Help me understand this idle prob please
 

I have been stumped by a very intermittent very minor miss/stumble at idle on my 92 500SEL for months. Otherwise this car runs unbelievably great but the stumble is annoying. I am as certain as one can be that there is no mechanical issue. I run premium, usually Vpower from Shell but when I add a bottle of octane booster (I used the Lucas brand) the problem never ever occurs.

Can someone tell me what if any adjustments can be made to the vehicle that would compensate for lower octane or whatever the additive does? I have tested the additive for about 6 full tanks now and the miss/stumble will come back intermittently only if I do not add a bottle and go away completely when I add it. I have let the car idle for an hour or more and not heard a single miss/stumble with a maybe 8 ounce bottle in 20 + gallons of gas.

I do not have enough hair left to pull anymore out over this but the car is getting real expensive to run adding $8 to a 75+ dollar fill-up. Thank goodness I can get 17-21 if I watch my foot.

BTW, I have done/replaced everything one can find on the net regarding idle issues including the switch in the transmission.

Any help, suggestions, guesses, or even voodoo cures greatly appreciated

Thanks

"Could be" a slightly bad fuel injector doesn't spray correctly w/o additive.

Thanks. Appreciate it. I have not changed them out so it is a great place to start.

does your car have a knock sensor? it could potentially be bad.

I am not sure. If it is bad it has not shown as a trouble code. Thanks for the tip it gives me another specific to search on.

THanks!