I'll go with George's theory that the plastic collar on the tank sending unit has not been properly installed, presumably after removing it to change the pre-pump. Look for signs of fluid stains on the (front) side and top of the tank. Even if no gas has been running out, there will be a strong gas odour, especially after a recent fill-up on a warm day.

If the collar is too loose it will obviously allow a leak, but that's not necessarily the problem. The big rubber gasket on the sending unit that fits inside the neck needs to be seated flush before putting the collar on and tightening it. The collar often doesn't have enough strength to force a crooked gasket to seat. If there is a noticeable gap between the inside of the collar and the face of the sender then this is very likely the problem. In any case, it's very easy to have this happen especially when working through the access hatch, well at least for me.

If the collar is forced it will easily expand and the threads will jump and start to strip before it is adequately tight. Although the spec says to just hand tighten the collar, I like to tap it an extra 1/8 turn when working through the access hatch as it's awkward to get a real good hand grip. Don't try to go much more than this or you'll start to jump threads. If the threads do get stripped, and it easily happens, you're supposed to replace the collar, but the following tip often works well.

Is there a ring clamp around the outside of the collar? My '89 didn't have one, but my 940 does. If not, go get a stainless steel one from your local hardware store, a 5" one as I recall (Home Despot had it). Partially tighten the ring clamp before hand tightening the collar to prevent the collar from expanding and jumping threads. Do the final tightening of the ring clamp after the collar is tight.