C63 Random Misfire I'm at a loose end please help

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Sep 28, 2019
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Hi all, first off I'd like to say yes I've searched, and this forum has provided lots of useful info in the past, but can't find the answer to my specific problem this time, and secondly I WILL PROMISE to update you all when I finally find the fault that is bugging the life out of me so that it helps anyone else with the same issue in future....

Now My Problem, please help if you can:- The car is a 2008 C63, and it has been running fine up until a few weeks ago when it seemed to temporarily misfire and lose power sounding like it was only on 7 cylinders. After a damn good thrashing it seemed to come back to life, and all was good for a week or so, then the same thing happened again, but this time it wouldn't go away.

I started by replacing the spark plugs, but when I took the airboxes off, the air filters were absolutely blocked up full of crud, so I replaced them aswell thinking all will be fine, and this will go like stink now....

Much to my dissapointment, when I drove it, it still had the misfire, and sounded off, but worse still, when I floored it, it started misfiring worse than ever, and then generated the check engine light, and shut off fuel injection to about 3 cylinders in order to protect itself.

I got back and put an icarsoft MBII scanner on it, and it showed the following codes:-
'Random Misfire'
'Misfire Cylinder 5 Detected'
'Misfire Cylinder 7 Detected'
'Misfire Cylinder 8 Detected'
Please note, all of these misfires are on bank 2 only (the left hand bank).

Since the fault only occured on the left hand bank, I decided to swap everything (in stages) i.e. Fuel injectors, coil packs, etc from left to right, to see if the fault moves, but the fault is STILL on the left hand bank.

Below is a list of things I have tried so far, and I am at a complete loss, so if you can come up with any help, it would be greatly appreciated:-
1) New air filters
2) New spark plugs (NGK ILZKAR7A10)
3) Cylinder comperssion test (Cylinders 6 and 1 190PSI, the rest all 230PSI)
4) Fuel Injectors swapped from between bank 1 and bank 2
5) Coil packs swapped from between bank 1 and bank 2
6) Ran car with Mass Air Meters disconnected, made no difference
7) Checked live data from Mass Air Meters, Manifold Absolute Pressure Sensor, Intake Air Temperature Sensor, Fuel Rail Pressure Sensor, 02 Sensors, and all OK.
8) Full strip down, inspection, and rebuild of inlet manifold, no leaks or cracks found, and variable intake length mechanism working OK. All reassembled with new gaskets and bolts.

To add further info, I have ran the car up the road while monitoring the fuel trims, and under partial load, while bank 1 (drivers side in the UK) short term fuel trims are going circa 7 positive (adding fuel), bank 2 (faulty bank) short term fuel trims are going circa -15 negative. This is resulting in a lean condition in bank 2, and subsequent misfire, buy WHY?

Mods are exhaust only:- Secondary cat delete replaced by crossover pipe, and resonator delete, and the car has covered at least 3000 miles without a problem since this..

Thanks in advance for your help.
Just a stab in the dark as i don't know your engine, but could it be a break in the wiring loom to the injectors. As I say I'm no expert, but you seem to have tried everything else
Hi, Thanks for the replies so far. There is no coolant loss, I have already swapped coilpacks over, and regarding the break in the injector wiring, I have listened to each injector individually using a stethascope, and I can hear them ticking away, but I haven't actually tested the wiring or signal strength, so I may have to revisit that one in order to be fully sure about it.

P.S. In the meantime keep the ideas coming in, cheers.
Right so you're getting lean running in one bank. Therefore you either have too little fuel or too much air entering that bank.

You've swapped injectors over so it likely isn't that.

You've checked for intake leaks and it isn't that.

The only thing I can think of then is that your lambda sensors are reading out and the ECU is trying to compensate for that. An exhaust leak could possibly be a cause too?
Bellow, are you suggesting the left primary cat may be blocked? You could be correct, I have not checked that yet, thanks....
Pillow, I had thought of the o2 sensors myself, but discounted it because the car should run in open loop at Wide Open Throttle (WOT), so that might have been the case at part load, but not at WOT.

I'm going to have another look at the car tomorrow afternoon, taking on board some of your ideas above. I may even swap the o2 sensors from left to right to fully prove the theory. I will report back with my findings, cheers.
Bellow, are you suggesting......

Below is where I was heading - differing signals from two distinct Lambda sensors and a swap over to confirm/disprove.

I'm going to have another look at the car tomorrow afternoon, taking on board some of your ideas above. I may even swap the o2 sensors from left to right to fully prove the theory. I will report back with my findings, cheers.

Open/closed loop - can only closed loop alter the fuel trim - which you are seeing?
Bellow, The system was running in closed loop at partial load while I was monitoring the fuel trims. I monitored the current signals from both wideband primary o2 sensors today, and both banks were very close to each other, the min value I saw was -0.14mA, and the max value when I closed the throttle at about 4000RPM was +2.60mA. I don't know a great deal about wideband o2 sensors, but since both sides looked very close, it gave me no clues what so ever.......

I have had another look at it tonight, and I swapped the priamry o2 sensors from one side to the other, then I reset the ecu to clear all historic fuel trim data (there were no fault codes, as it only produces them when I floor it and above about 4000revs, so I have been driving gently lately). I ran it up the road and put my foot down hard to see if the fault moved to the other bank, and it was exactly the same (check engine light came on with random misfire, misfire cyl 7, misfire cyl 8 codes), so left hand bank still faulty????

I didn't get chance to inspect the cats, but I will do that next unless anyone can come up with any better ideas? Could it be cam timing on left hand side? although surely there would be fault codes??????

Will keep you posted with my findings, cheers.
I don't know a great deal about wideband o2 sensors, .

Me neither - but I have a sneaking suspicion that it is Voltage that is the variable with them (not current).
Finally found the problem, at last, thanks to everyone for your help. The problem was, I took the exhaust off today to inspect the cats (they were aftermarket sport cats with a metallic catalyst), and the left cat had completely collapsed and gone down the exhaust pipe to a point where it blocked it up solid....... I'm surprised the car was even running when I saw how restricted it was!!!!!!

For now I have just removed the bits of broken catalyst (blockage) from the left pipe, and removed the cat from the right pipe while I was on (even though it looked OK) just in case it decided to do the same trick. So now I'm on the look out for a reasonably priced pair of primary cats. Thanks again everyone for your help, and hope this will help someone in future.

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