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04 grand prix gt

JunkBird69

JunkBird69

New member
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?
 






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.
 
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