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Matt
Matt, Mechanical Engineer
Category: BMW
Satisfied Customers: 20592
Experience:  BEng hons Mech engineering, in auto industry 22 years
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We have a BMW 525D 2003 with approx 100,000. we have aquired

Customer Question

We have a BMW 525D 2003 with approx 100,000. we have aquired a hesitation at approx 1700 revs onwards when warm, we have taken the car to BMW main dealers who could not make any suggestions. We then contacted a diesel tec firm who once again having plugged in the diagnostic machine no fault found.We spoke to a friend who advised us to return to the tec company and ask for a injector leak through test which we did and an injector was replaced fault but still there. It has now been sugested that we could try replacing to further injectors which were not as good as the new one, however this may not be the answer there could be other parts that need renewing. With todays technolegy why cannot the fault be found. Would you have any recommendations please?
Regards
Nigel
Submitted: 2 years ago.
Category: BMW
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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 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 a poorly pump will all result in insufficient fuel being delivered to the engine



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 the weather and engine temperature can effect them.

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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