Mercury Optimax 150 overheat at idle - 2CoolFishing
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Old 01-20-2021, 02:36 PM   #1
salty redneck
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Mercury Optimax 150 overheat at idle

Started getting continuous beep alarm while at idle on my 2010 Opti 150. This has happened about once a year for the last 4 or 5 years, and I always change out the water pump and seems to rectify the situation. Never gets hot while running, but always at idle after a long run.....like when I'm backing truck into water to load up. Changed water pump and thermostats (water pump only 2 months old). No dice. Changed poppet and cleaned strainer to air pump inlet. Same issue. Idle down and the alarm would come on. I could put it in neutral and bump the rpm's up and alarm would go off. Changed the poppet. Still alarming at idle. Unfortunately my boat is not equipped with SmartCraft gauges, so kinda guessing and throwing parts at it. Now it's personal.....and I'm ******....but there are definite advantages to being one of the most hard headed humans on the planet. Got my ohm meter, temperature sensor chart, laser thermometer and slapped the muffs on it. Sensor readings were matching pretty close to what the thermometer was telling me - temp got up above 190 when the alarm came on. Scratching my head and doubting everything I have learned about working on outboards for 40+ years. BTW, the water pressure improved after changing the poppet; idling at 3 psi and WOT 10+ psi. Good stream out of the tattle tale. Now I'm REALLY ******, so figuring it's acting like the thermostats were stuck, and absolutely not believing that 2 new thermostats were not functioning correctly, I removed the hoses from the thermostat housings and blew into hose.........nothing. Plugged up tighter than a snails rear. Took the hoses off where they terminate below the power head and a little muddy water came out. Hooked water hose up to fittings and couldn't get flow with 45 psi. Took a wire and poked and prodded, then hooked water hose back up. Finally blew the plug loose and flushed each fitting for 5 minutes. Hooked it back up and now she's running like a champ. Highest temp with laser thermometer is 161 near thermostat housing. I've read alot on the internet about Optimax's overheating at idle, and while I'm not saying this is the problem on all of them, it's certainly something to watch for. Also something I learned is the ~1X1 square hole directly below the flush fitting and tattle tale is where the water comes out when the thermostats open. If you are not getting steam/water out of this opening, the motor is below thermostat setpoint, thermostats are stuck, or there is a plug between the thermostat housings and this outlet. This can be masked by the fact that when RPM's increase, the poppet opens and allows flow out of the power head, cooling it even thought there is no flow past the thermostats. Something very important for Optimax DIY'ers is checking the strainer that screws in the lower port side of powerhead that feeds water to the air compressor. Mine had quite a bit of shell in it, and seeing as the compressor is $950 bucks, the last thing I want is having coolant flow limited to this. From the compressor it discharges out of the tattle tale, so if your water pressure appears normal but this stream is weak, CHECK THE STRAINER. FYI - it's plastic........don't get rough with it and DON'T overtighten. Another thing I learned is how easy it is to pull the lower cowling and how important this is in not exhausting your daily supply of "no-no" words when trouble shooting and working on cooling system issues. Hope this saves someone the weeks of heartache I just went through in trying to figure out this apparently common issue.
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Old 01-20-2021, 03:10 PM   #2
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I forgot to mention that I flush my motor religiously after each trip...........as soon as I back in the driveway. That made it hard for me to believe that it was a plugging issue, but sure enough!!
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Old 01-21-2021, 10:19 PM   #3
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a certified Mercury mechanic could probably figure it out
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