Engine Performance problem
1991 Chevy Caprice V8 Two Wheel Drive Automatic 148000 miles
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my wagon throws out a puff of black smoke every time i floor the gas pedal then goes away then at a stop sign i try it and it will do it again but it won't do it if i take off smoothly,i changed wires cap,rotor,plugs and fuel regulator,what can it be the engine runs good but will not pass smog as it runs too rich,please help.

Possible causes include a bad O2 sensor, excessive fuel pressure (bad fuel pressure regulator or plugged return line), leaky fuel injectors, dirty air filter or restricted air inlet, or a defective coolant sensor that prevents the engine management system from going into closed loop mode.

what would be the first thing you would do to figure this out?

Check the airfilter then fuel pressure could be excessive due to the regulator or clogged return line-