What was found
(CAN) DURING FIREFIGHTING OPERATIONS, WAS IN THE CIRCUIT AROUND A LAKE DOING THEIR PRE-WATER BOMBING LANDING CHECK, WHEN THE OTHER CL215 IN THE CIRCUIT RADIOED THAT ENG OIL & SMOKE WAS COMING FROM THE RT ENG OF THE FIRST CL215. THE CREW SHUTDOWN THE ENG & FEATHERED THE PROP. DECLARED AN EMER & MADE AN UNSCHEDULED LANDING BACK AT THE AIRPORT. INVESTIGATION DISCOVERED THAT THE CAP THAT SHOULD HAVE BEEN INSTALLED ON THE ENG PRE-OILING PORT WAS MISSING. THE ACFT HAD APPROX 30 HOURS SINCE ITS LAST MAJOR CHECK. IT APPEARS THAT THE CAP WAS NOT TIGHTENED ON PROPERLY AFTER PRE-OILING THE RADIAL ENG IN THE SPRING, PRIOR TO ITS FIRST START UP. NEW CAP WAS INSTALLED & ACFT RETURNED TO SERVICE.