E220 poor idle

d215yq

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Hey guys,

About 1 year and a half ago I had my only breakdown in my estate, when the MAF suddenly went. I had this replaced and ever since then the idle has been poor, like it idles a bit too low (it did this a bit before the new MAF but not as badly).

It idles typically at 600/650 unevenly and when you accelerate the revs instantly dip and there is a hesitation before they then increase as normal. Once above 1,000rpm the engine is smooth and powerful and no issues at all.

Occasionally when sat stopped in traffic the anti stall kicks in and the revs rise to 1200rpm. three times the car has actually stalled in a year and a half and 25k miles (twice just after filling with petrol!), so as i say this isn't a big problem. In the time since this has happened it's had new plugs, air filter, fuel filter, cleaned the butterfly valve, etc, to no affect.

I've got used to it but recently acquired a 220E as a spares car (as per here http://forums.mercedesclub.org.uk/showthread.php?t=116220 ), and this drives just so smoothly and idles perfectly. So the question is can anyone suggest to me bits to swap (that are easy to do) that might help mine become smoother...what about swapping the MAFs, or maybe the air intake pipes incase it has leaks? And is any of this risky...because at the moment i have two working cars and i don't want to jeopardise this by messing things up further!

Any thoughts appreciated! Thanks in advance!
 

BachelorDays

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Air leak would be an obvious thing to check. Any sign of any oil around in the pipes? Also, if you swap MAF and it works better, check for leakage before the MAF which might be damaging the MAF.
 

kth286

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'twice after filling with petrol' may be a clue, as the pressure/vacuum in tank was released when you took the filler cap off.

Is your MOT valve on the inner wing pulsing when engine up to operating temp.

You can feel it and hear it normally.

It may be stuck open and allowing petrol fumes into engine when it should not, thus causing feeling of stalling.

Fumes should only be allowed to be sucked into engine when certain parameters exist, and the valve should then open to allow this.
 
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d215yq

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'twice after filling with petrol' may be a clue, as the pressure/vacuum in tank was released when you took the filler cap off.

Is your MOT valve on the inner wing pulsing when engine up to operating temp.

You can feel it and hear it normally.

It may be stuck open and allowing petrol fumes into engine when it should not, thus causing feeling of stalling.

Fumes should only be allowed to be sucked into engine when certain parameters exist, and the valve should then open to allow this.

Thanks, is that the little plastic thing with "MOT" written on it and some switch to slide? So if i put my ear to it when the car is hot i should hear something? And should i experiment sliding the switch?

I forgot to mention there is a smell of fuel after hard acceleration/driving over 60mph/ or up hills. There is never any fuel smell when parked and engine off or idling so it's not a leak. Does this change anything. It's odd as on the two MOTS it's always passed the emissions at about 1/100th of the maximum allowed...and the mpg is very good (34mpg average).

Also the car idles perfectly at 800-1k rpm when under 70 degrees warming up.
 
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d215yq

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Air leak would be an obvious thing to check. Any sign of any oil around in the pipes? Also, if you swap MAF and it works better, check for leakage before the MAF which might be damaging the MAF.

Thanks, there was a slight bit of oil in the butterfly valve when i cleaned it a year ago. nothing too much though. I've inspected the pipes as best as i can and can't see any obvious leaks. Not sure how to verify there is no leak though other than visually?
 

kth286

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Thanks, is that the little plastic thing with "MOT" written on it and some switch to slide? So if i put my ear to it when the car is hot i should hear something? And should i experiment sliding the switch?

I forgot to mention there is a smell of fuel after hard acceleration/driving over 60mph/ or up hills. There is never any fuel smell when parked and engine off or idling so it's not a leak. Does this change anything. It's odd as on the two MOTS it's always passed the emissions at about 1/100th of the maximum allowed...and the mpg is very good (34mpg average).

Also the car idles perfectly at 800-1k rpm when under 70 degrees warming up.

When engine is up to temp you can hear it just by standing by side of car (bonnet up).

Put hand on it and you should feel a very strong pulse at approx. 2 beats per second.
 

BachelorDays

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Also the car idles perfectly at 800-1k rpm when under 70 degrees warming up.

