Sudden lift of tick over speed

inovermyhead

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On Sunday I took part in a BMW club social drive and subjected the old girl to 4 hours of driving in 40 degree weather, I have the 4 window air conditioning option, to say it was hot would be an understatement.
The car was fine all morning but after lunch the idle speed would not drop below about 1900 rpm which made the two hour drive home a bit of a chore.
The car runs an after market ecu( Haltec PS 2000) and this has been no problem for the last year or so and always ticked over at about 750 rpm, occasionally it would sit at 1200 ish but a blip on the throttle would always bring it back down, but this time it sat steadfastly at 1900 or so, I thought possibly it might be a ecu fault but having spoken to Haltec they think not, any thoughts?

Cheers. John
Also given this problem would I be best advised to change the oil/ coolant after two plus hours of driving?
 

nosmonkey

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I'd check the TPS to make sure it's reading correctly. Plug the ecu in to the laptop when engine is cold and see if something is amiss, very unlikely that the ECU has developed a fault, likely to be a sensor has had a fit.
 

Cornishman

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Can you explain the air leak issue and why it effects the idle speed please?
Is this on cars with carbs or injection?
Thanks
C
 

inovermyhead

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My car is injection, the diagnosis came back pretty quickly, all pointed to an air leak, as to why it drives up the idle speed , I have no idea, but some more mechanical minded members on here will probably be able to provide an answer.

Cheers. John
 

mulberryworks

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At idle the butterfly is closed and intake vacuum is high. A leak will add extra air making the mixture lean so the ECU sees that via the O2 sensor and adds fuel to get the mixture back to normal. More fuel means higher idle speed. When you step on the loud pedal, the butterfly opens and air comes in, decreasing the vacuum and making air from the leak a smaller percentage of the combustion chamber charge so the car runs more or less normally.
 

sfdon

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Well- either an Alpha N or a closed loop MSPNP will do the same.
Neither has an active O2 at idle. Or at any other throttle position.


Symptoms of a vacuum leak
A vacuum leak will cause the engine to run lean. In other words, there will be more air entering the engine than is needed to balance out the air/fuel ratio. This can result in the following:

  • Engine stalling
  • A rough running engine
  • Too fast of an idle



At idle the butterfly is closed and intake vacuum is high. A leak will add extra air making the mixture lean so the ECU sees that via the O2 sensor and adds fuel to get the mixture back to normal. More fuel means higher idle speed. When you step on the loud pedal, the butterfly opens and air comes in, decreasing the vacuum and making air from the leak a smaller percentage of the combustion chamber charge so the car runs more or less normally.
ell
 
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