This is a well known problem.

There's a several month old, huge thread, on this forum for the same problem.

Basically, the top cover on the drum unit is concave and coming in contact with imaging belt..... scraping off the image.

Two things you can try:
1. Remove the cover and operate without it. This makes it a little more difficult to get the drum in/out, but do-able.
2. Remove the cover and reshape the end closest to imaging belt end. Heat the cover enough to "just" allow it to be pliable, then cool it while holding in a convex position.

I've got some pictures that I'll post out here as soon as I find them.

Found the pictures.

Please View these 4 URL's: 1 2 3 4

Well, I took the cover to look at it and it doesn't really look warped at all.

In your first picture, it shows the symptom as streaks going all the way down the paper.  On what I have (copy and paste the link above), it's similar but only in a couple specific spots on each page.  Would that be caused by the same thing or might it be something different?

I'll try putting it in without the cover and doing a test print and see what happens I guess.

The KEY to this problem is the interval in which this occurs across the page.

Please note on the bottom left of that image is a note I added saying this was a worse case image. The picture you included is classic of this problem.

I have this issue, have had it since shortly after I got the printer. I don't print much, only up to 1100 pages at this point, but now every color print job that has content in the bottom half gets the lines. I went ahead and removed the back of the fuser cover and sure enough, the problem is gone, but I can't imagine that's a healthy solution. Considering this is CLEARLY design defect, what is Dell in a position to do to remedy this for those who have this problem?

You're preaching to the choir here...... Yes, this forum is moderated by Dell, but its contributors are people like you and me. Dell does have fairly liberal defects replacement policies, but the only way you'll get the thing replaced is if you call or email Dell tech support. I know a couple years ago they were replacing these drum units for this defect, but I think that program has ended. (Dell stopped shipping the 3100cn printer in July, 2006)

As for the "Drum Cover" being left off (no, it's not the back of the fuser cover)...... not really a problem as long as you don't let too much dust and dirt accumulate on top of your printer that might find it's way inside.

This sounds like my problem.  smudging on left side.  Where is the back of the fuser cover.  I looked up diagrams and it does not show this.

Thanks 

Jen