2003 G35 coupe was running fine and had given me consistently 19/25 mpg since new.
I changed spark plugs at 85000 miles and now city MPG is consistently 15-16.

Car seems to be a little more responsive based on seat of pants. But gas mileage has decreased by large margin.

Any ideas on what may have caused this sudden degradation in mileage?

Morris

 

Need to know what plug brand and type? Gap?

 

Did you change the heat range of your plugs? What model plug did you use?

 

What was the primary driving before...highway or city if that changed that is your answer.

 

used OEM plugs purchased from Nissan dealer parts counter
Gap was set to OEM spec

 

similar thing happened with the exception that i had done my brakes and put the car thru brake bedding procedure which really killed gas mileage. Need to take a 300+ mile road trip to improve fuel economy.

 

You erased the fuel trims by disconnecting the battery when you changed the plugs. It will get better.

 

How do you know this, did you check it with a gapper or feeler gauges?

 

do i need an ecu reset?