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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 21850
Experience:  BEng hons Mech engineering, in auto industry 22 years
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Mercedes clk 270 cdi.....2004 model. When driving the car

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Mercedes clk 270 cdi.....2004 model.
When driving the car the engine revs dip, this is noticed by the feel of the car, it's as if you are tapping on and off the accelerator pedal (not continuously but now and again) Especially noticed when pulling up a hill and also visually by seeing the rev needle momentarily dip and then return to normal. Also under gentle or normal acceleration there is a haze of white smoke from the exhaust, it's not thick smoke as in a headgasket problem.
Any thoughts or ideas would be welcome as my mechanic is not too sure....
Submitted: 1 month ago.
Category: Mercedes
Expert:  Nicola-mod replied 1 month ago.
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Customer: replied 1 month ago.
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Customer: replied 1 month ago.
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Expert:  Matt replied 1 month 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 or brake cleaner 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.

Air leaks are very temperature dependent as gaps can open up as things expand with temperature so depending on how the leak occurs ( on a hose joint with a clamp expansion will help seal and on a vacuum hose expansion ill make a leak worse) so leaks can be better or worse as the engine heats up.

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 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

It’s also worth cleaning the MAF sensor wires, as they can get coated with dirt over time which then offsets the reading
Use some contact cleaner or brake cleaner to spray onto the wires to remove the dirt – on no account touch the wires with anything as they are very fragile

Expert:  Matt replied 1 month ago.

Hi
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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 21850
Experience: BEng hons Mech engineering, in auto industry 22 years
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