Thread: 04 grand prix gt

Dont know what happened but ever since my recent tuneup my MPG's suck!!!! Went from 19-21mpg city 29-31 hwy, now im seeing 14mpg city and about 25hwy, only thing done to the car is an open element a/f and shift kit. any clue? Wonder if its the winter fuel there selling us?

Used NGK plugs, Plugs, wires, oil change, throttle body cleaning, still havent touched the fuel filter tho

No CEL no performance issues, just horrible mpg decrease.

also slight spark knock at part throttle under load

No i used NGK's not used plugs.lol

Tr55 at .060

With my '04, I get an average of 22-24 MPG driving a combo of city/hwy on winter gas. In the summer, it's more like 23-25 MPG. at least for me, winter blended gas isn't that bad. Are you really certain about your before numbers? Those are the best fuel econ numbers I've seen for a GP. There are a couple of threads/polls on this forum, and it seems like most get closer to 20 MPG. Driving habits I'm sure.

I know that the EGR valve sticking closed can hurt MPGs and cause knock. But I would think that should throw a code. Just double check that it's not loose or something. It's possible to bump it while doing plugs.

How many tanks of gas have you gone through? I wonder about a bad batch of gas. You could always fill up with premium and throw in a can of seafoam for good measure. Bad gas can cause both lower mileage and knocking.

I can't find the heat range on those plugs, but if they run hotter than stock, it could also be your problem. Looks like they should work though as I found people with Corvettes using them.