Narrative:

While on the ground in ord, we boarded the plane with a tight connection due to a plane change. When I examined the logbook, I noticed that the #2 engine 'a' loop of the engine overheat/fire protection system was inoperative per the MEL. I checked the loop by placing the #2 engine overheat detection switch in the 'a' position, and noticed that there was no fault light, confirming a suspicion that either 1) the loop actually worked but maintenance had not had enough time to investigate a previously failed test so simply deferred the loop, or 2) the wrong loop might have possibly/accidentally been entered in the amis as the faulty loop. I kept this thought in the back of my mind, placed the #2 engine overheat detection switch into the 'B' position as required by the MEL, and performed a fire test. The engine and APU overheat/fire protection system tested normally with the fire protection panel configured per the MEL, so I continued with my SOP's. The flight was normal until the descent into ZZZ. At approximately 15000 ft, with the throttles just about to hit their idle stops due to a descent, I noticed that both master cautions illuminated with an overheat/detection indication illuminated on the left system annunciator panel. I looked down at the fire protection panel and noticed that the engine 2 overheat light was illuminated. I asked my first officer to pull the qrc from the glare shield. As soon as he had opened the qrc, the engine 2 overheat light went out. I immediately suspected a problem with the engine overheat/fire loops. We did not run the qrc because the engine 2 overheat light went out so quickly. Both the first officer and I felt, after discussion, that an actual engine overheat did not exist. Upon leveloff from this descent, a min or two later, the throttles came up to cruise power and again the #2 engine overheat light briefly illuminated. I then selected the #2 engine overheat detection switch to the 'normal' position and the #2 engine overheat light immediately went out. Both the first officer and I felt, after discussion, that an actual engine overheat did not exist. I then gave the aircraft and the radios to the first officer to try to obtain a third opinion from maintenance. I then sent 2 or 3 very quickly typed (rushed) messages to maintenance explaining that we had had some very brief #2 engine overheat indications, but nothing appeared out of the ordinary except for the throttles being about 1 knob width apart at equal power settings, not uncommon on the B737-300/500 aircraft. In between the attempts of trying to contact dispatch and maintenance, I asked the purser to take a cursory look at the #2 engine to see if anything appeared unusual. The purser said that nothing appeared unusual. The response time from maintenance via ACARS was slow and with a midfield right downwind for runway 33L at ZZZ rapidly approaching, I decided that it was time to simply land the aircraft and deal with these previously observed spurious indications on the ground. There was little doubt in my mind or the first officer's that after having done multiple fire tests, after having seen only very few, spurious, brief illuminations of the engine #2 overheat light, after having seen no abnormal engine indications, after having not seen the engine #2 overheat light even illuminate briefly for the past several mins despite several power changes, and after having realized that I never received any engine #2 overheat indication with the engine #2 overheat detection switch in either 'normal' or 'a' and only very intermittently in 'B,' that an actual engine #2 overheat was extremely unlikely and that some sort of failure of the engine and APU overheat/fire protection panel had occurred. I called maintenance on my cell phone and again started to perform a fault/inoperative test while I waited for a maintenance person to answer. When I performed the fault/inoperative test this time, we received completely abnormal responses from the engine and APU fire protection panel for a fault/inoperative test. In addition to the normal fault/inoperative test indications, we received an engine #2 fire warning and an engine #2 overheat light with the fire bell. I released the fault/inoperative test button and attempted the same test with the same incorrect indications as the result. With maintenance now on the phone, I did some more fault/inoperative and engine and APU fire/overheat tests, with mixed results of success and failure. Obviously, something was very wrong with the engine and APU fire protection panel and maintenance immediately decided to ground the airplane. Callback conversation with reporter revealed the following information: the reporter stated the engine fire protection panel was suspect as the tests would pass and then fail with no positive conclusion. The flight crew received no details on the fix from maintenance.

