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Old 03-21-2011, 11:27 PM   #1 (permalink)
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Unhappy High Idle RPM in Idle when in P or N

In idle the RPM is in the 900-1100 range but if i put it in D or press the brake pedal when in P or N it drops down to normal 500-700. Any idea why idle speed is linked with the brakes?

The problem is more evident when the engine is cold and after it warms up RPM in Idle drop to about 800 range. I recently got the TPS changed and fuel pump was repaired.
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Old 03-22-2011, 06:40 AM   #2 (permalink)
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That sounds about normal to me.

If the car is otherwise running fine, why do you think something is wrong?

Just as a basic warm-up from a cold start you should let the engine come off of the initial fast idle before dropping it into gear and driving off.

Loaded (in gear) idle speed will always be lower than unloaded idle speed.

Hope that helps.
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Last edited by Check Codes; 03-22-2011 at 04:35 PM. Reason: Smartphone...sigh
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Old 03-22-2011, 08:11 AM   #3 (permalink)
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At first glance I think you may have reversed your thinking----loading at idel does what??
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Old 03-22-2011, 04:36 PM   #4 (permalink)
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^^^ Good catch. Posting from a 1.8" smartphone screen makes things look funny.
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Old 03-24-2011, 12:46 AM   #5 (permalink)
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Quote:
Originally Posted by gregs210 View Post
That sounds about normal to me.

If the car is otherwise running fine, why do you think something is wrong?

Just as a basic warm-up from a cold start you should let the engine come off of the initial fast idle before dropping it into gear and driving off.

Loaded (in gear) idle speed will always be lower than unloaded idle speed.

Hope that helps.
But why would the revs drop when i press the brake pedal even when in P or N and go back up if brake is released. How is engine speed related to the brakes? Its not something serious but just want to make sure its not a precursor to a bigger problem down the road
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Old 03-24-2011, 09:07 AM   #6 (permalink)
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When you press the brake, the vacuum booster for power brakes is in operation. So you will be drawing vacuum off the engine which can change the RPM. I have seen on a few of my cars the RPMs change a little, but not sure ever that much.
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Old 03-24-2011, 09:27 AM   #7 (permalink)
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>>In idle the RPM is in the 900-1100 range but if i put it in D or press the brake pedal
>>when in P or N it drops down to normal 500-700. Any idea why idle speed is linked with
>>the brakes?
  • 900-1100 is high for idle in P.
  • On my car, after warming up, the rpm dropped to 800-900 in P. While in P, pressing the brake had no effect to the rpm.
The rpm only dropped when shift to D, R, ...
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Old 03-26-2011, 08:55 AM   #8 (permalink)
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The OP has a diesel (CDI 270) so the idle speed will not be same as the gas/petrol model. Don't even know if the 270 is a 4, 5 or 6 cylinder and whether the brakes do use vacuum or a electric booster pump?

If vacuum, then it may be the booster going bad and allowing extra air in, instead of just using the vacuum to assist. Are your brakes working fine with a not too high an effort required? If effort is high, then brake booster seals may be bad and you are dumping extra air into the inlet system.
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Old 03-26-2011, 09:26 AM   #9 (permalink)
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Whoah, diesel car gives better mileage but gets complicated.
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Old 09-15-2011, 02:42 PM   #10 (permalink)
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Quote:
Originally Posted by carczar View Post
In idle the RPM is in the 900-1100 range but if i put it in D or press the brake pedal when in P or N it drops down to normal 500-700. Any idea why idle speed is linked with the brakes?

The problem is more evident when the engine is cold and after it warms up RPM in Idle drop to about 800 range. I recently got the TPS changed and fuel pump was repaired.
What was the resolution??
I am having a very similar problem with a 2002 E430 4Matic. Idle speed with brake off is about 1,750 in P or N. Depress the brake and it drops to 800. Clearly not normal. I've been doing some searching and discovered that there is a Idle Speed Control component that takes one of it's inputs from the brake switch.
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