I have a 2012 S 550 in my stall at work,customer's car and she is complaining of a miss under a load.I scanned for codes,showing a PO302,PO304 for codes.I did test drive too and did it how she explained it.This customer was with me when I test drove it.I did the simple checks,compression test on these two cylinders and that is good.Brand new spark plugs and they are dry.Coils are good,lots of spark out of them.It is driving me nuts.This customer is a great customer,pays her bill on the spot.I am thinking it is still spark plugs.Her brother installed the gimmick E3s in it and I know Mercedes Benz uses Bosch spark plugs for OEM spark plugs.

 

You should really put back the stock plugs.

Said that- if not the plugs check coils. I know- plenty of spark at idle, but once loaded up under acceleration they may falter. Last is wires. Check for higher than normal resistance.

Check her air filters too... And if all else fails- inspect fuel filter

 

I am going to try plugs first.I did an ohms test on the #2 and #4 coils,a little out of specs on this.Boss ordered the Bosch plugs for it which will be in tomorrow morning including a couple coils.I replaced the air filter last month when this customer brought it in for an oil change.Luckily this customer is not cheap at all,fix it if there is anything else wrong with it no questions asked.Sells houses and break downs with this car costs her money.I noticed an oil leak at the same time finding it and this is getting fixed also.

 

Runs good now,replaced the #2 and #4 coils,wires and spark plugs.I did find a good tear in the #2 and #4 plug wires also.Fixed the oil leak too.Customer is one happy lady,figured out my favorite brand of whiskey is Jim Beam.Gave me a bottle of Jim Beam Devil's Cut as a thank you gift,I put it in the trunk of my car right away now at home.

 

Thanks for the follow-up report. Reporting how these issues are closed out is helpful to the community