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Matt, Engineer
Category: Mercedes
Satisfied Customers: 21724
Experience:  Mechanical Engineer with 23 years experience in the auto industry, 8 yrs in formula 1 engine testing
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2005 c240 4matic. Idle is fluctuating by 200-300 rpm and the

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2005 c240 4matic. Idle is fluctuating by 200-300 rpm and the car is rocking. I changed the MAF, checked for vaccum leaks, changed plugs and coils and cleaned the throttle body. 02 sensor issue? Car has 160,000


Can I ask how you've checked for vacuum leaks? as it only take a small leak to give this sort of idle fluctuation

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

if you're confident that this is OK then I'd next check

Check for exhaust leaks before the O2 sensor as if this has been experiencing an exhaust leak then it will have been running very rich as the sensor will see the extra air pulled in as fresh / lean fuelling and be adding fuel to compensate.

Customer: replied 11 months ago.
I checked for vaccum leaks by removing the airfikters and placing my hand over the mass air flow sensor when the engine is running until the engine shuts off then kept my hand there and it remained full vaccum until I removed my hand or any vaccum line. I also just replaced the 02 sensors and the car is smoother but still idles rough and the needle goes up and down.

OK is the exhaust gas tight before the sensors?

a lack of fuel and could be a lack of fuel pressure from the electrical pump at the tank.

1st thing to try is to remove the fuel filler cap and then flick the ignition on whilst listening for the brief ‘buzz’ of the pump

If the connections to the pump are corroded or damaged then the pump could stop at any time or could not be running at full speed. Check that its relay switches in and out and the relay contacts are clean and bright – replace the relay if in any doubt and do the same for the fuse,

Ideally measure the pressure at the injector rail (should usually be around 3Bar / 45psi +) and if this is low check the pump as described and also consider changing the fuel filter as if this partially blocked this too will reduce pressure.

You can also get this issue with contaminated fuel , if there’s water or dirt in the tank then either the petrol floats on top of the water or the filter sock on the pump is choked up – so if the other things check out then I’d consider pulling out the fuel pump and cleaning the filter sock

Customer: replied 11 months ago.
One thing I will add before I check the fuel pressure is I took off everything all the way to the throttle Body and turned the ignition on (engine off) I could here the electronics of the throttle body moving back and forth, if I use a mirror you can see the throttle plate moving back and forth, could this be the issue? Or is this just because it's adapted to do so


the throttle body should only move back and forth once when you turn the ignition on

it should only move the once not keep doing so without any pedal input

so yes I'd say that there is something wrong with the throttle if its constantly moving


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