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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 22917
Experience:  BEng hons Mech engineering, in auto industry 22 years
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My 2003 clk 240 2.6 V6 for a few months when idling ticks over

Customer Question

My 2003 clk 240 2.6 V6 for a few months when idling ticks over lumpy, the revs drop and almost stalls but recovers to a normal idle rpm.. recently it sometimes doesn't recover.. but stalls, then will not fire up when cranking engine over. Engine starts if left for half light comes on dianostics shown 1st cam sensor fault (changed) Next time crancshaft sensor fault (changed) I had two cats cut out (one either side) about a yea ago.. But the engine has only just started stalling on idle (not every time) There seems to be some oil seeping from a joint around left side cat.
I have also noticed when trying to accelerate after negociating say a roundabout the engine doesn't respond as expected.. I have to depress the eccelerator further than usual.. by doing so the car sometimes changes down.. rather than pulling in a higher gear.
Submitted: 3 years ago.
Category: Mercedes
Expert:  Matt replied 3 years ago.

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

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 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 3 years ago.

Hi Matt,


I still have a fault with the car.. The engine managment light came on whilst driving and at that point I could feel the car was not running on all 6 cylinders (but this time engine didn't stall when I cam to a stop) The revs appeared slightly higher on tick over.. 800 rpm rather than approx 500rpm at idle. The tick over sounde and felt lumpy.. I could feel it was not firing on all 6 cylinders.??

The car engine restarts and runs sweet after it has been left for a while?


My mechanic is yet to come out and plug in his dianostics.


Could the problem be something as simple as a plug breaking down, which may account for the oily residue noticed around one exhaust down pipe under the vehicle.. Any other suggestions please .. apart from scrap it!!