In addition to the MOT valve check (why not just swap this with the other car?), swap the MAFs and see if that helps. The other thing you could try is running a fuel cleaner in the tank - maybe there's a slight back pressure which causes it to become rough when warm and close to idle.

Why would there be smell of petrol after hard acceleration or hard load unless the car was running rich at that time - again, fuel mixture or a back pressure, although a blocked MOT valve could be the cause of the vapour escaping too.
 
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d215yq

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UPDATE

The MOT valve is silent and cannot be felt to be pulsing one bit when the engine is idling and up to temp. Tried it 3 times after 20 mile journeys at 90 degrees temp.

Does this mean it's definitely dead or does it just pulse under some circumstances?

I'll swap them later next week, it is just unpluggung the two vacuum links and a wire i assume (that's how it looks!)?

Thanks again
 

kth286

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The MOT valve is silent and cannot be felt to be pulsing one bit when the engine is idling and up to temp. Tried it 3 times after 20 mile journeys at 90 degrees temp.

Does this mean it's definitely dead or does it just pulse under some circumstances?

I'll swap them later next week, it is just unpluggung the two vacuum links and a wire i assume (that's how it looks!)?

Thanks again

Yes - you need to change it, or try and unclog it with some cleaner.

Just two pipes and the electrical plug.
 

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Hi
Any chance of putting up a picture of this 'MOT' valve. Cheers.
 
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d215yq

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IT'S NEVER SO SIMPLE!

Well I changed the valves over in 5 mins...and the car seemed to run better, not perfect, but a little smoother and better. So after a 15mile drive I pull over and check the new valve and it's still not making any noise at all and no pulse can be felt, and the car still has the same symptoms (allbeit perhaps as a placebo i thought they were less pronounced).

So i'm guessing it's a wiring/sensor/ecu problem and I think that goes above my ability to repair...

So before I give up, is there anything simple that could be causing the car not to want to pulse the MOT valve that i can swap with the other car... and if not, is it better to leave the pipes "open" or "closed" given there's no way of switching between the states.

Finally, is this problem likely to be the cause of the fuel smell under acceleration, hesitation under acceleration from idle, lumpy idle, and very occasional stalling, or is it still worth switching everything on the air intake in front of the butterfly valve (this looks like an easy swap...but i don't want to do it unesecarily as i've heard the wiring to the maf doesn't like to be touched on this model)...

What do you think?
 
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d215yq

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Another thing, I just put the valve back on the donor car and at 82 degrees c (couldn't get it hotter as i'm not insured to drive it and that's all it shows after idling for 30 mins) there was also no sign of a pulse at all, just like my one. However, the car still idled/pulled off perfectly without pulsing the valve.

One thing, on both cars i can hear a ticking when the bonnet is opened, it's just the ticking is coming from the manifold/throttle body, NOT from the MOT valve itself. I have understood correctly it should be the MOT valve ticking and that you can feel to be pulsing?
 
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d215yq

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Hi
Is there another name for this MOT valve ?

Not sure but here's the picture, it is located on the passenger side wing next to the ABS unit.
 

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mattkh

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Not sure but here's the picture, it is located on the passenger side wing next to the ABS unit.
Hi
Thanks for the picture.
It is a purge valve. From time to time, it releases any pressure in the vent pipes of the fuel system into the inlet manifold.
I dont think it is related to your idle problem.
 

kth286

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Hi
Thanks for the picture.
It is a purge valve. From time to time, it releases any pressure in the vent pipes of the fuel system into the inlet manifold.
I dont think it is related to your idle problem.

They normally go faulty in the open position.

You don't think therefore, as the petrol tank fumes will now be continually sucked into engine (at times when they should not be) that there will be nil effect on engine idle/performance ?
 

Alex Crow

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One major point of note regarding the purge valve, as has been said above, it has the legend 'MOT' on top - this stands for motor, and shows which pipe should go to the engine.

If connected wrongly, you risk sucking the petrol tank flat...think on.
 

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Has the air flow plate been tampered with
 

mattkh

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Hi
One easy way to check the purge valve IF it is stuck in the OPEN position:

Remove it and blow through it, and check if any air is coming through at the other end.
 
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Alex Crow

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Back to post 1, do check the throttle return springs, one breaks and stops the idle microswitch being pressed - very easy to check.
 

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