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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > idle free rev jumpy



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      12-07-2012, 02:06 AM   #1
TOPEC
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idle free rev jumpy

i just noticed something weird with my car and dont think its normal, at idle, if i free rev the car, the rpms jump back and forth quite a bit, im pretty sure im holding a steady accelerator pedal position yet the engine revs jump up and down. ive captured 2 videos to show what im talking abt.

https://www.dropbox.com/s/8vaelt65gc...2021.25.07.mov

https://www.dropbox.com/s/cmjv91bl2l...2021.28.49.mov

car is a 2011 335is DCT with BMS DCI and BMS OCC

could this be a vacuum leak? the car drives fine, pulls hard, steady boost

any ideas?

thanks
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      12-07-2012, 09:01 AM   #2
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The one thing I do know is you need new brake pads but as far as the idle are you saying you are holding the gas pedal steady and the rpms aren't staying planted?
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      12-07-2012, 01:10 PM   #3
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You are intentionally holding the rpms at 1200 in the first video? If so its normal. They don't all do it but I've seen it here in the shop. It's either the vanos units adjusting to the just off idle condition or the throttle input varying slightly on its 5v cross reference.
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      12-07-2012, 02:13 PM   #4
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If that is idle, it is vac leak.
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      12-07-2012, 10:12 PM   #5
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maybe i wasnt clear on my first post? im trying to rev the engine and hold it steady, but the rpms keep jumping up and down while im holding the throttle steady. This is done while the car is stationary and in park mode.
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      12-07-2012, 10:15 PM   #6
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Quote:
Originally Posted by BMWN54tech View Post
You are intentionally holding the rpms at 1200 in the first video? If so its normal. They don't all do it but I've seen it here in the shop. It's either the vanos units adjusting to the just off idle condition or the throttle input varying slightly on its 5v cross reference.
i just held the throttle steady trying to hold the engine at a steady rpm. if this is throttle input variation, then this is a pretty BIG variation with the throttle held steady.
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      12-08-2012, 12:52 PM   #7
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Quote:
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i just held the throttle steady trying to hold the engine at a steady rpm. if this is throttle input variation, then this is a pretty BIG variation with the throttle held steady.
Its not about how big the variation is, is about the fact that rmp is going from a technically high idle rpm to off idle.

For example, notice how when you first start your car it idles at about 1200? Its programmed to do that, its running a high idle rpm. Its the same rpm the isid will set the car to run to heat up the cooling system for thermostat/water pump diag, its the same rpm the car runs to heat up the cats, its set in the car. When you free rev the car to that rpm its turning off high idle, but when it does rpms dip a little bit, which drops it back into high idle, and this process starts.

If something was wrong the car would tell you, trust me.
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      12-08-2012, 01:26 PM   #8
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Quote:
Originally Posted by BMWN54tech View Post
Its not about how big the variation is, is about the fact that rmp is going from a technically high idle rpm to off idle.

For example, notice how when you first start your car it idles at about 1200? Its programmed to do that, its running a high idle rpm. Its the same rpm the isid will set the car to run to heat up the cooling system for thermostat/water pump diag, its the same rpm the car runs to heat up the cats, its set in the car. When you free rev the car to that rpm its turning off high idle, but when it does rpms dip a little bit, which drops it back into high idle, and this process starts.

If something was wrong the car would tell you, trust me.
cool, never really thought of that, ill give it some higher revs and see if that happens again, thanks!
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