1996 Mercury 225 CXL, 0G408399, Quicksilver SS 14-3/4 x 21 prop, 1989 22’ Boston Whaler Outrage, this is a salt water boat that sits on a mooring and used a 1-4 days a week depending on weather and free time, gets flushed at the end of the year. Motor was given to me in spring 2020 with unknown hours (this one is counter rotating and someone bought the other with all the gauges and info) and have not had an issue till now. I've spent quite a bit of time digging through forums and the most common questions and answers to my issue have brought to this forum.
I noticed the pee stream was getting weak towards the end of last year, usually a race horse after tstats open up (last big exercise was pulling boats off a beach after a storm, had a knick in the prop so I’m sure it sucked up some sand and bits)… old impeller was totally fine but did a full water pump kit (impeller, gaskets, plates, shaft seal) this spring and started to get overheating alarm every few minutes or so at 1/2 throttle 2 people on board (mid 30s mph)… more passengers or faster speeds alarm would come on sooner, so I ran the boat for a while keeping it just barely on the throttle (15-20mph) with no issues.
Past few weeks I have been trying to further chase the issue. Rechecked the water pump, new thermostats and gaskets, new poppet, with no improvement. Took the thermostats out and back flushed with a garden hose and after a few minutes something released and went back to normal racehorse pee stream. It did improve and gained another ~10mph and can go longer till the over heat alarm comes in. Starboard bank runs ~15-20* hotter once warmed up (145port - 165starbord idling/ 155port - 230starbord right after alarm comes on ripping it around). Port side seems dead stable at any throttle and never reads over 155-ish. Starboard side right after alarm, 230 reading was at the sensor, bottom cylinder head was mid 160s, top cylinder head was 180s as well as right at the thermostat. The hot spot was right between the cylinder heads where the sensor is. I have Ohmed and checked the manual to see that it’s reading correctly even though I don’t have a gauge. After a minute of idling or turning the key off for five seconds the alarm turned itself off.
I have heard about issues in the exhaust plate; gasket issues leading to exhaust and or pressure entering the water intake, and carbon buildup. I don’t want to pull the boat and remove the powerhead midseason so I’m fine running it just on plane till I can pull it out in a in a couple months. My second thought is pulling the head to see if there is a chunk of something in one of the cavities or jackets? Is there one area I should start with and look at first? When I pulled the hoses after the thermostat to do a second flush and check for flow, I did notice the port has a smidge more flow and there was also light exhaust puffs coming up the hose to the thermostats, very minimal and equal both sides at idle, not sure at throttle or load but figured its worth a mention.
Water flow diagram for the 3L EFI but seems to be identical to my carbed if its any help
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Thanks for any ideas or suggestions
-Colin