A B737-200 #2 ENG PNEUMATIC PRE-COOLER VALVE WAS REPLACED AND PRIOR TO ENG RUN IT WAS DISCOVERED THE HIGH PRESSURE VALVE WAS LOCKED CLOSED.

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I WAS ASKED TO HELP CHANGE OUT A PRE-COOLER CTL VALVE AS PER REQUEST OF MAINT CTL. CHAPTER 36 HISTORY WAS RESEARCHED AND IT WAS SAID THAT WE HAVE BEEN EXPERIENCING A LARGER FAILURE RATE WITH THESE VALVES ON -700'S. TALKED TO THE CREW WHILE I STARTED OPENING #2 ENG. WE CHANGED THE PRE-COOLER CTL VALVE AND A SENSE LINE. WHEN ACTUATING THE VALVE; I COULD HEAR A DIAPHRAGM THAT SOUNDED DAMAGED AND DIDN'T FEEL RIGHT. CONFIDENT THAT THIS WAS AT FAULT; WE CLOSED UP FOR ENG RUN-UP. BEFORE PUSHBACK; WE NOTICED ENG BLEED AIR WAS ON MEL; SO WE HAD TO OPEN THE ENG AGAIN AND UNLOCK THE PRSOV. THE HIGH STAGE VALVE WAS MISIDENTED AS THE PRSOV. I DON'T KNOW IF THAT VALVE WAS LOCKED OUT; BUT I DID MAKE SURE IT (HIGH STAGE VALUE) WAS OPEN BEFORE CLOSING THE ENG AGAIN FOR ENG RUN-UP. THIS COULD HAVE BEEN PREVENTED IF A COMPLETE MAINT MANUAL CHK-OUT OF R&R OF PRE-COOLER CTL VALVE WOULD HAVE BEEN PERFORMED ALLOWING THE MEL TO BE CLRED CORRECTLY. ALSO; THE RUN-UP PAD HAD PARTIAL SNOW COVER ON IT CAUSING THE ACFT TO SLIDE FORWARD APPROX 10 FT WHEN THROTTLES WERE ADVANCED. DUE TO THIS DISTR; THE BLEED SYS WAS NOT CORRECTLY ISOLATED; HAD IT BEEN; THIS PROB WOULD HAVE BEEN PREVENTED.