remove the oil pump, pull the coupler out, then using a pair of needle nose pliers and pull the oil pump driven gear out, if you see melted plastic on the gear, you know whats going on. Pay attention to the coupler, it should fit loose within the hole, if its tight, you just found the cause. mercury finnaly figured out the coupler was swelling up and locking up the drive gear, they eliminated the sensor and went with a smaller coupler on late model 2.5 EFI's and carb motors
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