Have Boat Questions? Ask a Boat Repair Expert.
Hi and thank you for your question.
If one engine is overheating and the other one isn't, it sounds like there is a restriction in the cooling line (hose) running from the jet pump to the engine. I would suggest you disconnect the cooling line at the engine and blow compressed air through the line backwards towards the jet pump. It should flow fairly easily, but keeping in mind it is appx. 6-8 ft. in length so there will be a bit of restriction. A best case scenario would be it seems to be restricted at first and then clears. Since the jet pump drive is acting like a big vacuum cleaner sucking up anything within about 2' of the intake, you could have picked up a shell or a pebble or even some sand clogging the line.
The overheating on the flush hose begs the question, at how many RPM's? A garden hose does not flow enough GPH to cool the engine at high RPM's. It should not overheat at idle indefinitely but at mid throttle and up it certainly might and is not necessarily indicative of a problem. If is is overheating at an idle, then it sounds like a blown head gasket and not a cooling system problem. IF that is the case, you need to do a compression test on the engine and see what that shows.
Let me know what you find.