THE PLT WAS ON A FLT TO TEST A NEW LIQUID COOLED ENG & ITS RADIATOR SIZE & LOCATION. ON PREVIOUS FLTS, HE HAD NOTED INADEQUATE COOLING. COMPANY PERSONNEL HAD MOVED THE RADIATOR TO IMPROVED ITS EFFICIENCY. ON THE PREVIOUS FLT, THE PLT NOTED THAT THE TEMP GAGE HAD FAILED. HE LANDED ON THE COMPANY'S GRASS STRIP, & WITH ASSISTANCE, DRAINED THE COOLANT SYS, CHANGED THE GAGE & REPORTEDLY RESERVICED THE SYS WITH COOLANT THAT HAD BEEN DRAINED & SAVED. THE PLT STATED THAT PRIOR TO TAKEOFF, HE OPENED THE RADIATOR CAP & HAD SEEN COOLANT IN THE NECK OF THE UPPER SURGE TANK. AFTER TAKING OFF, HE NOTED AN EXCESSIVELY HI COOLANT TEMP. HE REDUCED THE POWER TO PREVENT HEAT DAMAGE TO THE ENG & MANEUVERED TO LAND. ACCORDING TO WITNESSES, THE ACFT SHUDDERED, PITCHED NOSE DOWN & CRASHED. AN EXAM OF THE ACFT REVEALED NO PREIMPACT FAILURE. ONLY ABOUT 2 CUPS OF COOLANT WAS FOUND IN THE SYS. THE COOLING SYS HELD ABOUT 1 GAL OF COOLANT. A 1 GAL CONTAINER USED TO DRAIN & SERVICE THE ACFT WAS FOUND ABOUT 5/8 FUL OF COOLANT.