Before replacing that EGR valve, clear your codes and disconnect the vacuum line from EGR. If you don't get a code the next time you run, it probably means the vacuum control solenoid isn't working correctly or you have something hooked up wrong. FPR line going to EGR and vice versa.
Vacuum line should be going from EGR valve to vacuum tree and then to the EGR vacuum solenoid on the backside of your passenger side strut tower. The lone one on top.
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351W 89 Mustang GT Convertible