MECHANIC REPORTS PILOT NOTICED HOT AIR COMING FROM THE LEADING EDGE SLATS ON THE LEFT WING. SLATS WERE EXTREMELY HOT; COULD NOT BE TOUCHED. WING TAI VALVE FROZEN IN THE MID-OPEN POS AND NO INDICATION IN THE B737-400 COCKPIT.

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RESPONDED TO A CALLOUT ON ACFT X. THIS WAS AN RON ACFT AND DURING THE PREFLT WALKAROUND THE PLT NOTICED HOT AIR COMING FROM THE LEADING EDGE SLATS ON THE L-HAND WING. SEVERAL TECHNICIANS AND I RESPONDED AND SUSPECTED THE L-HAND WING TAI VALVE. THE LEADING EDGE SLATS WERE EXTREMELY HOT AND COULD NOT BE TOUCHED; WE IMMEDIATELY TURNED OFF APU BLEED AND VERIFIED L-HAND WING ANTI-ICE SWITCH WAS IN OFF POS. WE REMOVED THE PANEL TO GAIN ACCESS AND FOUND THE VALVE FROZEN IN THE MID-OPEN POS. 'MANUAL OVERRIDE LEVER WOULD NOT MOVE.' WE VERIFIED THE L-HAND WING ANTI-ICE SWITCH WAS INDEED OFF BUT NOTICED THE INDICATOR LIGHT WAS ALSO OFF. THIS WAS TROUBLESOME BECAUSE PER X DESCRIPTION AND OP THE INDICATOR LIGHT SHOULD HAVE BEEN BRIGHT TO INFORM THE PLTS THE VALVE WAS IN A POS THAT DID NOT AGREE WITH SWITCH POS. I PROCEEDED TO CHANGE THE VALVE AND OPS CHK REF X ALL OPS NORMAL. AT THIS POINT THE VALVE AND INDICATION ARE GOOD AND MY CONCERN WAS WITH POSSIBLE HEAT DAMAGE TO THE LEADING EDGE. MAINT CTL WAS BROUGHT IN TO TRY AND FIND A CONDITIONAL INSPECTION FOR THIS SITUATION AND THEY RPTED THAT THERE WERE NONE. I FELT SOME TYPE OF INSPECTION HAD TO BE ACCOMPLISHED BECAUSE AT A MINIMUM THAT WING WAS COOKING ALL MORNING FROM APU BLEED; AND WITH FALSE INDICATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED THIS WAS A SERIOUS SITUATION FOR MULTIPLE REASONS. WITH THE LEFT WING TAI VALVE FROZEN IN THE MID-OPEN POSITION; ON THE B737-400; HOT BLEED AIR FROM THE APU OR ENGINES; IF RUNNING; WILL BE PRESENT IN THE LEFT WING. THERE ARE NO OVERHEAT SENSING ELEMENTS IN THE WING LEADING EDGES; OUTBOARD OF EITHER ENGINE. THE ONLY OVERHEAT PROTECTION/DETECTION IN THIS AREA; IS THE THERMAL SWITCH IN THE TAI VALVE ITSELF THAT SHOULD CLOSE THE VALVE; WHEN THE APU OR ENGINE BLEED AIR REACHES 255 DEGREES FAHRENHEIT IN THE WING DUCT. REPORTER STATED IF THE VALVE SEIZES IN THE MID-OPEN POSITION; AS THIS ONE DID; THE THERMAL PROTECTION BECOMES NON-EXISTENT. THE EXCESSIVE HEAT BUILDUP ALSO CAUSED THE TAI VALVE INTERNAL POSITION SENSOR FOR THE COCKPIT BLUE LIGHT TO FAIL ELECTRICALLY; RESULTING IN THE BLUE LIGHT GOING OUT; INDICATING VALVE HAD CLOSED; WHICH WAS NOT THE CASE. ALTHOUGH THERE IS A VISUAL AND NON-DESTRUCTIVE TESTING (NDT) INSPECTION PROCEDURE REQUIRED TO INSPECT FOR HEAT OR METAL DAMAGE; IF ONE OF THE SLAT TELESCOPING TAI DUCTS SHOULD RUPTURE; NO PROCEDURE CURRENTLY EXISTS FOR A FAILED TAI VALVE IN THE OPEN POSITION. REPORTER STATED THE ACFT WAS RELEASED WITHOUT ANY VISUAL INSPECTION OF THE FIXED WING SURFACES; WHICH ARE COMPOSITE MATERIAL AND SUSCEPTIBLE TO EXCESSIVE HEAT. THE SLATS ARE ALUMINUM. AFTER RETURNING FROM HIS DAYS OFF; REPORTER STATED HE INSISTED HIS COMPANY DO AT LEAST A VISUAL INSPECTION OF THE FIXED WING LEADING EDGE COMPOSITE MATERIAL; WHICH WAS ACCOMPLISHED LATER.