What was found
ACRID SMELL IN COCKPIT WITH APU RUNNING. CREW HAD MENTIONED THIS SMELL WAS NOT PRESENT UNTIL THE APU WAS RUNNING WHILE TAXIING IN. THEY SEEMED TO OF ALREADY ISOLATED IT TO THE APU FOR IS SAYING THEY COULD DUPLICATE IT WITH THE APU RUNNING AND HOW IT WOULD GO AWAY WITH THE APU OFF. PERFORMED FAULT ISOLATION IAW FIM 21-00 TASK-806 AND AMM 05-51-56. NO FUMES; SMOKE; OR UN-USUAL ODORS NOTED WITH APU RUNNING AND APU BLEED RUNNING BOTH PACKS. A/C OK FOR SERVICE.