I have a 2004 W211 E320 and few weeks ago my engine started misfiring. I changed my spark plugs i know its not ignition coils they've been replaced too. My only guess is the cats. Missfire occurs when i go 3500rpms and up the engine starts making weird noises and eventually shuts off after like 2 mins. I changed my primary O2 sensors too. Any help would be appreciated. The car starts shaking too and the engine won't accept any throttle.

Comments

It could be the cats but first I would check the fault codes. Have you done that yet? 

What are the codes? 

 

Bad cats would cause the symptoms you describe but the car shouldn't shut off. It may still be a bad coil. How did you eliminate that possibility. It also can be a dirty MAF. 

I have a P0300 code for random missfire and p0301,2,3,6, and 5 which indicates which cylinder is misfiring and p0432 which is cats below threshold.
There is p0410 it says It's secondary air injection and i have no idea what that one means or where is it located
I went to autozone just now hoping that they're gonna give me the reason why is my engine misfiring but it said no solution. I have no idea what to do next and my car shut off only once btw as i was pulling over to shut it off and turn it back on because then the engine comes back to normal. I appreciate you trying to help me. What do you think is it cats should i change my O2 sensors first?

You have misfire codes for both cats. Which makes me think it is not the cats. Very unlikely they both have gone bad at the same time. What you can do is unplug the upstream o2 sensor and see if car drives better. Or take the car to a muffler shop to do a backpressure test. If you have a Exhaust Back Pressure Test  gauge you can perform the test yourself. Remove upstream O2 sensor and plug the test kit. That will allow you to tell if the cats are clogged. 

My guess would be a bad mass air flow sensor or maybe a vacuum issue.

 

Btw, the M112 is very solid engine. So don't give up yet.  

Thank you very much and im most definitely not giving up it's my baby that keeps braking all the time ??

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