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Mini cooper: When I let off the gas, coasting to a stop the…

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When I let off the...
When I let off the gas, coasting to a stop the mini jerks like it is about to stall. Is this normal? Would a mechanic be able to adjust something? I is a 2012 cooper automatic.
Submitted: 2 years ago.Category: BMW
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Answered in 3 hours by:
6/25/2015
BMW Mechanic: Matt, Engineer replied 2 years ago
Matt
Matt, Engineer
Category: BMW
Satisfied Customers: 23,199
Experience: Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
Verified
Hello
If you don't have a check engine light this sounds like it could be
an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.
As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.
Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.
The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.
It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine
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BMW Mechanic: Matt, Engineer replied 2 years ago
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