussion Starter #5
Chuck,

I have just checked this for you. The response of the engine revs to throttle position with the clutch in is identical on my car whether I have a gear selected or not. I tried holding the revs at 1500, 2k, 3k and 4k rpm and then releasing the throttle with the clutch in with the gear stick at neutral, 1st gear and 2nd gear position; no difference in the rate of revs falling at all.

My car is a stock 2001 model with 131k miles and was not completely up to temperature; yellow tachs starting at 5500rpm.

Hope that helps my friend:)
Awesome! That's what I thought would be the case since it seems highly unusual on my car. For example, if I'm going along slowly in 2nd and let off the throttle, I can feel it slightly pulling still (but not at the throttle opening I was just at) and then feel the throttle letting completely go. For some reason it is hanging up like this when a gear is selected.

Thanks,
Chuck

P.S. If anyone else gets a chance to check this, it would be nice to have confirmation.
 

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No problem - I have just thought of something; is your air conditioning turned on?

On my old Nissan if I had the air con on, sometimes it would drive itself below 1500rpm because the ecu was applying so much throttle to compensate for the draw of the pump. Obviously the M5 has plenty of low down power to cope with the air con without us really noticing, but it might be worth checking if your air is on in case the computer is over compensating for some reason.
 

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Discussion Starter #7
No problem - I have just thought of something; is your air conditioning turned on?

On my old Nissan if I had the air con on, sometimes it would drive itself below 1500rpm because the ecu was applying so much throttle to compensate for the draw of the pump. Obviously the M5 has plenty of low down power to cope with the air con without us really noticing, but it might be worth checking if your air is on in case the computer is over compensating for some reason.
Yep, that was the first thing I thought about, and there is absolutely no difference when the AC is on or off. The only thing that makes this happen is putting the car in gear. Weird.
 

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Yep, that was the first thing I thought about, and there is absolutely no difference when the AC is on or off. The only thing that makes this happen is putting the car in gear. Weird.

I don't have the rev hang in gear, but I noticed a slowness before I added the UD pulleys. I did the headers at the same time, but my sense was the pulleys allowed the car to rev quicker and the revs to return quicker.

Having said that, I don't know how the car knows which gear the car is in. I guess the ECU might since there is a soft limiter at 6500 in 6th, and of course reverse trips out the reverse lights.

Can't remember, stock ECU? And after the recent discussion, I am sure you checked for a CDV!:thumbsup:

Regards,
Jerry
 

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i've never noticed this either, and will check it on my way out tonight. I am not convinced that the ecu knows what gear the car is in.
 

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Nope.......

Chuck - 'found your post interesting; so went and checked mine when fully warm. No difference in RPM fall-off at all. 'Did all the tests you described. No difference.
 

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Discussion Starter #14
Wow, this is really weird. Everyone here doesn't have the issue except rneedham1979 and me (so far). The ECU (stock in my case Jerry) is stock in my car.

We ate dinner last night with Redshift and his wife, so I tried this on his car. His had absolutely the same response whether the car was in gear or not. I then tried mine again, and we could easily see the issue as I described above. Redshift's car is a 2001 built in the same month as mine (July 2001), and his VIN is actually really close to mine. So...

I'm guessing there was an update to the s/w somewhere along the line that my car did not get. I vaguely remember something in the past that could have been about this subject, and I seem to recall a TSB or such might have been issued about it? Does that ring any bells with anyone? A s/w update that BMW did on M5s sometime back in the 2002 timeframe?

rneedham1979 (Ryan is it?), I'm having my dealer look at the car on Monday, so I'll post back with what they find.

Thanks to all for checking your cars -- I really appreciate the help!

Chuck
 

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Discussion Starter #15 (Edited)
i've never noticed this either, and will check it on my way out tonight. I am not convinced that the ecu knows what gear the car is in.
Yeah, it definitely knows what gear the car is in. Here's a link to a diagram where you can see #11, "gear identification switch". Sometime back I had a discussion with a major aftermarket BMW engine builder, and he talked about how even the ignition advance curve is different depending on the gear since 1st gear can take a lot more ignition advance earlier in the rpm range than say 4th gear can....stuff like that. Makes my old school knowledge of tuning Weber sidedrafts and changing points a bit of a moot point I think. :1zhelp: :)

Chuck - 'found your post interesting; so went and checked mine when fully warm. No difference in RPM fall-off at all. 'Did all the tests you described. No difference.
Thanks Dan! The thought of that sweet S3 nicely responding to the throttle without this problem is very pleasing. :) My car is at the shop today, so I should know something soon, fingers crossed.

Chuck
 

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Discussion Starter #17
Chuck, are you saying that we all should see the OP's symptoms? Now you have me thinking. I will have to try mine again!
No, not at all. I was just pointing out that the car does know what gear you have selected (i.e. it electronically knows the state of the gear shift mechanism). I don't think any M5 "should" behave like this. Redshift's car didn't do what mine does, DRM's didn't, etc.
 

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Very interesting thread!

Do you think it could be something up with the clutch itself?

I'll try this out on my car later on and report my findings.

I have noticed what you described on other BMW's before, they seem to take longer to rev back down to idle for some reason - although Richard (Irish M5) didn't notice anything on his 540??

On our 325ci when going down the gears the revs do seem to take longer to wind down than on the M5 - I'll check this again also.

Could this be related to the CDV?? (if we have one on our M5s - I stopped reading that thread!)

Richie.
 

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Discussion Starter #19
Very interesting thread!

Do you think it could be something up with the clutch itself?

I'll try this out on my car later on and report my findings.

I have noticed what you described on other BMW's before, they seem to take longer to rev back down to idle for some reason - although Richard (Irish M5) didn't notice anything on his 540??

On our 325ci when going down the gears the revs do seem to take longer to wind down than on the M5 - I'll check this again also.

Could this be related to the CDV?? (if we have one on our M5s - I stopped reading that thread!)

Richie.
No, I don't think it is related to the clutch at all. It is purely an electronically related change in the throttle/engine behavior.

The E39 M5 was not manufactured with a CDV, but this has nothing to do with the clutch. This is all done with the car sitting still.

This is not standard operation for an M5...I'll get to the bottom of the issue soon I hope.
 

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No, I don't think it is related to the clutch at all. It is purely an electronically related change in the throttle/engine behavior.

The E39 M5 was not manufactured with a CDV, but this has nothing to do with the clutch. This is all done with the car sitting still.

This is not standard operation for an M5...I'll get to the bottom of the issue soon I hope.
I hope so too - nothing worse than a problem like that.

Could it be to do with the gear position indicator switch? Perhaps that itself is faulty?

How about a vacuum leak somewhere?

I don't mean to throw random suggestions here - I am sure you have spent some time trying to diagnose before posting.

Best of luck,

Richie.
 
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