A B737-200 CAPT HAS UNCERTAINTY OF AUTOPLT PERFORMANCE ON ILS CAPTURE MODE BUT MAKES NO WRITE-UP FOR 3 LEGS.

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I HAD CAUSE TO SUSPECT THE SYS DUE TO POOR PERFORMANCE I EXPERIENCED FROM THE SYS ON MY FIRST LEG AS PF. I WAS NOT CERTAIN THAT THE AUTOPLT WAS TO BLAME; HOWEVER; BECAUSE IT COULD HAVE BEEN CAUSED BY THE ILS AT THAT ARPT (INTERFERENCE) OR BY MY OWN USE OF THE SYS; I HAD USED 'MANUAL' TO ATTAIN THE CAPTURE. SINCE I HAD TIME; ON THE GND AT THE ARPT; I CALLED A MECH AND DISCUSSED THE PROB; BUT THEY DON'T DO -200 AUTOPLTS THERE. SO I DECIDED TO TRY IT AGAIN; ON THE WAY TO PHX; TO OBTAIN MORE OF A CERTAINTY REGARDING THE SOURCE OF THE POSSIBLE PROB -- ME; THE ARPT EQUIP; OR THE AUTOPLT. AS IT TURNED OUT; I DIDN'T GET A CHANCE TO CHK OUT THE SYS ON EITHER OF THE LAST 2 LEGS; DUE TO NO ILS AT PHX RWY 8 AND AT BUR DUE TO TASK SATURATION (TFC; CIRCLING TO RWY 15; ETC). TURNING THE PLANE OVER TO THE RELIEVING CAPT AT PHX; I MENTIONED MY OBSERVATIONS REGARDING THE AUTOPLT AND AGAIN HAD CALLED FOR MECH. I WAS REQUESTED TO MAKE A LOGBOOK ENTRY BY THIS CAPT AND DID SO. IN RETROSPECT; I SHOULD NOT HAVE MADE AN ENTRY SINCE I WAS NOT CERTAIN THAT THE SYS WAS AT FAULT. IT MAY APPEAR THAT I WAS 'CARRYING' THE DISCREPANCY; BUT IN MY MIND I WAS NOT SUFFICIENTLY CERTAIN THAT A PROB EXISTED. I SHOULD HAVE MADE THIS MORE CLR TO THE ONCOMING CAPT.