Backfire from the engine of 1989 Ford Taurus 30 engine not the show?

If you are getting this condition without setting any fault codes, I would first check the EGR valve for leak by when no vacuum is applied. Air leaks in the hose between the mass airflow sensor and the throttle body could also be a cause. Check for a stopped up fuel filter. Ensure fuel pressure is at 45-50 psi without vacuum line connected to fuel pressure regulator, and 30-35 psi with the line connected. If it is not, the fuel pressure regulator may be the culprit. If the engine has many miles, a badly worn cam can cause backfire as well. This condition should set faults and render a check engine situation however.