W163 ml320 rough after service?

daveb76

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My ML320 Failed its MOT on lambda by 0.03. I had no proof that the previous owner had done anything more than a oil change recently so decided that a set of plugs and an air filter my just help it tip over into the pass.
I have changed the plugs ( very worn!) with Bosch ones, whilts removing the leads i found that number one lead had not been replaced correctly in the past and the connection to the coil was bent over and very corroded, i tried to straighten and clean but it snaped of as soon as i straightened the connection. i have fitted the plugs and exhanged the the damaged lead with one i had in the garage off a vw vr6 - the rating reading on the vw lead is 5kohms and the merc 2kohms-. i also changed the air filter at the same time.
When i restarted the car it cut out within seconds, when i resarted i noticed the air pump in the centre of the manifold was running flat out, i restarted a few times until eventualy the car now idles ok but is lacking on power and was very hard / imposible to rev. i disconnected the maf and tried to give it a rev but made little or no difference. After leaving to idle for a while the car is now driving ok but lacks power. i have only driven it round the block a couple of times.

I have an odb fault code reader that i pluged in and no faults recorded.

Do you know if the ecu could have changed / addapted its values to compensate for the very worn plugs and faulty lead and just needs a good drive to re learn the values ( im a ex VAG tech and this could be true on VAG stuff and also I know Vauxhalls do the same) or is it something else.

Whilst speaking to merc about ordering a replacement lead they have told me that there is more than one type of lead on my car, but couldnt tell me what the differnces are - i am assuming its just the lenght that vary- so couldnt have got them muddled up?

Thanks in advance
 

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In general on MBs the O2 sensor will put the engine lamp on if the O2 sensor shifts too far from its stored range,, The maf can cause this to happen,,when the O2 sensor try to adjust for better fueling.

If the air pump is running then it is too rich, The MAF may have failed rather than just be out of spec,and this is usually the case when it runs the same with it as it does unplugged
 

Alex Crow

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yes, the leads are just different lengths. the rocker cover says which length for each plug, it has letters for reference, eg k,g etc.

i doubt that the worn/new plugs swap has caused your problem, more like one or more of your 12x ht leads - certainly that vw one needs changing. but with this you will get p03** codes eventually, and a definate juddering miss-fire as the cylinder shut down kicks in.
 
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daveb76

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ok thanks, think il try a set of leads 1st then, only put the vw one on to move out the garage, do the coil packs generate fault codes if they become faulty?
 

Alex Crow

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in my experience (it's my day job), the coil packs are fairly solid. some change the coils and leads at the same time and conclude it was a coil pack fault, but it is more likely to be leads 10/1.

you may not have miss-firing though, it is usually fairly distinct when it shuts down a cylinder, i am sure you would realise.

have you by any change disconnected the battery? it could be a failing maf sensor that had its adaptation erased by powering down the ecu! ie a maf fault.
 
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daveb76

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hi yes i had the battery off as i did a lower ball joint so removed the hub with abs sensor - to avoid putting a fault on the abs! so you think it could be a maf sensor rather than the leads? i have just been out to the car and although starts and idles fine will not rev, i can just about get it through the rev range, if i leave it to idle and get warm this improves. i have checked everything in the vacinty of the plugs and air box etc - no splits in pipes. wires off or damaged etc.

the idle is pretty smooth and whilst trying to rev is not really fealing like a missfire but more like a fuel starvation fealing. i guess the missfire would be apparent on idle.

my first thoughts were that by giving it a service and changing the damaged lead it may have adapted its self to run as best poss over time and now it would need to re-learn, but going back to it tonight is back to how it was when i first reconnected the batt and fired up.!???
 

MB TECH WARRINGTON

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it does sound air mass related, cleaning very rarely fixes completely

removing abs sensors is fine with battery connected,the abs light will not come on,
 

Alex Crow

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removing abs sensors is fine with battery connected,the abs light will not come on,

good advice, if only we could back date it ;)

i am sure then that THE MOST PROBABLE cause based on the information IN THIS THREAD is the air mass sensor. did anyone notice the caveat in the previous sentence?? :D

as i said, the ht lead/coil pack failing scenario will very quickly lead to an obvious miss-fire. you have worked with engine management before dave, so i hope you understand the cylinder shut down facility of the ecu. it is based on miss-firing counts, and used in order to protect catalytic convertors. expect p03** codes to appear if so.

the more likely scenario however is that the ecu's self adaptation at idle and part load etc has been erased by the power down. i have known just this happening on an ml320 after changing the srs module (das stipulates dissconnecting the battery). in time and use the maf sensor goes out of range slowly and the self adaptation corrects as it goes. when you wipe adaptation values back to default mid settings it barely accepts any throttle.

go for a few test runs and get back with the fault codes it will log, eg p0170 p0174 p0150 etc etc.
 
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daveb76

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ok just been back out to it, started perfectly, would not rev - has def got a missfire when reved - managed to clear the missfire whilst in park but still not fealing 100%. pluged fault code reader in and got P0136 o2 sensor circuit malfunction bank 1 sensor 2. and P0141 02 sensor Heater circuit malfunction bank 1 sensor 2.

cleared faults and gave it a run round the block, lack of power due to missfire, pluged fcr back in and no faults found.

I am going to put a set of leads on tomorrow and go from there as i really need to change that red vw one! I am thinking that the sensor may have had so much unburnt fuel over it whilst i was trying to clear the missfire is thrown a fault up. so fingers crossed.
bit of a strange one as really didnt feal like a missfire earlier, i guess it could have been a bit of both, dosent help that ive been looking after our 6 week old tonight whilst trying to pop out to the car! that was the 1st proper drive of it ive just had since the problem.
thnaks for your advise alexander and post back tomorrow when new leads are on.
 

Alex Crow

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you will not get excessive unburnt fuel through due to ignition miss-firing. if it is miss-firing it will shut down cylinders. it sounds more like very poor overall running - weak rather than rich which will give your symptoms, not your codes though.

the chances are that your ecu will have had stored codes before you disconnected the battery, all gone now.

btw those codes you do have are for the DOWNSTREAM sensor after the cat, not the upstream that you may be mistaking it for.

i would suggest you do not erase any more codes. for some faults to be logges as codes your ecu needs to see them on more than one driving cycle, erasing stored codes will also erase pending codes and events.

if you try disconnecting the maf sensor and running without it may restore performance at higher throttle openings, if so suspect the maf.

i hope those leads help, but you would have had p03**codes from that test run with an ignition miss-fire.
 
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daveb76

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All sorted and now running better and more responsive than it ever has since ive owned it!

Changed the leads as one was def u/s, this made no difference, i got an maf at the same time based on alexanders comments.

changed the maf and it has transformed the car. so pretty sure this will also help the with the lambda reading it failed the mot on.

Big thanks to alexander for you help and knowledge.
 

Alex Crow

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aww shucks dave, me making your day made my day.
 

schammer

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Hello, reading this thread and seemed like you were right people to ask so was wondering what advice you could give me for the fault codes on my ml55...CEL came on yesterday and had codes read today and got p0150 p0155 p0300 p0305 p0307...it still seems to run fine with no noticible misfire and my first instinct was MAF however reading the codes on a friends diagnostics it's indicating o2 sensors bank 2 i presume front and back, could this be a problem with the maf tripping these sensors? many thanks in advance. its done 67k now and coming up for a service however i looking back through service history i dont think the spark plugs have been replaced are they due a change also.

Sam
 

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