Engine Thermostat and MPG

[dantheman1]

I had a check engine light pop up a few months ago when it had around 61k miles. Took it in, they read the code and said it need a new thermostat. I didn't have the time then to have it fixed at the time so I drove it with the engine light for another month or so. While the engine light was on, i did noticed engine idle wasn't exactly smooth and on colder mornings, engine would be especially rough. During this time though, I was still getting around 14 mpg, even with my roof rack. Took it in last week to have them change it out and mpg nose dived. The engine is definitely smoother and feels as though it even has some more power, but mpg is now at 10.3 (and getting lower the more I drive it). Any ideas? Could they have installed the new thermostat improperly? I can put up with now for a bit (while gas is still $1.75), but is there a way I can check or fix it myself without another trip to the dealer?
Thanks!

 

[Houm_WA]

I doubt it. I would reset the MPG computer a few times and see if you can't get a few consistent readings on-highway and then off just to make sure you're not psyching yourself out. Heck I seldom get less than 20 MPG anymore on I-5.

 

[richpike]

It could also be the shift in gasoline. Not sure when they changed to "winter gas" where you are, but that might have an impact (we usually see a 2-3mpg drop when they switch to the winter blend).

-Rich

 

[dantheman1]

Update: I reset the mpg indicator before i drove to houston last night and I got around 16.3mpg... so I guess I was just paranoid about it. Still puzzled as to how the mpg got so low so quickly, but I guess it's fine now. Thanks y'all for the input.