Drop in mileage?

I posted about a week ago, but I didn't get any responses. Let me try again and hopefully, somebody who knows more about cars than I do can help me.
I have a 07 Hyundai Elantra SE with auto transmission. Since I bought it, the car has consistently gotten about 27-28 mpg for mostly city mileage. However, recently, with the same driving, it's dropped to about 25 or below. This coincided with a yellow indicator lamp that, according to the dealer, produced two codes: P0304 (intermittent misfire in cylinder #4 spark plug) and P0442 (small evaporative leak). The dealer replaced the spark plug and tightened the gas tank cap, but, the mileage hasn't improved and I've driven sufficient miles to see a difference. Anybody know whether the first code wasn't produced addressed by the replacement of the spark plug and that's why the mileage hasn't improved? I haven't noticed any performance difference in terms of smoothness of acceleration or power.
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On Sat, 26 Jul 2008 14:02:23 -0700 (PDT), Fantine

Are you using the A/C more often?
Charles
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On Jul 26, 8:05 pm, Charles wrote:

No more than I did last summer, and I didn't see the drop in mileage then.
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I would check the obvious first, meaning take the car back to the dealer and have the plug checked again - sparkplug wires too. It sounds like the mileage dropped off abruptly, but I would still check tires/pressure, air filter, oil (is it clean?) and wheel alignment.
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Oil was changed when the spark plug was "fixed," and the tire pressure is OK. I also am going to another gas station now; I was buying at the cheapest name brand I could find, but maybe there was a reason the case was so cheap. So, back to the dealer we go. Are all spark plugs the same, BTW, or are there better brands or types? (You can see I know next to nothing about cars.) Thanks.
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Fantine wrote: Are all spark plugs

There is definitely a difference in gas at different stations but it usually isn't much in regards to fuels mileage. There are times when NEW plugs are not necessarily working plugs! Stick with factory plugs and you will be fine.
Clay
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I should be able to get the car to the dealer tomorrow, so hopefully, they'll be able to say what's going on. Frustrating to have gas mileage go south on a car that has under 10,000 miles on it.
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Always the possibility that you were getting gas without Ethanol previously and you are now getting it with ethanol. ethanol cuts down the milage.
Jim
wrote:

Oil was changed when the spark plug was "fixed," and the tire pressure is OK. I also am going to another gas station now; I was buying at the cheapest name brand I could find, but maybe there was a reason the case was so cheap. So, back to the dealer we go. Are all spark plugs the same, BTW, or are there better brands or types? (You can see I know next to nothing about cars.) Thanks.
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I'm pretty sure that all the gas that is sold in my area is E-10, so that shouldn't be the difference.
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Responded in your other thread before I read this one.
If you haven't reset your fuel economy reading, you may want to do that first. If it hasn't been reset since the problem, then idling time in the shop and data from when the problem was occurring are still included.
The conditions that would cause a P0304 (misfire on cylinder #4) will easily cause a significant decrease in fuel economy if they continue. The conditions that cause a P0442, on the other hand, will only cause a very small, if even noticeable, loss in fuel economy. P0442 indicates that there's a small fuel vapor leak.
I'm not saying there's nothing wrong in your, but in the vast majority of cases where people complain about fuel economy, I can find nothing wrong with the vehicle.
It's also safe to presume that if your check engine lamp hasn't come back on, that the misfire and vapor leak have been fixed.
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I did re-set the economy when I left the dealership. In fact, in the weeks since the problem occurred, I've done more highway driving than I usually do, so if anything, I probably should be getting better mileage than before, but that's not the case. And I'm the proverbial LOL (little old lady) driver who doesn't gun the engine and accelerates gently.
I was afraid that you were going to say that if the check engine lamp was off, the misfire was fixed. I have an appointment about five minutes from the dealership tomorrow, so I might as well go there and ask them to check the car out. Thanks for your help, and I'll report back.
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One more thing to check is the front brakes which might be dragging. A simple way to check this is to remove the front wheel covers when the car is cold and then drive the car for about three to five miles, preferably on the highway with as few stops as possible. Touch each front wheel and center hub where the axle end is. Both should be no more than slightly warmer than air temp, and both sides should be at the same temp. If either side is hot to the touch, then the brakes are dragging on that side, not enough to pull the car to that side or to make it feel sluggish, but enough to affect gas mileage.
The rear brakes could be doing the same, but it's unlikely with a car that new.
Bob
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