Replace the spark plugs. If the check engine light continues replace the coils on 6 & 7 or swap the coils to other cylinders to see if the misfire follows the coil.

Fuel of different octanes have different ignition characteristics. My guess is that 6 & 7 have weak/marginal ignition that just squeak by on the burn characteristic of premium/higher octane. Run the proper octane from a top-tier brand in the car.

No I don't think it is injector related although 14,000 miles of regular has not been keeping them as clean as premium.

Good luck!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Yes - I finally got to doing this test. Switched the coil packs between #5 and #7.

Magic - the CEL reported that the misfires (from P0307 to P0305) moved from #7 to #5.

I think we have the root cause.

Thanks so much for your patience.

Comment: I have changed the spark plugs and the wires, but missed changing the coil packs.

So - replacing the coil packs now is fair.

Suggestion to All Reading this. Changing the coil packs when you change the plugs (I was penny wise pound foolish)

This is a M113.940 engine, Model: E430 Year: 2002

__________________
Thanks for the help
Bill Fisher

'86 560SL (186K) - Now a 'classic' : Registered as an Historic Vehicle
02 E430 in the stable
'14 LS460 (Lexus)
- - - - -
'95 E420 (198K) found a new home
'99 E320 (80K) (gave it's life for me as we hit a bumper)
'95 E420 (231K) Sold to a happy buyer, new to Mercedes
'90 300E (65K) Sold to an Mercedes Lover
'92 190E (215K) - retired to the salvage yard
'93 500SEL (214K) - Moved to another family, still runs like a young pup

I never understand the logic of running regular in the m113 engine and other modern engines that call for premium. Of course your car runs fine on regular. You have knock sensors that adjust the timing, but your fuel economy goes down during the process.......and there goes the few bucks that you saved at the pump.

I seem to get 29 MPG either way.

Thanks for the suggestion. I am going forward with premium from now on.

__________________
Thanks for the help
Bill Fisher

'86 560SL (186K) - Now a 'classic' : Registered as an Historic Vehicle
02 E430 in the stable
'14 LS460 (Lexus)
- - - - -
'95 E420 (198K) found a new home
'99 E320 (80K) (gave it's life for me as we hit a bumper)
'95 E420 (231K) Sold to a happy buyer, new to Mercedes
'90 300E (65K) Sold to an Mercedes Lover
'92 190E (215K) - retired to the salvage yard
'93 500SEL (214K) - Moved to another family, still runs like a young pup