I HAVE A 1990 6 CYLINDER JEEP CHEROKEE THAT RAN HOT AND SITTING STILL THE OIL PRESSURE DROPS FORM 40 TO 20.IT NOW RUNS HOT AS SOON AS I START IT(HARDER TO START).COULD IT BE THE OIL PUMP OR JUST THE THERMOSTAT?IT DOES HAVE A TAPPING NOISE IN THE ENGINE.BUT BEFORE NOW IT WAS RUNNING FINE.