Will not turnover

It also does not leak any, and doesnt smell lke gas at all, when I start it, it will crank if I keep holding the key but once I let go it goes off

1991 FROD EXPLORER : 100.000 MILES EGINE SIZE IS A 4.0, 6-CYLINDER:AT FIRST MY CAR WOULD JUST CLICK WHEN I WOULD TURN THE KEY AND IT JSUT DID THIS ALLL OF A SUDDEN, SO I REPACED THE STARTER, STARTER SOLENOID, BATTERY, BATTERY CABLES, I RELIEVED PRESSURE FROM THE RELIEVE PRESSURE VALVE AND NO FUEL CAME OUT, I CHECKED THE EMERGENCY FUEL SHUT- OFF VALVE AND STILL NOTHING! NOW IT WILL TURN BUT ONCE I RELEASE THE KEY IT WILL JUST SHUT OFF. SO YOU CAN PRETTY MUCH ELIMINATE THIS THINGS TO DO FROM YOUR LIST. I STILL DO NOT KNOW WHAT IT IS, ANY BODY HAVE ANY IDEAS?
____HISTORY__
OF MY CAR: I HAVE HAD A NEW TRANSMISSION PLACED IN IT, U-JOINTS, MUFFLER, CATALATIC CONVERTER, STARTER, BATTERY, BATTERY CABLES, STARTER SOLENOID, SPARKPLUGS, SPARKPLUG WIRES, OILFLITER, AIRFILTER A COMPLETE TUNE-UP.