Suddent drop in MPG's, and slight misfire. No codes

Hi all!

I'm trying to diagnose an issue with my Mom's car. She noticed her MPG's dropped off from about 44-43 down to 33-32 all of a sudden. I've driven it around some and I noticed a slight misfire when idling. This doesn't affect drive ability but you can hear the misfire clearly.
I replaced the Spark plugs already, but that hasn't helped. Old ones were pretty bad, so that needed to happen anyways.
Next thing I'm thinking is that is could be the Coils? But I'm not a fan of just throwing parts at a problem without diagnosing things. Does anyone have a test for the coils? Do they often fail 100% suddenly, or could a bad coil cause a slight misfire?

I appreciate any help!

 

Ah cool man, I'll go pull em out and check tomorrow. Thanks

 

It's about 140k .

 

So I haven't got around to pulling the plugs again yet. But the MPG's have risen up to about 37-38!

Could the ECU have a delayed response to the new plugs I put in already? I'm thinking that since the old plugs were really bad, they weren't able to burn all the fuel in the combustion chamber. And because of that the ECU was compensating by throwing more fuel into the cylinders. Then I replaced the plugs and the ECU was still running on the old fuel patterns. That's why it took about 50 miles of driving for the MPG's to go back up?
I'll keep watching it and see what happens.