1976 OMC 165 Overheating

diogee2

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Alright 1976 OMC 165 overheating at odd intervals with other strange symptoms. Temp will get stable at 160 ish and then slowley start to rise and inlet hose felt hot so i thought i was sucking up exhaust in the swivel housing. I replaced the thermostat and checked the seals and repacked the swivel housing and it was fine. But i put on a clear hose on the inlet line from the outdrive to motor. This time no air bubbles coming thru but after 10 minutes it seems to build up back pressure and stop the water flow. I shut it off and alot of air, steam,and bubbles blow backwards thru the inlet line. Once stable i start it again and water flushes thru cools it to 160ish and slowly builds pressure again and stopping flow. No idea where to start with this one. Thanks ahead of time.
 

southkogs

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Have you changed your impeller? If that's not done properly, you can suck all kinda' stuff up. The other thing to see is if the circulation pump on the engine itself is moving water.
 

diogee2

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I changed the inpellar a season or two ago. With the clear hose on you can see the impelar moving water to the motor ill have to put a clear piece on the outlet hose from the moror to manifold to check flow and bubbles i guess. I was just puzzled by the back flow of steam and water after the motor was shutoff and why the water flowed fine initially then a back pressure built causing almost like a vapor lock situation stopping water flow and the over heating.
 

southkogs

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My initial thoughts is that the impeller is shot ... two seasons is about as long as I like to let 'em go. Is the temp rising when you put the throttle to it, or is happening just sitting there?
 

diogee2

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Initially at idle it runs fine i can see water coming thru the hose from the impellar to the motor then after about 10 minutes you can see the flow stop as if there is back pressure and it overheats. Then once i shut it off the water and bubbles blow backwards from the motor to outdrive. Once the pressure blows off i can start it up and water flow is normal for another 10 min then startes all over with back pressure.
 

southkogs

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Is the circulation pump on the engine itself moving water?
 

diogee2

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Alright circulation pump moves water fine. I ordered the impellar kit and new seals for the swivel housing. I replaced all the hoses with clear ones to watch it and on the garden hose it ran fine and stayed cool but as soon as i turned the motor off and water off the manifold heated up the water inside and blew water back into thermostat housing and into the supply line instead of out thru the riser/exhaust. It seemed like the riser might be blocked so i pulled it and it was pretty bad inside restricting water flow. Picking up a new one of those today. My questions now are, is the risor the culprit in my original overheating problem and when i removed the thermostat cap to check it i noted that the water port that goes thru to the manifold is pretty small and would limit the amount of water until the thermostat opened. Is this normal or has anyone moded it to make it a bit bigger?
 

southkogs

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Sorry - I brain faded a little on this: are you on the lake when this is happening or on a hose in the driveway? Or "yes." :)
 

diogee2

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Ha. On the hose all is fine until i shut it off then there is a blow back of steam from manifold thru thermistat housing into supply line but only after i shut off the water. When i am in the bay it overheats after about 10 mins at idle and the water just stops flowing liike its vapor locked. The minute i turn off the motor it does the same back flow from manifold to supply line. I think my riser was blocked with a weak impellar but thats a guess i have all the pieces will swap out romorrow and try again.
 

southkogs

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Gotcha' ... I'm back to impeller. Though it does sound like somethin' mighta' been up with your riser. Good luck.
 
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