Normal city-style driving - ML drives great - smooth idle but with only a slight rocking when stationary, smooth acceleration - no problems.
Driving fast, then suddenly push the accelerator pedal to the floor and get the revs up to around 5000, then I get the problem.....

Problem is that cylinder #6 is fails and ML idle is quite rough - i.e. disconnect the #6 cyl spark leads gives NO change in idle.
Turning the engine off and waiting for several seconds then starting again - engine idle is back to smooth idle again.
Fixes tried: new plugs, new plug leads, new coil pack, new air-flow sensor (sensor in the air filter hose between filter housing and manifold), new crankcase sensor.

Diagnostic tests were run (did not note error codes !!!) and all referred to cyl #6 and potential damage to cat. conv.

Engine has 107,000km. I took ownership at 106,000km's.
Cat converters were removed and welded around the pipe seams when car went in to MB (Melbourne)for 100k service - it was then that service people noticed cat. conv. seams were cracked and decided to weld them.

PLEASE, PLEASE, any ideas.... could it be a faulty/blocked cat converter? or faulty control unit? something else?

Andrew....

I would start with fresh spark plugs...switching coil OR spark plug wires to a different cylinder.

Has the fuel filter been replaced?

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
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Fitted new spark plugs - Bosch FR8-DPP-33 Platinum.
Swapped coils around - eg from #6 to #3
No improvement.

Don't know if Fuel Filter has been changed (but I would guess Merc Benz workshop would have renewed Fuel Filter in normal services.
Where is Fuel Filter located?
My guess is if Fuel Filter is blocked, then problem would be in all rev ranges...??
Anything else I can consider?

i had exact same problem on a customers ML, we went through all the steps you did and more, turned out to be the crank sensor. if you replace it you need to have the system sensor gear adaptation properly reset and relearned using the factory tool. if you just replace the sensor you may still have the problem since the sensor gear adaptation is still set. some one may know if disconnecting the battery may do it, but i do not know. there is a sequence you need to follow to relearn the adaptation once it has been reset. i think it is 3rd gear under 2000 rpm for 30 seconds,

Thank you so very much for your information.

Can someone PLEASE advise me how to make the crank sensor relearn ?

Would I cause any damage or problem if I disconnected the battery for a few minutes - if this could reset the crank sensor ?

Make sure you have the code for the radio. Disconnecting the battery will activate the theft feature in the radio and you WILL need the code. Usually the dealer will provide your code on proof of ownership, which should also contain the vin number.

Jim in Phoenix

Have the radio code handy and will try disconnecting the battery.
HOWEVER, still keen to find out how else to reset the crank sensor - eg. the 2000rpm thing....

also be sure to remove the key from the ignition before disconnecting the battery or charging the battery. to do as i said, replace the current sensor, then reset the adaptation (i recommend dealer or shop with factory tool), place the transmission in third gear if it is an auto, (shift up to third if it is a manual), not exceeding 2000 rpm, and drive that way for 30 seconds, that will relearn the adaptation. do not try with the old sensor, as it will likely fail

You guys are just simply GREAT !!!! Thank you.
Please let me know if you come up with other suggestions - I would be much obliged - here in down under Melbourne, Australia.....

Sorry.... just not sure here....
Can I reset the already fitted crank sensor or should I only do the reset whith a new one?

new one only, the old one is defective.

Finally had the problem cleared up ----
I was told by the workshop that the computer (control module ???) was fouled up with error codes that could not ordinarily be cleared up.

Mechanic dit some 10 passes of clearing out the control unit's error codes.

And that..... ended this wreched saga.


To all you guys wo responded to this post - THANK YOU. YOU OVERWHELMED ME.

andrew.