You should have let the tank empty out before adding the fresh gas, epecially if you aren't buying quality gas (toptiergas.com).

My MB had about 8 gallons of 'bad' gas in it (from sitting) which I put in my '00 Sierra. Truck ran like crap, hard start, stumble, etc... basically the driveability you're describing and only got slightly better when I added 10 gallons of premium (only requires 87).

This time I ran the tank almost dry before refilling, and full tank of 87 had it back to normal.

Still, it wouldn't be a bad idea to check and see if your pumps are running, how they sound & what kind of current they're drawing.

If you haven't changed the fuel filter(s) lately, that might be something else to look at.

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.