Bill T, I am an advanced emission repair specialist, my specialty is not Ford, but am pretty sharp about the lingo and systems, what confuses me about your case, is it sounds like your failing the evaporative emissions test, thing is, besides the gas cap, this was generally not part of the test until 1996, when vehicles came equipped with their own evap test systems and would set fault codes, there are some codes that can be set on your year model 1991, and if there are any codes I would like to know them. Generally speaking, in most systems, as you drive the fuel vapor system will be either pressurized by an external pump, or have a vacuum put on it, usually through engine vacuum, there is a pressure sensor in the fuel tank, and once the self test starts, the pressure must stay within limits or a code is set. On 1996 and newer vehicles, these limits are extremely narrow, and most of the time are due to deteriorated vent hoses, sticking purge valves or perforated charcoal cannisters. Any extra info would be helpful in me helping you.
As to disabling the CEL, that won't fly anymore, at least here in TX, that is part of the test, if light does not work, no pass, and then on 1996 and newer vehicles, there are these pesky readiness monitors, they are there to keep someone from resetting the CEL, and getting a inspection before the light comes back on, the monitors are subsystem tests that occur while driving to verify the repair was successful.