What year sporty? If it's a OBDI car 95 and before you can pull the code using a jumper, for OBDII, 96 onwards you will need a scanner, you can also get it scanned at almost any major US parts store.

The CEL is the key, so get the code read, it could tell you the problem straight away, it may be something as simple as a faulty gas cap or blocked vent pipe, as you said it only happens when the tank is low.

08' Jeep Wrangler
64' Plymouth Sport Fury 383
58' Triumph TR-3A