Idle speed varying on ML 320 cdi

Smart320

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Have just acquired an 2007 ML 320 cdi which is in very good condition with full MB service history , only 42 k miles and serviced only 500 mls ago.
The idle speed ( about 900 ) has been checked after I noted before collection that I felt it was too high ( was OK according to the garage who sold it to me )
Have done about 300 miles in it and it still idles at about 800 to 900 revs generally but today was at 1100 when I stopped after driving 60 miles . On restart a couple of minutes later it was down to 850. Also when using a full throttle there seems to be a bit of a vibration through the throttle pedal. Engine seems to be pulling well and is definitely not misfiring and is very smooth otherwise. Getting about 31 mpg on motorway at 65/70 mph , 32 mpg cross country on A roads at 45 mph ave.
As it is going back next week to have Glow plug / or g/p controller changed as light stays on for 1 minute after starting and a non working Parktronic fixed there is another problem I would like to get it sorted at the same time.
Maybe I would be best to get it on Star at an Indi before going to the garage
Any thoughts?
 
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Smart320

Smart320

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Does not have a DPF, thank goodness
 

mersum1es

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Have you checked how much alternator is charging, does voltage varies when idle goes upndown? Freewheel pulley of alternator ok? Have you tried to shut down AC, does it affect?

Usually vibrating idle at cold engine is because of changes in fuel pressure, can you monitor fuel pressure reading now (some torque OBD versions may do that)? At least ask your indie to do that. And check the condition of injectors.
 
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Smart320

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Problem is not at cold idle 1100, but once warming up drops normally to 900 , hot usually 800 but often 900, and was at 1100 yesterday until stopped and restarted engine, then 800. Will check effect of switching aircon on / off but from memory revs drop a little as compressor starts. Looks like a visit to the Indi to get them to check on Star.
 

kid-jensen

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Check if you can get the idle speed down by pulling the accelerator pedal up.

If it reduces (to say 600-700 where mine is) then the accelerator "fly-by-wire" sender is probably acting-up...

Obviously, it could also be something more random, like a piece of trim or carpet affecting accelerator travel....
 
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Smart320

Smart320

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Will give it a try later today, carpet and mats not interfering with pedal
 
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Smart320

Smart320

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Tried it and idle reduced from about 900 to 700 , revved it up and it then settled at 700 again. After a short drive was back to 850. Given the variability of idle speed I will ask that the seller changes the electronic throttle sender, I gather it is not expensive and is a quick change
 
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Smart320

Smart320

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Garage is taking a look, they think Throttle body probably needs cleaning but say they will check fault codes before doing anything. They have already fixed the Parktronic by fitting a new sensor, working perfectly now. They replaced all 6 glow plugs as had problem with glow plug light staying on, found 2 were u/s. Said better to have all new as others likely to fail, no charge at all! They have now ordered a Glow Plug Control unit as Glow Plug light back on again after starting, ( starts more smoothly now though ). I am absolutely delighted with the response by this second hand car dealer to my problems, no hassle, anxious to make sure the car is 100% free of problems. Would buy from them again. ( Timhaywardcarsales.co.uk , located in Cardiff and specialise in used Mercedes, Bmw, Porsche ,Volvo , I found them by chance and based on my experience have been very lucky to have found a dealer who is honest and a bit of a 'petrolhead'
 

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