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)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator

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