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Original NASA ASRS Text

Title: A B737-300 WAS DISPATCHED WITH #2 ENG 'A' LOOP FIRE-OVERHEAT DEFERRED AS INOP PER THE MEL. AT TERMINATION OF FLT, TESTS AGAIN REVEALED MIXED TEST RESULTS ON BOTH LOOPS.

Narrative: WHILE ON THE GND IN ORD, WE BOARDED THE PLANE WITH A TIGHT CONNECTION DUE TO A PLANE CHANGE. WHEN I EXAMINED THE LOGBOOK, I NOTICED THAT THE #2 ENG 'A' LOOP OF THE ENG OVERHEAT/FIRE PROTECTION SYS WAS INOP PER THE MEL. I CHKED THE LOOP BY PLACING THE #2 ENG OVERHEAT DETECTION SWITCH IN THE 'A' POS, AND NOTICED THAT THERE WAS NO FAULT LIGHT, CONFIRMING A SUSPICION THAT EITHER 1) THE LOOP ACTUALLY WORKED BUT MAINT HAD NOT HAD ENOUGH TIME TO INVESTIGATE A PREVIOUSLY FAILED TEST SO SIMPLY DEFERRED THE LOOP, OR 2) THE WRONG LOOP MIGHT HAVE POSSIBLY/ACCIDENTALLY BEEN ENTERED IN THE AMIS AS THE FAULTY LOOP. I KEPT THIS THOUGHT IN THE BACK OF MY MIND, PLACED THE #2 ENG OVERHEAT DETECTION SWITCH INTO THE 'B' POS AS REQUIRED BY THE MEL, AND PERFORMED A FIRE TEST. THE ENG AND APU OVERHEAT/FIRE PROTECTION SYS TESTED NORMALLY WITH THE FIRE PROTECTION PANEL CONFIGURED PER THE MEL, SO I CONTINUED WITH MY SOP'S. THE FLT WAS NORMAL UNTIL THE DSCNT INTO ZZZ. AT APPROX 15000 FT, WITH THE THROTTLES JUST ABOUT TO HIT THEIR IDLE STOPS DUE TO A DSCNT, I NOTICED THAT BOTH MASTER CAUTIONS ILLUMINATED WITH AN OVERHEAT/DETECTION INDICATION ILLUMINATED ON THE L SYS ANNUNCIATOR PANEL. I LOOKED DOWN AT THE FIRE PROTECTION PANEL AND NOTICED THAT THE ENG 2 OVERHEAT LIGHT WAS ILLUMINATED. I ASKED MY FO TO PULL THE QRC FROM THE GLARE SHIELD. AS SOON AS HE HAD OPENED THE QRC, THE ENG 2 OVERHEAT LIGHT WENT OUT. I IMMEDIATELY SUSPECTED A PROB WITH THE ENG OVERHEAT/FIRE LOOPS. WE DID NOT RUN THE QRC BECAUSE THE ENG 2 OVERHEAT LIGHT WENT OUT SO QUICKLY. BOTH THE FO AND I FELT, AFTER DISCUSSION, THAT AN ACTUAL ENG OVERHEAT DID NOT EXIST. UPON LEVELOFF FROM THIS DSCNT, A MIN OR TWO LATER, THE THROTTLES CAME UP TO CRUISE PWR AND AGAIN THE #2 ENG OVERHEAT LIGHT BRIEFLY ILLUMINATED. I THEN SELECTED THE #2 ENG OVERHEAT DETECTION SWITCH TO THE 'NORMAL' POS AND THE #2 ENG OVERHEAT LIGHT IMMEDIATELY WENT OUT. BOTH THE FO AND I FELT, AFTER DISCUSSION, THAT AN ACTUAL ENG OVERHEAT DID NOT EXIST. I THEN GAVE THE ACFT AND THE RADIOS TO THE FO TO TRY TO OBTAIN A THIRD OPINION FROM MAINT. I THEN SENT 2 OR 3 VERY QUICKLY TYPED (RUSHED) MESSAGES TO MAINT EXPLAINING