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Matt
Matt, Mechanical Engineer
Category: BMW
Satisfied Customers: 21849
Experience:  BEng hons Mech engineering, in auto industry 22 years
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Hi - My 330i 6-speed auto has developed an intermittent issue,

Customer Question

Hi - My 330i 6-speed auto has developed an intermittent issue, when under moderate, but relatively quickly applied load (e.g. starting a 90 deg turn, or the start of a hill), the car will cut power - (no traction control or EWL tho) - which can be restored by lifting the throttle and reapplying. Engine doesnt rev during the power loss - any ideas as local dealers cannot answer and want to take on a test drive, where it may or may not arise. No fault codes registered apparently.
Submitted: 2 years ago.
Category: BMW
Expert:  Matt replied 2 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.
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