I drive an '06 with about 60k miles on it, all periodic maintenance done& the miles are almost all highway... So, aboutthree months ago my check engine light comes onsolid, I have the dealer pull the codes & they tell me that it was just a loose gas cap, reset the codes, and send me on my merry way. All fine & dandy, EXCEPT that ever since then I'd be lucky to get 35mpg on the highway, with no change in driving habits, overall weather patterns,average speed or route, type of gasoline, etc ...whereas before I had been getting 42mpg+ easy! This is really driving me insane, because that's like 10mpg gone, evaporated into this air! I had another dealer check things out & also an independent shop, and neither could find anything in terms of filters, codes, etc .... The only thing that I am noticing is that I seem to get the "check gas cap" message more and more frequently, even though I crank the damn thinguntil it's clicking forlike 20-30 seconds ... So, I am at a huge loss to explain what could be causing this significant drop in fuel economy, and I am really bummed out ...[X(]

Any thoughts, ideas, suggestions? Thank you much!

 

i had the same problem only happened once, and it was when i fueled up with the car running

 

have you replaced ur air filter, i just recalled the same problem with mine and that helped the mpg

 

Get a new gascap. If it caused the engine to run differently for a bit, I'd suspect something like deposits on the sparkplugs or an O2 sensor or some other sensor that got affected somehow.

Also check your tires for lowpressure and brakes for dragging. They could be off just by coincidence.

 

Yes, those are all great suggestions, but why the gas cap? And, also - wouldn't a check engine light come on for a bad O2 sensor? Just some random thoughts ... Finally, you know, I kind of feel half bad bitching about my "problem" because I still get better than 80% of cars on the road in terms of mpg, but compared to what it was before, it's like 10 mpg less ....

WHat does the O2 sensor do anyhow, and is it pricey to replace?

 

I've heard if you don't tighten your gas cap tight enough, like 3 clicks or something, it will throw the check engine light. I'd try disconnecting the negative battery terminal and leave it off for about 15mins(give everything a chance to discharge), or speed up the process by disconnecting both, negative first, and touching them together for about 15sec(This will delete all memory stored in the electrical system, as well as drain capacitors). Then hook it back up and see if it changes anything.

O2 sensors can get extremely pricey, e****ally if you have it done at the shop. No, the light won't always come on from a bad o2.
Basically it monitors/adjusts your air/fuel ratio. I'm pretty sure your car has 2 of them, don't quote me on that though.