Have a traverse been noticing not running smooth and MPG has dropped the other day my wife was leaving home going up a hill CEL flashed and went off, I pulled codes and got a P0304. A few months back I replaced Plugs car has 125K on it now, I see alot of people with this but different results. Was going to order new coil packs but is this wasting money is there something else I should be looking at for the cause of this. Seems MPG used to avg on highway about 21 now seems the best I can get is 16mpg Any help would be much appreciated.

 

i would inspect the plug and wire for number 4 cylinder and make sure its all good and has a good connection. if it looks ok i would then run a compression check on all cylinders to compare number 4. if that checks out then maybe swap that plug with another and same with the coil/wire to see if the code follows.

 

After thinking about it last year when I had an issue it was P0303 and I replaced the plugs and thinking then these are hard to replace on the rear so I moved that coil to the front number 4. Do you think it could be the coil, do the coils have intermittent issues usually or fail all together.

 

i can see something electronic having an intermittent issue, like if it can handle X amount of load but with a higher demand of a load it might start acting up

 

if the misfire followed the coil; replace the coil...do it pronto. driving a misfiring engine will damage the catalytic convertors.