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Neutral/Clutch disengagement and idle while moving

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97gsxIA

10+ Year Contributor
418
44
Apr 22, 2011
Des Moines, Iowa
Ok so from my understanding, it is completely normal for a car to have a higher idle while moving in neutral(1300-1400) until coming to a stop (where it returns to normal 900-1000) Whether it be to keep it high for power steering, or whatever.

My curiosity spikes when I notice that the idle returns to normal when I push my clutch in while moving (in neutral) instead of staying up in the 1300-1400 range. (Let me emphasize that it returns to normal... it does NOT bog or go below 900rpm when pressing in clutch)

I'm wondering why exactly this occurs, specifically when my clutch is disengaged.

Assume everything is in working order (at face value).
In this instance, its on my GSX that has a built 6 bolt swap (400 miles on it) new clutch, new trans, new flywheel, new just about everything with ALL clearances including step height in spec, Throttle body and sensors are all dialed in with less than 2,000 miles on them (previously rebuilt). No clutch drag up to 7K rpm test.
Crank end play is in spec, so please save me from rolling my eyes at crank walk comments.
The car runs great, pulls great and has been tuned by me. MAFcomp matches airflow, WBfactor is <3% through out power band. So everything is well within working order, and I can't be more pleased with how the build has turned out. I am just very curious.

Any insight on this?
 
Well once you disengage the clutch from the flywheel, there is nothing to tell the engine what RPM to be at besides the speed sensor, and I don't think they're related.

My car falls back to normal idle (800 rpm) when I push the clutch in. As far as I know, that's how it's supposed to be. I could be wrong though.
 
Well at least I'm not the only one who has noticed this then. The only related posts I've seen for this "issue" have been concerning BOV recirculation and crank end play (crankwalk)... Which just isnt the case here. MY idle is well tuned with my LrdnIdleAdj around 140ish, my ISC at 36, CombinedFT <2% deviation from 0. TPS is new and tested within spec, speed sensor... well that could be the only thing up for question as it is indeed old. I'm using my TPS to control my idle switch... So I mean everything points to it being normal...

But there has to be a reason why. Whoever made our ECUs didn't just get up one morning and decide to do this for shits and giggles.
 
I don't think there's a reason honestly. If you think about how a manual transmission works, once the clutch is disengaged (clutch pedal pushed in), there is very little connecting the drivetrain (wheels, transmission, etc) to the engine. So it doesn't really have a reason to idle anywhere other than what the idle is set to, since it doesn't know the wheels are moving.
 
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