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Matt, Mechanical Engineer
Category: UK Car
Satisfied Customers: 20374
Experience:  BEng hons Mech engineering, in auto industry 22 years
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My W212 M274 is running lean at idle : 18,000 miles done. 2L

Customer Question

Hi, my W212 M274 is running lean at idle
JA: How many miles are on the car? What size is the engine?
Customer: 18,000 miles done. 2L
JA: What is the model of your '2147481646 Mercedes?
Customer: E250 petrol
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: been trying tp
JA: Anything else you think the mechanic should know?
Customer: it is intermittent.. happens only when idle speed is at 700rpm with Air con turned on. if engine idles below 700rpm it's fine with air con turned on. Zero issues without turning ON the air con.
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the Mercedes Mechanic about your situation and then connect you two.
Submitted: 4 months ago.
Category: UK Car
Expert:  Matt replied 4 months 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 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 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 4 months ago.
Fault code history shows P2187.
Engine check light appears once in a while with the above code stored in history.We've done most of the leakage check and found nothing, including a leak down test and smoke test.As this issue came active only after a Intake Cam Sprocket changed when it had loud rattling crank up on cold start especially. We've even reopened the cam covers to check the timings. Everything's fine and correct.
Nothing found damaged, missing or loose.Items that we tried changed were:
Oxygen sensor before the Cat..and
MAF sensor
To our surprise, the Oxygen sensor that was out of the car appeared White in color.Throttle body has been cleaned too.The car will wobble and doesn't feel right on idle, but not always even when the engine is hot. Sometimes there sometimes not.
But, it doesn't happen at all WITHOUT turning on the air con.
I could feel the wobble/tremble on idle sometimes and will always managed to easily solve it immediately by turning off the air con.what else did we missed?The car is factory standard and had done only 18000 miles to date since June 2014.
Expert:  Matt replied 4 months ago.


OK something else to check is 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.

this can lead to catalyst failure as the extra fuel can re-light inside and melt the catalyst

The best way to find an exhaust leak is to run the engine from cold and feel around the exhaust joints with your bare hands - you'll have about 30 seconds before it all gets too hot and you should feel any escaping gas blowing over your fingers

Customer: replied 4 months ago.
Done it.
Nothing leaks from the exhaust manifold to before the Cat.The air con matters in this case. I was thinking maybe it could be air con related.A leak is always a leak no matter if air con is turned on or off.But the system just runs well without any intermittent issues without the air con.
Customer: replied 4 months ago.
Oh yes, it happens every morning cold starting, for 2 secs and gone, without air con.
Expert:  Matt replied 4 months ago.


the extra load of the AC is probably dragging the engine down to below the point where it can idle

so I'd pop the drive belt off and turn the AC pump over by hand to feel how it turns as it shouldn't be overly stiff

also the idle issue may be there all the time but the systems can cope without the AC load

so next replace the spark plugs if any of them fail this checklist:

Expert:  Matt replied 4 months ago.


do you still need help?

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