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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 22931
Experience:  BEng hons Mech engineering, in auto industry 22 years
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I have recently purchased a cls 320. I have full service

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I have recently purchased a cls 320. I have full service history so know it has genuine mileage.
Shortly after I purchased it I started to get an odd occasion of Kangaroo juice and what I can only explain as limp home mode.
The dealer I purchased it from had allowed it to run almost empty of Diesel.
I explained this to a Mercedes specialist in Milton Keynes where I live and arranged for it to go in to be looked at.
Diagnosis was a fault to the exhaust back pressure monitor which was changed but the intermittent problem is still there.
The car has not yet none 53,000 miles and needless to say, I paid quite a bit of money for it as it is in excellent condition.
What do I do or who do I best go to to get this resolved?


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 flattenedpiece 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 idle speed control motor / valve 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 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

Customer: replied 1 year ago.
Hi Matt,
I'm no expert but am pretty good with my hands.
Who could I trust to do this work without ripping me off?


any small garage can carry this out

ideally if you need me to locate an Mercedes independent Specialist then this will ensure the best service

just let me know your whereabouts

Matt and other Mercedes Specialists are ready to help you
Customer: replied 1 year ago.
I am in Milton Keynes, my post code is MK4 2GE.
Thank you.
Customer: replied 1 year ago.
Hi Matt,
I have read through your answer a few times now and don't feel that it something I should do.
A job best left to an expert.
My post code is MK4 2GE.
I would be grateful if you could recommend a suitable garage.
Kind regards,



there's a couple of independent Merc specialists in MK


or in luton