A B737-300 WITH THE L WING AIRFOIL ANTI-ICE VALVE INOP LOCKED OPEN AND DEFERRED; THE L WING OVERHEATED WHEN THE APU BLEED AIR WAS USED FOR STARTING.

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UPON ARRIVING AT THE ACFT AT APPROX XX00 AM IN ZZZ WE NOTICED SEVERAL CEILING PANELS WERE REMOVED INSIDE THE PAX CABIN AND BOTH ENG AND APU BLEED SWITCHES HAD PLACARDS WARNING 'NOT TO OPERATE.' LOGBOOK NOT IN THE ACFT. AFTER A PERIOD OF TIME; SEVERAL MECHS ARRIVED AND INFORMED US THAT THE ACFT WAS GOING TO BE REMOVED FROM THE GATE FOR OPERATIONAL TEST OF THE PNEUMATIC AND PRESSURIZATION SYS. INBOUND CREW THAT NIGHT HAD WRITTEN A DISCREPANCY FOR THE L WING ANTI-ICE VALVE AND A LOUD NOISE NEAR MID CABIN DURING FLT. WHEN WE REBOARDED THE ACFT; WE FOUND THAT THE L WING ANTI-ICE VALVE WAS ALREADY LOCKED OPEN AND PLACED ON MEL AND MAINT WAS FINISHING WITH THE LOGBOOK. PRIOR TO BOARDING THE ACFT I WAS WATCHING THE MECH TECHNICIAN MANUALLY OPEN THE WING ANTI- ICE VALVE AND DISCONNECT THE CANNON PLUG SECURING IT WITH A TIE WRAP. AFTER REVIEWING THE LOGBOOK AND THOROUGHLY READING THROUGH THE MEL PROC; THE FO AND I HAD SOME QUESTIONS AS TO THE PROCS AND LIMITATIONS PROVIDED UNDER THE ACTION AND EXCEPTIONS COLUMN OF THE MEL. 1) HOW COULD WE COMPLY WITH THE MEL PROC THAT CALLED FOR THE ANTI-ICE VALVE IN QUESTION TO BE CLOSED DURING THE #2 ENG START WHEN MAINT HAD JUST FINISHED LOCKING THE VALVE OPEN AND SECURED THE ACCESS PANEL. 2) WHAT IS THE PURPOSE FOR THE XBLEED START OF THE #1 ENG. 3) SINCE THE PROC WOULD REQUIRE STARTING AT LEAST 1 ENG WITH THE WING ANTI-ICE VALVE OPEN; HOW WERE WE GOING TO OPERATE OUT OF ZZZ WHERE THEIR METAR WAS ALREADY RPTING A TEMP OF 12 DEGS C; EXCEEDING THE ACFT TAI LIMITATION OF 10 DEGS C. MANIFOLD MUST BE DEPRESSURIZED ANYTIME TEMP IS ABOVE 10 DEGS C. (HOW COULD WE START THE #1 ENG WITHOUT EXCEEDING THE LIMITATION IN THE MEL PROC.) I CONTACTED XYZ IN MAINT CTL REF THESE QUESTIONS; THIS WAS AFTER CONVERSING WITH ZZZ MAINT AT GREAT LENGTH ABOUT THE ABOVE ISSUES OVER THE RADIO BEFORE OPTING TO TAKE THE MEL BOOK INTO OPS AND COMMUNICATING WITH YYY MAINT VIA THE PHONE. AFTER CONSIDERABLE TIME ON THE PHONE WITH XYZ IN YYY MAINT CTL; I WAS ASSURED THAT THE CURRENT TEMP IN ZZZ WOULD PROVIDE NECESSARY PRESSURE AND TEMP PROTECTION FOR ENG START IN ZZZ AND HE WOULD NOTIFY ZZZ. THE FO AND I DISCUSSED AT GREAT LENGTH THE IMPLICATIONS OF FLYING THE ACFT TO ZZZ WITHIN THE LIMITATIONS OF THE MEL. WE DECIDED THAT WE HAD EXPLAINED OUR POS CLEARLY TO YYY MAINT AND WERE TOLD QUITE SIMPLY THAT WE WERE 'OK TO LEAVE ZZZ.' AFTER PUSHBACK WE ATTEMPTED TO START THE #2 ENG BUT ONLY MADE 15% N2 WITH 12 PSI SHOWING ON THE DUCT PRESSURE GAUGE. I ABORTED THE START AND CLOSED THE APU BLEED VALVE. IT WAS