Alright, I'm finally throwing my hands in the air and asking for ANY help or clues as to what to chase next. I've googled for hours and hours on end.

I'll try to keep it as brief as I can. Going to give complete history just in case anything helps.

Bought a used 2007 ML63 with 80k miles. It was mostly ok, but did get an occasional misfire code. Seemed to be when raining out. Didn't think much of it, and certainly could be totally unrelated. I did have an oil leak, though, and seemed to be fairly substantial. I also noticed the dipstick kept popping up when driving. I replaced the crankcase vent valve, but the old one, when I took it out, looked like new.

I decided, since I had planned on inspecting the head bolts anyway, I could just do that sooner rather than later and be able to take a closer look at things, replace valve cover gaskets, etc.

Head bolts were original, so I ordered up new bolts and, since I'm there, new lifters and plugs. And, of course, the timing tool set. Replacing the bolts seemed to go well enough. I had a couple of the lousy coolant return connectors break, so that was a PITA, and I had the plastic vacuum line that runs along the driver's side valve cover break, so I patched that with a rubber hose. I did something really stupid and followed a previous timing mark someone put on the pulley, which turned out to be 10 degrees after TDC. So when I put it all back together and ran it the first time, it was not happy. Took it apart again and realized my stupid mistake. Fixed the timing. By now, I realized the battery had had it. I would leave the car for a while (was working on it in my spare time) and come back to a dead battery. So a new battery went in at this time as well. Back together, still didn't run right. Codes I pulled were P0019, P0040, and misfires on cylinders 5, 6, 7, 8.

So, O2 signals swapped. Huh? I never touched the O2 sensors. Then I have the crankshaft sensor and cam bank 2 sensor b not agreeing with each other. I have rechecked timing TWICE since, and it checks out. I have now replaced ALL FOUR cam sensors. I have cleaned all the connectors. I even took apart the driver's side again just to recheck timing but also to make sure I didn't have any wires pinched. I was never fully happy with how the exhaust cam sensor plug was seating. Seemed hard to push in. Noticed today that a piece of the orange seal was torn and getting in the way, so I fixed that and now it clips in nicely. Today, however, a new code appeared. P0410, the secondary air pump. Huh. What now? And why did this not come up before?

Does the combination of P0019, P0040, and P0410 mean anything to anybody?? What the heck did I do wrong that could cause these? A vacuum leak? But I don't think that makes sense for the sensors. Seems like an electrical issue. But now that air pump. That could obviously be a vacuum issue. I'm not even sure what the broken vacuum line I patched is for. I keep thinking it is certainly possible that replacing the battery resulted in blowing a fuse. But what fuse would throw those codes?

I hope I remembered everything, and hope some knowledgable folks have read this far.

 

Retrace your steps carefully and check your work.

If a vehicle of this advanced age was mine I would throw parts at it indiscriminately. Crank sensor, O2 sensors, plugs, coils, coil boots, the whole enchilada

You didn’t talk about the fuel system. Tell us more about the fuel system. Did you open, move or change any fuel related parts?

How does the engine idle? How does it sound at higher rpm while in park?

Misfire on a few cylinders suggests a local issue such as spark or fuel, or a sensor that manages spark or fuel.

Does the M156 suffer from oil wicking into the wiring harness? It emanates from the position sensors and cam magnets and can cause misfire and CEL. This affects later engines M27*.

 

I have not touched the fuel system.

The fact that it is the entire driver's side bank of cylinders should eliminate certain issues. I mean, if anything really was wrong with the O2 sensors, that would cause both sides to misfire. Same with the fuel system. There is only 1 crank sensor on this engine, right? Or is there actually a left and a right?

MB always has crazy vacuum systems that befuddle me. So we have this secondary air pump that feeds what looks like a left and right ... I don't know ... solenoid?? valve?? Maybe that is bad on just the driver's side? What symptoms would that produce?

 

One crank sensor. Secondary air pumps air into the exhaust to stoke the catalyst and burn off hydrocarbons to meet emissions regulations. I don't see a failed secondary air pump causing misfire. CEL maybe but not misfire.

Spark-fuel-air. Something is not right, or the vehicle thinks it's not right, which might point to a sensor. Time for a compression test on the driver's side?

 

I'm no expert on the M156 (learning fast!) but is running these engines at 10 degrees out enough to touch the valves on the pistons???. As said compression check first.