1200

After having her for a few days i noticed maf wasnt plugged in, plugged it in fault code the lot. Got a second hand one. Fitted it ran like cack but no maf codes. Changed the chain tensioner (new) still cack. Changed vvt motor (second hand) still cack Then got code for misfire cylinder 4, plugs...

Hi there all. Could anyone tell me why my car revs up to about 1200 rpm and back down to 800 rpm when sitting idle please