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Matt, Mechanical Engineer
Category: BMW
Satisfied Customers: 22732
Experience:  BEng hons Mech engineering, in auto industry 22 years
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after turning ignition on engine revs to over 3000 , remove

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after turning ignition on engine revs to over 3000 , remove throttle cable does not change this, we had been running car with just 1 knock sensor in place and this is occurring after putting the 2nd 1 back, new throttle put in a couple of months ago, when you disconnect air flow meter the revs drop to 1500 and when you re connect it the revs go way up ? what is problem?


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.

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

Might also be worth cleaning out the throttle body as these get clogged up with carbon and some brake cleaner washed through helps free things off.

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.

if the engine condition is the same then chances are the meter or the connection to it is faulty

Air leaks are very temperature dependent as gaps can open or close up as things expand with heat, so they can be effected by the weather and engine temperature.

This leads them to be quite intermittent in the case of mild leaks

its also worth cleaning the airflow meter by removing it from the car and spraying the exposed sensor wires inside the tube with a brake or switch cleaner - ensure that the cleaner is one of the old fashioned, non Eco type that does not leave a residue

On no account touch the sensor wires with anything physical as they are extremely fragile

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Customer: replied 3 years ago.

Hi thanks for your advice, changed a breather hose on manifold, still revving but not so bad, when I remove air flow plug the engine then cuts out. Slightly worried knock sensors, not connected to the correct set of cylinders, not sure what effect this might have. Will check other hoses with the propane spray. am assuming as it is new the throttle control module is ok. can't drive it at moment, think may have to get it to a diagnostic computer, am sure it is likely something to do with air flow, very strong smell of petrol, after turning on. Thanks again for help. Cheers.

Hello Trevor

having the knock sensors on the wrong cylinder will cause the ECU to misdiagnose a misfire on what cylinder but thats about it

its not that sophissitcated to retard spark on just 1 cylinder it'll just pull them all back