some thoughts
I've got the same engine in my '87 H31, and I've been through the raw water cooling system from top to bottom in the last year or two to solve an overheating problem. They can be kind of sticky... like the old saying that 'you find it in the last place you look.' 1) From the sound of your problem, since your strainer was clogged, it's possible that there might be a fair amount of weed growing near the intake. This would be an easy fix... ask your diver. On my boat, the raw water line to the impeller will actually siphon freely if it's primed and unobstructed. [see note 7] 2) The line between the raw water strainer and the impeller can partially collapse if it's old, or if it's been replaced with ordinary 'heater hose.' You might consider replacing it with reinforced hose. 3) Your impeller is probably working OK. You probably, like most folks, replaced it at the first sign of trouble. Try this: get a couple of feet of hose (5/8"?) and hook it up to the intake side of the impeller, put the other end in a gallon container full of water, and see what happens. The water should be pumped out of the container within a couple of minutes. This takes the raw water intake plumbing upstream 'out of the loop,' so you might want to try this first. 4) Conversely, connect the hose to the output side of the impeller. Put the other end into an empty container. Run the motor for a few seconds and there should be a substantial flow of water. If not, your problems lie upstream. 5) A clogged heat exchanger will always cause overheat problems. It's easy to pull of the forward end cap and inspect visually. Remember to get new O-rings, etc, for reassembly. Note that this problem would not be likely to produce 'burping.' Unless you're drinking beer. Seriously, though, the 'burping' could be a result of an unusual mix of raw water and exhaust gas, so it could be almost anything. 6) Another common problem could be the 'nipple valve,' as I believe some people call it. It's the elbow fitting that directs the raw water from the heat exchanger into the exhaust via the mixing elbow. The mixing elbow itself may also be clogged (worst case scenario in my view.) Take the hose and attach it to the intake end of the heat exchanger and force water through the engine using a garden hose and a spray nozzle (preferably a spray nozzle with a bullet shaped nose.) Water should flow relatively freely, within reason. If not, one of the three items listed above may be clogged: The nipple valve (easy), the heat exchanger (relativelly easy) or the mixing elbow (relatively hard). 7) The same garden hose/spray nozzle check can also be used to check and clear items 'upstream' of the impleller. Stick the nozzle into the hose that attaches to the intake side of the impeller. Water should, again, flow freely. This, being my last suggestion, might also be tried first. Then, the last place you look might be the first place you look. [see note 1] Lastly, since I have a different boat, all of the above might be gibberish. In that case, I apologize. Good luck- Rick J
