2001 CLK320 with 135K miles ran fine with original spark plugs but over time mileage dropped from 22-23 mpg to 18-19 mpg in city driving. Since spark plugs were long over due (at 100K miles) I got them replaced with Bosch 9652 FR8DII33X Iridium spark plugs. This should have improved mileage, right?

To my horror and surprise the mileage has dropped to 17-18 mpg. Car has a new fuel filter too. Shouldn't the mileage be going up with new spark plugs? Bosch Iridium 9652 plugs are compatible with 2001 CLK320 and supposedly better than the Bosch Platinum 7422 plug (newer version of the original plug).

I am trying to figure out what could have gone wrong here. Are new spark plugs not really compatible ?

Any one experienced such scenario?

thanks

 

They are not really a mileage affecting thing. Will affect power though especially if one or two is not working.
Mileage thing would be O2 sensors, MAF etc. Any sensor that helps tell the ecu what the engine conditions are can affect the fuel mileage.