Overheating MR-1 HO Engine, Stuck Thermostat, and Housing Corrosion.

[SX230HO]

So my new to me 2005 SX230 HO fired right up after I bypassed a faulty lanyard kill switch. While running the engines in the driveway with a garden hose hooked up for a few minutes with some revving here and there to get the engines warmed up to verify new oil levels, the port side engine overheated. I assumed it was the high outdoor temps, low garden hose pressure, and too much revving that caused it to overheat so I put it in the water the next day. Unfortunately, we were in the water less than 5 minutes when the same port side engine overheated while docking the boat.

Once home, I removed the thermostat housings and noticed a ton of calcium build up and corrosion on the housings. Here is a photo of the two side by side, but does it make sense that the overheating engine has the thermostat "plunger/plate" sticking out? I assumed sticking out would indicate the passage is open and water can flow, but I also heard running without a thermostat or one stuck open doesn't provide enough back pressure to keep cool water inside the block long enough to cool the engine. Does that make sense why just the port side engine would overheat? And if this really is stuck open, I don't understand why water wasn't coming out of the stern drain hole from the large diameter hose coming out of the thermostat housing. I'm concerned that might imply water isn't getting through the engine block at all and may be clogged internally.





I have already ordered two replacement thermostats (the cheaper 6CE-12411-00-00 version). Unfortunately, after several rounds of a heated vinegar and water solution inside my ultrasonic cleaner and lots of wire brushing, I was able to get it down to bare metal but I see a lot of corrosion, pitting, a poor thermostat sealing surface, and even a break in one down tube. I assume it's time to replace them as well? I have found several used MR-1 HO housings on eBay that look to be in much better shape at a decent price. I'd rather go used because a new assembly will be much more money.



I'm concerned how much corrosion I'm seeing. The previous owner ran it in brackish water, but said he always flushed it. Should I be concerned internal cooling channels inside the engine block have similar pitting and corrosion? And is it possible some passages are blocked? I was thinking of running the engines and flushing water through a garden hose without the thermostat housings installed yet to allow water to flow out through the system to verify water is flowing properly. Would Salt-Away solution being flushed through help even though the boat was last run two summers ago? It might be a moot point.

Also worth mentioning is that the pickup screens in the jet nozzles/impellers are not clogged and I unblocked the pee holes on the side (2 out of 4 were clogged up).

 

[Neutron]

So my new to me 2005 SX230 HO fired right up after I bypassed a faulty lanyard kill switch. While running the engines in the driveway with a garden hose hooked up for a few minutes with some revving here and there to get the engines warmed up to verify new oil levels, the port side engine overheated. I assumed it was the high outdoor temps, low garden hose pressure, and too much revving that caused it to overheat so I put it in the water the next day. Unfortunately, we were in the water less than 5 minutes when the same port side engine overheated while docking the boat.

Once home, I removed the thermostat housings and noticed a ton of calcium build up and corrosion on the housings. Here is a photo of the two side by side, but does it make sense that the overheating engine has the thermostat "plunger/plate" sticking out? I assumed sticking out would indicate the passage is open and water can flow, but I also heard running without a thermostat or one stuck open doesn't provide enough back pressure to keep cool water inside the block long enough to cool the engine. Does that make sense why just the port side engine would overheat? And if this really is stuck open, I don't understand why water wasn't coming out of the stern drain hole from the large diameter hose coming out of the thermostat housing. I'm concerned that might imply water isn't getting through the engine block at all and may be clogged internally.

View attachment 236775View attachment 236777

View attachment 236778

I have already ordered two replacement thermostats (the cheaper 6CE-12411-00-00 version). Unfortunately, after several rounds of a heated vinegar and water solution inside my ultrasonic cleaner and lots of wire brushing, I was able to get it down to bare metal but I see a lot of corrosion, pitting, a poor thermostat sealing surface, and even a break in one down tube. I assume it's time to replace them as well? I have found several used MR-1 HO housings on eBay that look to be in much better shape at a decent price. I'd rather go used because a new assembly will be much more money.

View attachment 236779

I'm concerned how much corrosion I'm seeing. The previous owner ran it in brackish water, but said he always flushed it. Should I be concerned internal cooling channels inside the engine block have similar pitting and corrosion? And is it possible some passages are blocked? I was thinking of running the engines and flushing water through a garden hose without the thermostat housings installed yet to allow water to flow out through the system to verify water is flowing properly. Would Salt-Away solution being flushed through help even though the boat was last run two summers ago? It might be a moot point.

Also worth mentioning is that the pickup screens in the jet nozzles/impellers are not clogged and I unblocked the pee holes on the side (2 out of 4 were clogged up).

Brackish is just as corrosive as salt. And yes the rest of the cooling passages are just as bad. All you can hope for is new thermostats and housing keep it from overheating.
Exhaust manifold that is restricted will make it overheat fast since the water enters there first and travels through the engine after

 

[SX230HO]

Brackish is just as corrosive as salt. And yes the rest of the cooling passages are just as bad. All you can hope for is new thermostats and housing keep it from overheating.
Exhaust manifold that is restricted will make it overheat fast since the water enters there first and travels through the engine after

Thanks for the reply. A new thermostat and housing on the overheating port side engine won't help me since water wasn't even making its way into the housing and out the back of the boat. This is the thermostat that was stuck open and water wasn't flowing out the drain hose and the stern drain hole while the engine was running and the garden hose flush port was hooked up.

I've been reading up on CrankyGypsy's closed loop flush procedure, but was hoping to start with something more simple like RYDLYME Descaler and a regular flush. I've also heard reverse flushing through the engine block and exhaust separately would be more thorough in pushing out stuck crud and deposits. I'm concerned about getting water/descaler into the cylinders and still trying to figure out the best way to do this.

 

[Neutron]

Thanks for the reply. A new thermostat and housing on the overheating port side engine won't help me since water wasn't even making its way into the housing and out the back of the boat. This is the thermostat that was stuck open and water wasn't flowing out the drain hose and the stern drain hole while the engine was running and the garden hose flush port was hooked up.

I've been reading up on CrankyGypsy's closed loop flush procedure, but was hoping to start with something more simple like RYDLYME Descaler and a regular flush. I've also heard reverse flushing through the engine block and exhaust separately would be more thorough in pushing out stuck crud and deposits. I'm concerned about getting water/descaler into the cylinders and still trying to figure out the best way to do this.

Doubt it will work. Looking at that thermostat, you most likely have to start with removing the exhaust manifold and inspecting the water jackets. They are most likely packed and no amount of flushing will break it up. The head side could be clogged as well.
Check your t fitting that comes from the pump and inspect it for restriction

 

[Speedling]

My boat was originally brackish owner used. It was bad. Same overheat issue. I replaced one thermostat and tore into the exhaust pretty decently to physically scrap away everything I could. In Bimini right now and the other side (this is many years later. Like 10?) Is now giving an occasional overheat. Water is going through all 4 pee holes. Is yours? If on water it's going through all 4 then you are probably "fine" for a time but should work on asap. Thermoatats are cheaper than engines.