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Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 20395
Experience:  BEng hons Mech engineering, in auto industry 22 years
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Merc S 320L Cdi -2007

Resolved Question:

my 2007 S class W221 320Lcdi seems to rev quite high when moving from stop and gears do not change till it hits 2500 revs. The pick up is not as smooth as it should be and the revs seem to fluctuate quite a bit

Submitted: 2 years ago.
Category: Car
Expert:  Matt replied 2 years ago.

I'd agree that cleaning the air sensor is a good idea and this could also 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.
Customer: replied 2 years ago.


Thank you for your reply. Is there anything else that can cause this problem?

Expert:  Matt replied 2 years ago.
Hello Rasmin

If the engine load sensing ( air flwo meter) is Ok then its worth checking the basics of the transmisson

I'd take a look at the colour of the transmission oil it should be a dark red colour if its black then its needs changing together with its oil filter. Also check its level with the gearbox warm and just after you've moved the selector through all the positions leaving it for a few seconds in each one and then back to park.

You can top up the box through the side fill plug if required and make sure you use the correct oil for your car as the fluids are not all the same. Some manufacturers recommended a change of the type of fluid - Speak to the local dealer and parts dept - they will advise you on the updated fluid to use..

If that’s all OK then check all the electrical connections of the gearbox, remove each connector in turn and spray with contact cleaner. Check that the wiring to the gearbox is in good condition with no chafing or frayed wire

If this is OK then check the selector lever linkage / cable under the car for any broken parts and or missing bolts as if this is coming adrift then it can rub on the car body or just bind up

Then - you should top up if low and re try it - or, if its level is ok - and condition is ok - then the next step is to drain it off - open it up and clean the filters inside -- refill and retry it..

Often - just doing the above cures many troubles with these - and you certainly can't condemn the unit without doing the above...
Matt, Mechanical Engineer
Category: Car
Satisfied Customers: 20395
Experience: BEng hons Mech engineering, in auto industry 22 years
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