My 1994 Miata has great pressure starting cold. The longer I drive , the oil pressure drops and I get that TAP,TAP,TAP. Cool off ,pressure is fine, tap is gone. Any ideas?

 

YOU NEED TO HAVE YOUR OIL PRESSURE PROPERLY TESTED. AT NORMAL OPERATING TEMPERATURE AT 1000RPM 15-28PSI; AT 3000RPM
43-56PSI. YOUR MIATA HAS A OIL PRESSURE SWITCH, WHICH CONVERTS PRESSURE TO ELECTRICAL SIGNAL TO THE GAUGE, MAY NOT
BE READING PROPERLY.
HOW LONG SINCE LAST OIL CHANGE? WHAT VISCOSITY OIL ARE YOU USING? SHOULD BE SAE 10W30 THIS TIME OF YEAR.
AS FOR THE TAPPING; IF SHARP PITCH SOUND, COULD BE YOUR HYDRAULIC LASH ADJUSTERS BLEEDING DOWN.
GET THIS CHECKED OUT SOON, HLA'S RUN ABOUT $60 EACH AND YOU HAVE 16 OF THEM. PLUS OTHER DAMAGE TO CAMSHAFTS COULD OCCUR
IF CYLINDER HEAD IS NOT GETTING ENOUGH OIL.