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      03-05-2013, 12:01 PM   #1
TT6mT
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F30 N55 6mt idle at stop light

Is it normal for the N55 6mt idle to jump around when coming to a stop, or when pressing the clutch pedal?

1. When the car comes to a complete stop at a light, the RPM will go to about 500rpm, then jump up to around 1k, then back down to 600. (clutch pressed in)

2. Car is idling at the stop light at around 600rpm, when I press the clutch pedal it'll jump to 800 then settle back down to 600.
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      03-06-2013, 07:06 AM   #2
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I've noticed it jump a little like that in mine about 200 rpm when you hit the clutch. I assume it from the engine being released from the drag in the trans. It unloads the engine. It's riding the throw out bearing instead of the entire clutch assembly. Please correct me if I'm wrong.
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      03-06-2013, 11:59 AM   #3
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The engine is still under load in neutral? I thought the spike in RPM is from some kind of electricity load from pressing the clutch.

How about when you come to a stop, does the RPM jump up to about 1k rpm, then drop?

Thanks for responding, I'm trying to figure out if the behavior is normal.
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      03-06-2013, 12:56 PM   #4
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Quote:
Originally Posted by TT6mT View Post
The engine is still under load in neutral? I thought the spike in RPM is from some kind of electricity load from pressing the clutch.

How about when you come to a stop, does the RPM jump up to about 1k rpm, then drop?

Thanks for responding, I'm trying to figure out if the behavior is normal.
Well if you're off the clutch pedal, then the plates are together. It may not be turning the drive (not in gear), but it's connected. So it's a load albeit a small one... I assume this is what you're seeing.

My E46 had similar behavior, as does my F30 (both w/ manual gearboxes)

edit: sorry, I didn't mean to suggest there was an electrical load.
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      03-07-2013, 10:38 AM   #5
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I don't think there's an electrical load by pressing the clutch. I thought that was a hydraulic thing.

Ill try to see if I can make the issue happen in my car and isolate it down for ya. But I have seen it move the rpm around by pushing the clutch. Is it a lot? No. But also coming to a stop, a touch of power and you can knock it out of gear without the clutch. The touch of gas takes the tension off the drive train from the rears slowing the car and gives the opportunity. See if when you come to a stop then what the engine is doing. It should come back to idle as you coast, using the breaks to stop. Shouldn't have to jerk it out of gear that way. It almost pops out for me

Another thing I have noticed was in cruise control, without touching the breaks and down shifting the revs went up, but them stopping shortly there after my rpms went to 2k before I put her back to normal. I can't get it to do it consistently, but it did happen. I'm just tossing that out there as it has to do with slowing down.

Ill try to get mine to do it for ya bud and monitor the results.
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      03-25-2013, 08:21 PM   #6
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The clutch is hydraulic, so there's no change in electrical load when operating it. However, it's completely normal for your engine RPM to vary slightly if you press and release the clutch while at a stop and in neutral.

Your transmission has two shafts: the input shaft, and the output shaft. The input shaft has all the gears for each of the forward ratios (1-6), and some sort of reverser (different transmissions implement this slightly differently). When you're in neutral, and the clutch is out, the input shaft actually spins in the transmission. None of the gears are "engaged", but they're still riding on the input shaft. They're "coasting" if you will.

The thing is, your transmission is full of gear oil, which is thick stuff. I've had the transmission out of more than one car, and I can tell you that turning the input shaft of a transmission that is in neutral requires a reasonable amount of force when it's cold. As the transmission warms up, this effect is reduced, but it's still enough to affect engine RPM at idle.

What you're observing is completely normal. You might be interested in checking out this HowStuffWorks.com article on manual transmissions. It's amazing how all this stuff works:

http://www.howstuffworks.com/transmission.htm
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      03-29-2013, 03:23 AM   #7
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Thanks Bradley. That was a nice site.
I've faced that my car is working as designed. It's actually idling smoother after it was only driven by my lady for a week.
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