hi guys and gals. hoping i could get some help or pointers on this issue, i got a 2004 ext cab silverado z71 5.7 ltr. went to lake of the woods ice fishing over new years, it was 30 below most of the time. i started the truck and let it run a few times a day. the day we left the chk engine light was on and flashing at some points, it had a rough idle, never died, but power was normal when you hit the gas. i put a can of heet in when i gassed up. mileage was around 7mpg, after the heet, and a hr or so, mileage returned to normal. but the check engine light is still on and flashes at times, still rough idle, mileage still varies. Also noticed some grayish black stuff coming from the tail pipe. any help would be appreciated. happy new year to all chevy owners!!!!!

 

when the mil is flashing, the truck is misfiring and catalytic convertor damage is occurring. do not drive the truck for any duration when the mil flashes. get the codes read pronto and post back.