What was found
WHEN STARTING ENGINES WITH APU RUNNING, APU BLEED AIR OFF, HEAVY SMELL OF JET EXHAUST IN CABIN AND COCKPIT. CREW NOTICED THAT WHEN THE APU BLEED AIR KNOB WAS SELECTED OFF, THE WHITE APU BLD VALUE CLOSED CAS MESSAGE NEVER APPEARED. REPLACED APU BLEED AIR SHUTOFF VALVE. OPS CHECK GOOD. REF. AMM 36-10-30 AND 71-00-00.