QUESTIONABLE ABOUT WHETHER OR NOT 30 PSI WOULD BE ENOUGH PRESSURE TO START. I TOLD THE FO TO CONTACT ZZZ MAINT VIA RADIO. AFTER CONTACTING ZZZ MAINT; THEY REQUESTED WE CYCLE THE APU BLEED SWITCH SEVERAL TIMES. MAINT THEN REQUESTED WE TRY SEVERAL OTHER BLEED SWITCH POS WHILE TRYING TO ISOLATE A POSSIBLE BLEED LEAK IN THE PNEUMATIC SYS. THEY INSISTED THAT THE ENG SHOULD START WITH THE WING ANTI-ICE VALVE LOCKED OPEN; THAT THERE SHOULD BE SUFFICIENT AIR PRESSURE. GND CTL CALLED AND INFORMED US THAT FLOW CTL WAS NOW IN PROGRESS FOR ZZZ AND TO STATE OUR INTENTIONS. THE FO WAS TRYING TO COMMUNICATE WITH MAINT AND OPS RELAYING INFO TO ME WHILE I WAS MONITORING GND CTL. I WAS TALKING TO THE TUG DRIVER FOR A TOW TO THE GATE AND GATHERING INFO AS TO WHERE TO PARK THE ACFT. DURING THIS TIME THE APU BLEED VALVE WAS LEFT OPEN. AS WE WERE BEING TOWED BACK TO THE GATE; MAINT PERSONNEL ON THE GND WERE TRYING TO GET OUR ATTN. FLT ATTENDANTS RPTED THAT SOMETHING IN THE CABIN SMELLED VERY HOT. I OPENED MY WINDOW AND MECHS WERE TELLING US TO CLOSE THE APU BLEED VALVE. A TREMENDOUS AMOUNT OF HEAT WAS RADIATING OFF OF L WING LEADING EDGE. PAX NOTICED SMOKE FROM L WING AREA. SEVERAL PAX DEPLANED AFTER SEEING THE SMOKE AS WE OPENED THE L FORWARD EXIT. I WAS CONCERNED AS TO THE DAMAGE TO THE L WING LEADING EDGE DEVICES BECAUSE THERE IS NO OVERHEAT PROTECTION WITH THE ANTI-ICE VALVE LOCKED OPEN AND CANNON PLUG DISCONNECTED. ZZZ MAINT EXTENDED THE LEADING EDGE DEVICESAND NO VISIBLE DAMAGE WAS NOTICED. A MAINT TECHNICIAN INFORMED ME THAT WHILE I WAS IN OPS; THEY HAD HOOKED UP AN AIR START CART AND INSUFFICIENT AIR PRESSURE WAS AVAILABLE FOR START. AT FIRST; THEY KEPT INSISTING THAT THE ANTI-ICE VALVE WAS LOCKED CLOSED; BUT THEY WERE LOOKING AT THE WRONG VALVE. I WENT OUTSIDE TO THE LOCATION OF THE ACCESS PANEL WHICH COVERS THE WING ANTI-ICE VALVE AND THEY HAD OPENED A PANEL RIGHT NEXT TO THE CORRECT ACCESS PANEL COVERING THE WING VALVE. THE LEADING EDGE WAS THEN INSPECTED FOR DAMAGE AGAIN PER A PHONE COM WITH BOEING; AS EXPLAINED TO ME BY THE MAINT MGR IN ZZZ WHO MET WITH ME IN THE CREW LOUNGE. THEY INSISTED THAT THE ACFT WAS RETURNED TO SVC AND A FULL BOAT OF PAX WAS WAITING TO DEPART. UPON FURTHER REVIEW OF THE LOGBOOK; NO CORRECTIVE ACTION OR INSPECTIONS HAD BEEN LOGGED. THERE WAS NO TECHNICAL DATA TO SUPPORT MAINT'S INSPECTION OF THE ACFT FOR DAMAGE. MY DISCREPANCY; RECORDED IN THE LOGBOOK; INDICATED POSSIBLE OVERHEAT TO THE L LEADING EDGE DEVICES; BECAUSE OF THE VALVE BEING LOCKED OPEN DURING INITIAL START ATTEMPT. AFTER DISCUSSING THIS SIT WITH MORE THAN A DOZEN INDIVIDUALS IN DISPATCH; SYS OP CTL; MAINT CTL; AND FLT OPS; ABC IN THE OFFICE OF THE CHIEF PLT; SUPPORTED MY DECISION NOT TO ACCEPT THE ACFT FOR DISPATCH.