What was found
PRESSURIZATION SPIKES +/- 1500 FPM ON TAXI, ACFT PRESSURIZED TO .2 PSID ON GROUND. UNABLE TO CONTROL PRESSURIZATION WITH ALT CONTROLLER IN DUMP. MANUAL FULL OPEN & FIRST OFFICERS FORWARD OUTFLOW VALVE FULL OPEN. MX RAN NR 1 & NR 2 ENGINES & TAXIED ACFT WITH PRESSURE CONTROL IN NORMAL/AUTO MODE, NR 1 & NR 2 BLEEDS ON & SET FLOW CONTROL TO MIDWAY. ACFT MOMENTARILY WENT TO +500 FPM & BACK TO 0 AT ENGINES BLEEDS ON SELECTION. PRESSURIZED ACFT IN MANUAL MODE TO 3 PSI DIFFERENTIAL AND BACK TO 0 IAW DHC-8 200 AMM TASK 21-30-00-780-801. OPERATIONAL CHECK GOOD. NO PARTS REPLACED. ACFT RETURNED TO SERVICE.