need help in understanding why Serial.println makes LED to flicker

short description 2xIR sensors connected to arduino+motorshield rev3 if without serial.println LED burns nicely when sensor sees obstacle if i add serial print to code it starts to flicker when sensor sees obstacle and in serial print it looks 1110001110001111

more detailed in sensor part of forum http://forum.arduino.cc/index.php?topic=368962.msg2553848#msg2553848

more detailed in sensor part of forum

Better not to cross-post