I have had an ongoing check engine light problem on my '95 E320. I have checked vacuum lines, cleaned the pipe leading up to the valve and finally replaced the valve.

It was good for about 1000 miles and then lit up again. I had last filled up at a full serve station and when I checked the cap it was not fully tightened to the second "position". I reset the light under the hood and have my fingers crossed. Do you think that was the problem?