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Chasing my tail with overheating!


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Wondering if anyone has had similar issues with overheating. I have a 1999 Mercruiser 7.4 MPI with a 160deg. thermostat. Normal operating temp. at higher RPM's has 

always held steady at 175deg. I noticed that when I was winterizing it in the fall, I wasn't seeing water flow out of the two ports at the gimbal housing the way it should. I

replaced the raw water pump housing, impeller and wear plate after backflushing the system several times. Good water flow after that and finished winterization. Come

spring, seemed good for the first couple short trips. Ran out to the nuke plant from I-Bay to fish and while I was motoring back, the temp climbed enough to set the alarm

off and the engine compartment was full of white smoke. Didn't want to chance it so I got towed back. Belt looked fine and still not sure where the smoke came from. I noticed

a small drip coming from water pump. Pulled it apart and it was already showing signs of wear. Replaced the impeller, wear plate and housing again. Now, any time I go over

3000RPM or so, the temp starts to climb above 175deg so I back off. I heard the shaft seals in the raw water pump could be sucking air. I replaced the "entire" pump assembly

hoping to squash the problem. Better, but similar. Coupled a clear hose onto the input hose going into the thermostat manifold and I can see dime sized air bubbles passing

through so I know it is drawing air in from somewhere, just can't chase it down. My current situation is that i can hold 3800RPM and it seems to hold a temp around 180deg....

as long as I get to 3800RPM very gradually. If I try to plane quickly, the temp climbs right up. All I can think is that the suction is collapsing a hose or something and is restricting

the volume of water? With the clear hose attached and discharging into my fish box, it has a consistent flow of water but it does not increase proportionally when I increase the

RPM as I expected it would. It actually looses pressure. I replaced all of the bellows and upgraded the water intake hose/hard plastic elbow in the Spring, new thermostat,

pulled risers (both free and clear), pulled circ. pump (looks fine), completely new raw water pump, I backflushed the system multiple times and checked the screens in the

oil coolers, all clear. I even brought my compressor down and pressure tested everything inside of the boat while spraying every connection with soapy water, nothing. While

I had the compressor there, I blew out all of the lines including back through the outdrive in case there were any mussels or anything. Everything seems unrestricted. I still

fish multiple times a week but with this restriction. Any words of wisdom are appreciated!




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I had a similar situation could not figure out what issue was. I have a closed cooled system and the water canister seal had a small rip in it. Just enough to pull air in system. Would make the motor get warm because my water pump would loose prime. My guess you are getting air somewhere. 

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I had a similar problem with my 4.3 Alpha One. Drove me crazy for months. Turned out there was some corrosion in the lower unit that was letting exhaust gases into the water intake. I could stay below 3000 rpm all day no problem but as soon as I went above that my temp would spike and fluctuate heavily.

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