ML320 tricky issue

[curlyearly]

Hi all,
Have a recurring problem on my ML320. Just turned 50K miles. Always serviced. 50/50 main dealer / local garage.
　
Symptoms...
　
At tick over there’s a slight hesitation, a bit like a loose plug lead but more subtle. Its more noticeable in "D" at tick over than when I put it in "N".
　
Sometimes, say 1 in 20, when I pull off its like a plug/s is not firing. Certainly the car will struggle to pull away, round-a-bouts and pulling out of a junction are an issue. Then, after, 30 seconds is clears itself. I can then drive 2-300 hundred miles without a glitch. But. Pull over, leave for fifteen minutes and restart and the 30 second hesitation is back.
　
The car has been to three garages (my local garage who I have used for years, a MB specialist garage and MB them selves). Items changed, in some cases by all as they like to use their preferred brand lines are; plugs, plug leads, lambda sensor, crank sensor, air sensor. I’ve spent hundred of pounds without success.
　
Now before I list some codes thrown up by a hand held computer I have I want to add the car runs on LPG (gas). The conversion was done three years (30K miles) ago with good success. When I run the car on gas the symptoms are exaggerated but I am old its harder for an engine to run on gas so they would be. Indeed the main MB dealer disconnected the gas system in full without success and agreed "it probably was not that".
　
OK. The hand held computer I have is a "Memoscan". The four recurring codes I have are...
　
PO135. Pd 02/04. Generic 02 sensor circuit malfunction (bank 1 sensor 1).
PO155. Pd 03/04. Generic 02 sensor heater circuit malfunction (bank 2 sensor 1)
PO131. PD 04/04. Generic 02 sensor circuit low volts (bank 1 sensor 1)
PO306. Pd 01/04. Generic cylinder 6 misfire detected.
　
Out of interest, with the bonnet open and looking into the engine bay. Which is cylinder 6?
　
I’m a bit lost to be honest. Three garages are lost too. The car can go for a couple of months running great (petrol or gas) then play up. Perhaps a clue, or red herring, it always seems more troublesome in damp weather?
　
I welcome your thoughts, advice and hopefully solution.
　
Cheers all.
Earl

 

[SJX]

I would suspect either low tension or high tension sides of one of the coil packs. Either a break down in the coil insulation or the wiring to or from it (HT lead). Indicators from your post are that it isnt the plugs or leads so more likley the coil itself or the low tension multiconnection plug/wiring to it. Possibly No 6 Cylinder, You will need to get hold of one of these (used is a good start) and play by substitution. Most will all call out - get it read on Star first... but sounds as if you have spent enough already ! CU1147 MERCEDES ML320 3.2 Ignition Coil Pack | eBay

Info robbed from another post:

ML320
The firing order is 1-4-3-6-2-5.
#1 is on Pasenger side Front
#2 is on Pasenger side Center
#3 is on Pasenger side Rear
#4 is on Driver side Front
#5 is on Driver side Center
#6 is on Driver side Rear

Good luck me owd, from sunny Bucknall !

Steve