Brian, are you sure it's gas? The reason I ask is because a sheen in
the water is frequently caused by a leak in a transmission, or engine,
oil cooler. All it takes is a pin hole in the cooler. Keep an eye on
the transmission fluid and engine oil level, if you have a cooler for
each, or a combination cooler, if the level is dropping very slowly
(only when engine is running) remove the cooler and have it pressure
checked. If that's not it how is the engine temp.? Is it a little too
cool. A cold running engine tends to not burn all the fuel, leaves a
sheen in the water. As Brian indicated, you could have a cylinder not
firing. That's easy to check, with the engine running at idle, with a
pair of pliers, pull the end of the wires out of the distributer cap
one at a time. If the engine runs rough when a wire is lifted up, or
the RPM's drop, that cylinder is firing, if their is no change, that
cylinder is bad.