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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