I think you got the timers mixed up. The 250 mS was after power-up. The pulse was 20 uS.
delay(250); // or more to make sure power is up on the sensors.
pinMode (pinX, OUTPUT);
digitalWrite (pinX, HIGH);
delayMicroseconds(20); // Keep the line high for 20 microseconds
pinMode (pinX, INPUT); // now high impedance
digitalWrite(pinX, LOW); // Make sure the internal pull-up is off
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