E200 auto Estate car

[JamesNeil]

Hi, my much loved "97/98" automatic car has suddenly become less fuel efficient and I sometimes hear a little "pop" backfire when I press down hard on the accelerator pedal. I can also hear a vibrating noise from underneath that I have not heard before, my son thinks it is from the exhaust but I am not sure. I also have some pinking going up hill where using premier fuel has made no difference. Any ideas or help would be appreciated.