THAT WE HAD HAD SOME VERY BRIEF #2 ENG OVERHEAT INDICATIONS, BUT NOTHING APPEARED OUT OF THE ORDINARY EXCEPT FOR THE THROTTLES BEING ABOUT 1 KNOB WIDTH APART AT EQUAL PWR SETTINGS, NOT UNCOMMON ON THE B737-300/500 ACFT. IN BTWN THE ATTEMPTS OF TRYING TO CONTACT DISPATCH AND MAINT, I ASKED THE PURSER TO TAKE A CURSORY LOOK AT THE #2 ENG TO SEE IF ANYTHING APPEARED UNUSUAL. THE PURSER SAID THAT NOTHING APPEARED UNUSUAL. THE RESPONSE TIME FROM MAINT VIA ACARS WAS SLOW AND WITH A MIDFIELD R DOWNWIND FOR RWY 33L AT ZZZ RAPIDLY APCHING, I DECIDED THAT IT WAS TIME TO SIMPLY LAND THE ACFT AND DEAL WITH THESE PREVIOUSLY OBSERVED SPURIOUS INDICATIONS ON THE GND. THERE WAS LITTLE DOUBT IN MY MIND OR THE FO'S THAT AFTER HAVING DONE MULTIPLE FIRE TESTS, AFTER HAVING SEEN ONLY VERY FEW, SPURIOUS, BRIEF ILLUMINATIONS OF THE ENG #2 OVERHEAT LIGHT, AFTER HAVING SEEN NO ABNORMAL ENG INDICATIONS, AFTER HAVING NOT SEEN THE ENG #2 OVERHEAT LIGHT EVEN ILLUMINATE BRIEFLY FOR THE PAST SEVERAL MINS DESPITE SEVERAL PWR CHANGES, AND AFTER HAVING REALIZED THAT I NEVER RECEIVED ANY ENG #2 OVERHEAT INDICATION WITH THE ENG #2 OVERHEAT DETECTION SWITCH IN EITHER 'NORMAL' OR 'A' AND ONLY VERY INTERMITTENTLY IN 'B,' THAT AN ACTUAL ENG #2 OVERHEAT WAS EXTREMELY UNLIKELY AND THAT SOME SORT OF FAILURE OF THE ENG AND APU OVERHEAT/FIRE PROTECTION PANEL HAD OCCURRED. I CALLED MAINT ON MY CELL PHONE AND AGAIN STARTED TO PERFORM A FAULT/INOP TEST WHILE I WAITED FOR A MAINT PERSON TO ANSWER. WHEN I PERFORMED THE FAULT/INOP TEST THIS TIME, WE RECEIVED COMPLETELY ABNORMAL RESPONSES FROM THE ENG AND APU FIRE PROTECTION PANEL FOR A FAULT/INOP TEST. IN ADDITION TO THE NORMAL FAULT/INOP TEST INDICATIONS, WE RECEIVED AN ENG #2 FIRE WARNING AND AN ENG #2 OVERHEAT LIGHT WITH THE FIRE BELL. I RELEASED THE FAULT/INOP TEST BUTTON AND ATTEMPTED THE SAME TEST WITH THE SAME INCORRECT INDICATIONS AS THE RESULT. WITH MAINT NOW ON THE PHONE, I DID SOME MORE FAULT/INOP AND ENG AND APU FIRE/OVERHEAT TESTS, WITH MIXED RESULTS OF SUCCESS AND FAILURE. OBVIOUSLY, SOMETHING WAS VERY WRONG WITH THE ENG AND APU FIRE PROTECTION PANEL AND MAINT IMMEDIATELY DECIDED TO GND THE AIRPLANE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE ENG FIRE PROTECTION PANEL WAS SUSPECT AS THE TESTS WOULD PASS AND THEN FAIL WITH NO POSITIVE CONCLUSION. THE FLT CREW RECEIVED NO DETAILS ON THE FIX FROM MAINT.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.