Hotspotter

I accidentally put 89 instead of 91 in my c300, but when i noticed i put 89 it was at $24 and then i switched it to 91 which ended up costing me $34. What should I do now?

fabbrisd1

Absolutely no worries - nothing bad will happen ! Rest easy !

Hotspotter

Will i need to do anything? Wont the engine light turn on? According to what i have read from the internet.

diamondsilver16

No nothing at all will happen for one partial tankful. If there does happen to be any pre-detonation (knock) due to the 89 octane, the engine management systems (anti knock sensor, ECU, etc.) will adjust the engine timing to prevent any negative effects.

I saw a guy pumping 87 into his M-B here in Cali the other day. Highly unrecommended to do regularly as the long term effects (multiple tankfuls) can damage valves and pistons due to the firing/detonation that occurs too early in the combustion cycle. The engine can only compensate so much over the long term.

Hotspotter

Alright then. I always put 91 for my c300, it was because i was driving my friends car which uses 89, so when i added mine, i put 89 by accident.

Hotspotter

I read online that it will adjust to the 89, will i need to do anything to readjust it back to 91?

dwpc

No. Your engine computer senses the combustion constantly and adjusts automatically for the octane. The only negative from occasional use of lower octane is some engine performance loss.

TYB

If you're really worried about it, just do a few partial fill-ups of premium as you use up the gas to further dilute the mid-grade you put in. Some stations sell even higher octane you could use to really mix with what's in the tank, but no need to go overboard.

netboy

sometime when i got a c300 or cla250 loaners, i put 87 gas, it saves me a few bucks.
yeah, i know some will say what about the next owner?
hey, when you buy a loaner, you kinda expected the worst.