COULD BE INCORRECT AIR AND FUEL RATIO (TOO RICH), FAILED FUEL PRESSURE REGUALTOR, INJECTOR PROBLEM OR BAD RINGS WILL CAUSE THIS.
I WOULD FIRST SUGGEST MORE FREQUENT OIL CHANGES TILL PROBLEM IS LOCATED.
PULL VACUUM LINE OFF fpr AND CHECK FOR STRONG FUEL ODOR THERE. IF REGULATOR RUPTURES IT WILL SEND FUEL INTO LINE INTO INTAKE AND CAUSE THE AIR FUEL RATIO TO BE WAY OFF. AND COULD WASH THE CYLINDERS DOWN.
TO CHECK THE INJECTORS AT REST RUN MOTOR, THEN SHUT OFF AND PINCH OFF OR BLOCK RETURN LINE TO TANK. PUT FUEL GUAGE ON SCHRADER VALVE AND OBSERVE FOR 30 MIN. THERE SHOULD BE LITTLE OR NO BLEED DOWN IN THAT TIME. YOU MIGHT HAVE TO REPEAT THIS TEST AS HOOKING GUAGE TO LINE MAY LOWER PRESSURE. IF THERE IS BLEED DOWN THEN PINCH OFF SUPPLY LINE (OR BLOCK IT) YOU CAN ISOLATE THE BLEED DOWN TO VARIOUS PARTS OF THE FUEL SYSTEM.
IF INJECTORS ARE HANGING OPEN WHEN RUNNING THATS A MORE COMPLEX TEST.