CEL came on after Fill-Up
 

I have a 2000 Mercury Marquis.

I have not been driving the car much lately, and it was sitting in the street in the cold for a few days.

Today, I went to the Shell gas station because the tank was a little less than a quarter of a tank left of gas. Prior to this, I filled up at a newly opened independent not brand name gas station, and then put in Dry Gas afterwards because of the cold weather.

After I filled up at Shell with Regular, and drove away, the car started to shake slightly, and then the CEL light came on.

I drove to the supermarket, shopped, went back to the car, started it ok but the CEL came on again but this time no engine shaking. It ran more smoothly again.

The CEL is on when I start the car.

Thoughts or recommendations? Thanks.

Scanned for trouble codes?...your descriptions could be anything from a clogged fuel filter, vacuum leak...etc.

I have seen quality of gas issues, once in a honda accord with gas from a Virginia Beach 7/11 that was watered down and the car hardly wanted to run, in a toyota truck from a local food mart pump-trash in the gas itself clogged that filter quickly. The good ole GM doesn't seem as sensitive but it did require a filter change once sooner than needed due to quality of gas issues.

Get back with the trouble codes and I am sure the folks here can help :)

My code tester indicated P307 misfire in cylinder 7.

Last night, I drove the car on the highway for 75 miles 65-70 mph, stopping twice to do other things. I also put in a good fuel additive prior to this drive. The car never hesitated or shook again.

That being said, I turned off the CEL with the Code Reader when I stopped in the shopping center. When I restarted the car, it started smoothly, and no CEL came on. No problems with acceleration either.

I stopped one more time for food. Restarted the car, and no CEL.

I think it was the fuel I put into the car at the new gas station prior to the Shell Station.

What do you think? Bad gas?

Sounds like it, good to hear it is running smooth now :). You might want to check/or just replace the fuel filter to see it is partially clogged up now/restricted flow etc.