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W245 B1700 petrol 2005/6 cylinder misfire

2931 Views 22 Replies 4 Participants Last post by  Bnut
Hello folks,

This is w245 B1700 petrol 2005/6 with no history, basic runaround.

Cylinder four has a miss determined by pulling HT, removed plug - gap looks huge at 1.1mm although OEM plugs are set to 0,9m which I assume is just under 1mm, not 100% sure as separated by a comma, perhaps denoting something different?

If someone could confirm correct electrode gap that would be great, ordered a set of Bosch which should be here tomorrow.

The coil packs which feed 3/4 has two coils, are these independent of each other, i.e could one coil fail (out of the two) leaving the other working fine?

Complete newbie on this stuff as owned a S124 dinosaur for an age

Thanks for any pointers.
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Plugs arrived early, popped those in (real job getting caps off) no change - taped cylinder 4 plug to engine and started, plenty of spark so looks like coil pack okay.

The catalytic convertor is starting to whiff, possibly due to unburnt fuel making its way back ?
It looks as though fuel is being burnt inside the cat which is getting super hot.

I may have answered my own question regarding coils, it looks like each plug has one coil each independent of the other, this may explain cylinder 4 miss while cylinder 3 is still okay, it would also explain unburnt fuel inside cat - ordered replacement along with code reader.
Auto part Engine Muffler Carburetor Automotive engine part

Checking the throttle body butterfly for operation, it opens under throttle (which is weak) but not a huge amount when revving.

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The box in the first picture is a close up of coil pack feeding cylinders 3 and 4, the diagram etched into the surface illustrates HT lead configuration (as below pic)

This I believe is what is happening to mine, the rubber exhaust hangers are burning.


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It tested my little grey cells too, last time I touched a petrol car they had mechanical petrol injection :ROFLMAO:

That said, once you get over the unusual layout and start to identify each part not much has changed. It still uses spark plugs, coils (four of them) along with a set of pistons housed in a water jacket.

I did find it difficult to rev the engine whilst tinkering due to no physical linkage, had to get my neighbour out to help with that one.

DEVOURS, I purchased a set of Bosch mention in your post, these have 0,9mm which is around your measurement of 0.38"

MB David, there's a good overview of cats getting (very) hot due to engine issues, hoping mine is an easy fix with a new coil.

Catalytic Converter Glowing Red Hot

Will keep you guys posted.
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The grey cells were wiring again, decided to check part numbers on both coils - if same swap over which I did, no change in fact all cylinders are pretty much the same, just very low power, disconnecting each cylinder in turn doesn't make much difference.

It really smells of petrol fumes from over fuelling, my attention turned to the variable intake which has a vac operated switch to one side - mine has a split boot but also noting attached, i wondered if some kind member could check there's to see if anything (or it's blocked off) is connected?

If the intake is closed shut could explain issues as mentioned.

This video at 4.21 describes the part but cannot hear audio to make out what is is?

4.21 vacuum part




Courtesy of doit vehicles
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Okay, scrap the request above as vacuum tested the bridge circuit which appears to be working fine.

One thing I did come across were other owners reporting the exact same faults but remain undetected by Star software, the Q & A around this is on page 146 here;

Bert Rowe's A class info

The format is a little unusual but I found the info very helpful.

Following their diagnosis I removed the camshaft sensor which made absolutely no difference at all, this could indicate it's faulty - new one ordered.

Also removed the crankcase sensor breaking off the nut head whist doing so, will attempt to extract that tomorrow. The magnetic coil head was caked in debris which came off fairly easily but ordered a replacement just the same.

Also received scanner today which picked all of the stored faults which were identical to reported above, very impressed with functionality and price, able to read live data too.

Camshaft sensor


Crank sensor

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In case you are like me, "how do these things work,..and do they wear out?"

The 'Hall' effect
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DEVOURS, thank you.

Over the years I have had a few of these go mainly wheel mounted for ASR/ABS, it crossed my mind these sensors may have a limited life due to the integral fixed magnet?

they do seem to stop working without any physical wear.
From what I have read the magnetic properties are not inexhaustible, although happy to be corrected on this. If this is the case perhaps Hall type sensors should be built into service schedule for periodic replacement.

The sheer number available on ebay suggests this to be the case.

Its also worth mentioning original part OEM numbers seem obsolete, hoping the parts ordered fit and work as per description.
That's good to hear!

Bit of a disappointing day my end, fitted both crankshaft sensor and cam sensor, reassembled after cleaning everything then erased codes.

No difference at all, still exactly the same very rich petrol mixture which you can smell from outside, only running on two pots too. Reconnected reader, no codes showing or stored?

Removing the MAP makes little to no difference, can only think it maybe bad injectors but would that not spring a code?

Edit: went out for another try with code reader, this time codes for cylinder misfire on 2 and 4 plus multiple misfires.

It needs to be a fault which allows fuel into the cat/exhaust, perhaps injectors - is it possible to swap them around without removing intake manifold?

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Managed to remove the fuel rail together with injectors.

Couple of questions, are these injectors serviceable as several guides suggest blockage may have occurred?


Could someone confirm if numer one pot is opposite end to transmission (drivers side UK)

The scan reader reports cylinder miss 2 and 4, I assume that's 2nd from left, and forth - cheers,

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I hoped by popping out the fuel rail and turning over the engine it would show any defective spray patterns, unfortunately to get the rail out virtually all of the sensors have to be unplugged rendering the engine unstartable.

The other option was to swap 2 and 4 with 1 and 3 to see if codes changed, again the whole engine has to be reassembled to get it started.

As an interim solution I have ordered four second hand injectors which should help diagnose the problem and may even last for 5-10k of miles (they are guaranteed working)..

It's not ideal but I then have a spare set of injectors which can be cleaned and tested, keep you posted.
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Well that's me stumped!

Injectors arrived today which looked in good shaped, Mercedes too - fitted and reassembled in a couple of hours, started almost first turn of the key ...and you guessed it no change.

So that's new plugs, crank and cam sensor together with four injectors, will pop out in a bit and check for new codes if any.

Identical codes, P0302 and P0304, cylinder misfire 2,3


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Changed thread title to more accurately reflect content.

Bit of an iupdate

On the ignition side I had already swapped over both coils leaving HT leads in-situ, this was done a while back along with new plugs trying easy things first. Also pulled plug 4 lead and tested with spark plug (engine running), plenty of spark, in fact almost continuous which I put down to high efficiency engine.

That in my mind ruled out lack of spark but decided (run out of things to change/buy :ROFLMAO:) to swap out HT lead 4 for 3 and clear codes. Total surprise on my part to see code had moved to 3, tried again with 1, same thing which suggests faulty or incorrect lead (even though it knocks out a fair spark having had a few jolts myself while fault finding).

It's also possible the leads maybe incorrect for the car providing too much resistance (or too little), gone are the days when you just bought four foot of HT cable and cut it to length :ROFLMAO: :ROFLMAO::ROFLMAO:

Anyhow, old leads didn't have any OEM numbers so ordered a generic set - can always change later if poor quality.

Hopefully next post will be a thumbs up!
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Thanks, fitted leads and....

...........runs like a top (y)

Thankfully I didn't buy new injectors so parts bill still well under $100 including tester.

Intermotor is a new brand for me but leads look very good quality (not sure if available US).

Cheers for the replies.

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