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Matt
Matt, Mechanical Engineer
Category: BMW
Satisfied Customers: 22145
Experience:  BEng hons Mech engineering, in auto industry 22 years
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Hi I have a 07 plate 325i coupe and when I take my foot off

Customer Question

Hi I have a 07 plate 325i coupe and when I take my foot off the accelerator its makes a really loud screeching noise, not all the time, not when its stationery just when I lift my foot off the gas.
Submitted: 3 years ago.
Category: BMW
Expert:  Shantal-Mod replied 3 years ago.
Hello,
I'm Shantal and I'm a moderator for this topic.
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Customer: replied 3 years ago.
Do I still have to pay if you don't find anyone to answer the question?
Expert:  Matt replied 3 years ago.

Hello

This 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.

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.

Customer: replied 3 years ago.
Hi Matt,
Thank you for your reply , but the noise is really loud !! I mean everyone stops and looks. Would a air leak make that much noise?
Expert:  Matt replied 3 years ago.
Hello

if the leak is pulling in over a torn rubber hose then yes as its acts like reed in a wood instrument

they can make a lot of noise sometimes as engines do move a lot of air
Customer: replied 3 years ago.
Hi Matt, I've spoken to my macanic, on your reply and he says as the engine ticks over and runs smooth, he thinks it's not an air leak, but I do feel it's a hard one to solve as this does not happen all the time , once every 100 miles and only when my foot lifts off the accerlator . Any other theory's on what could cause this loud screeching?
Expert:  Matt replied 3 years ago.
Hello

I'd also try removing the auxiliary drive belt and turning every pulley and ancillary ( power steering pump , alternator etc) by hand. All should turn easily (with the exception of the AC pump) with no grittyness or wobbly pulleys- If you do find a sticky pump / alternator then it should be replaced or repaired

If the engine is stone cold you can briefly run it with no belts to see if the noise changes or has gone away, this will help show if an ancillary is the issue