AFTER ABT 4.3 HRS ON AN OVERWATER FLT, THE CREW RPRTD THEY HAD 15 MIN OF FUEL REMAINING. SUBSEQUENTLY, 3 OF 4 ENGS LOST PWR FM FUEL EXHAUSTION. DRG AN EMERG LNDG, THE ACFT HIT A LEVEE & WAS EXTENSIVELY DMGD. MOST OF THE WRECKAGE CAME TO REST IN A DAINAGE CANAL. BODIES OF THE CREW WERE RCVRD FM THE WATER ON 7/22/88. DRG AN INV, NO FUEL WAS FND IN THE FUEL TANKS & NO FUEL SPILL WAS EVIDENT. RECORDS SHOWED THE ACFT HAD DEPD EL SALVADOR WITH 7 HRS OF FUEL. NO REASON FOR THE LOSS OF 2.7 HRS OF FUEL WAS VERIFIED; BUT ABT 1 MONTH AFTER THE ACDNT, AN EMPLOYEE OF THE OPERATOR RPRTD FINDING AN OPENDRAIN VLV IN THE WRECKAGE, INSIDE THE #4 ENG NACELLE. A METALLURGICAL EXAM INDCD THE VLV HAD BEEN IN AN OPEN PSN FOR AN EXTDD TIME. NO GND PSNL SAW FUEL DRAINING FM THE ACFT DRG START, TAXI OR TKOF. THE CREW HAD NO CTL OF THE VLV IN FLT. THERE WAS EVIDENCE THE #1, #2 & #3 ENGS WERE NOT PROVIDING PWR DRG IMPACT. ALL X-FEED VLV CTLS WERE FND IN X-FEED PSNS. ETHANOL WAS FND IN THE PLT'S & COPLT'S BLOOD, BUT THERE WAS EVIDENCE THAT IT WAS THE RESULTED OF POSTMORTEM CHANGES.