Mercruiser 350 MPI Overheating Problems

[lvecch]

Good morning,

I have a Mercruiser 350 MPI, 2005. I have always ran it without the termostat, I am in Venezuela, so the water is pretty warm. The temp gauge never moved neither a bit. But since some months ago, while running at 3000 to 4000 RPM the temp begun to rise a bit every time I ride the boat. So far, running at 4000 RPM, the temp rises up to 165?, then when I slow down the temp goes down. I have already changed the elbows and the manifolds are OK. I inspected the hoses and they are OK. I found some pieces of an old impeller in the check valve but I removed them. The impeller looks OK. I have not inspected the circulating pump nor the ways were the water goes inside the engine. But I think even when the water cannot circulate well through the engine, it can be hot but the temp sender should be fresh because the water coming from the seawater pump goes directly to the termostat housing refreshing it.

The engine is still rising its temp at 3000 to 4000 RPM. So please somebody can advise me were to look to troubleshoot? Can the problem be the circulating pump or some obstruction inside the engine?

Thanks in advance for your help.

 

[agallant80]

Re: Mercruiser 350 MPI Overheating Problems

I would start by putting the thermostat back in. I tried to run mine without the thermostat and the guardian system started going off about poor water flow. I think its needed to build pressure in the system so water circulates proper. I know you said that you have been running without the thermostat without issue but I would get the system back to its default configuration before I started ripping things apart.

 

[NHGuy]

Re: Mercruiser 350 MPI Overheating Problems

Welcome to our forum!
I agree with putting the stat in. That might not solve your temp creep though.
It wouldn't hurt to back flush the water system from the thermostat entry toward the water pickup, there are a couple of corners in there that could hold a piece of impeller.
I assume you don't have to winterize where you keep the boat, so maybe you haven't needed to drain your block.
It would probably be smart to pull the block drains which we use in cold places each winter. Don't do that in the water though because they are below the water line and you could sink your boat.
The drains are low on each side, they are little T shaped petcocks. Remove them completely from the block and poke in the holes with a coat hanger end or something. There will be a skim of rust in there that you need to break through. That could possibly release some crud that holds back cooling.
You could also remove the hoses and check for obstructions. If the hoses haven't been off in a long while you'll want a hose remover pick to free them from the metal fittings they slide onto. You just stick the end of the pick between the metal of the hard part and the rubber of the hose. Then slide the pick around inside the hose end as much as you can reach. Otherwise they can be a bear to take off because they stick to the metal.
Also when you have the thermostat housing off make sure nothing is lodged in it either. They have a few passages that could catch debris.
If that doesn't fix it come back and give us a report!

 

[Scott Danforth]

Re: Mercruiser 350 MPI Overheating Problems

if you do not have a thermostat, you will at a minimum need a restriction orifice.

I would put a new thermostat in, flush out the motor, lines, oil coolers, and check your raw water impeller

 

[alldodge]

Re: Mercruiser 350 MPI Overheating Problems

Good morning,

I have a Mercruiser 350 MPI, 2005. I have always ran it without the termostat, I am in Venezuela, so the water is pretty warm. The temp gauge never moved neither a bit. But since some months ago, while running at 3000 to 4000 RPM the temp begun to rise a bit every time I ride the boat. So far, running at 4000 RPM, the temp rises up to 165?, then when I slow down the temp goes down. I have already changed the elbows and the manifolds are OK. I inspected the hoses and they are OK. I found some pieces of an old impeller in the check valve but I removed them. The impeller looks OK. I have not inspected the circulating pump nor the ways were the water goes inside the engine. But I think even when the water cannot circulate well through the engine, it can be hot but the temp sender should be fresh because the water coming from the seawater pump goes directly to the termostat housing refreshing it.

The engine is still rising its temp at 3000 to 4000 RPM. So please somebody can advise me were to look to troubleshoot? Can the problem be the circulating pump or some obstruction inside the engine?

Thanks in advance for your help.

:welcome: to iboats

I'm not sure you have a problem. The MPI should have a 160 degree thermostat, and having it go to 165 is not a problem when running on plane. The warm water of your area may cause it to come up a bit more which would not concern me. If you started getting up to 170 then I would look into it further. Your engine must reach 160 before it will run correctly.

 

[lvecch]

Re: Mercruiser 350 MPI Overheating Problems

Welcome to our forum!
I agree with putting the stat in. That might not solve your temp creep though.
It wouldn't hurt to back flush the water system from the thermostat entry toward the water pickup, there are a couple of corners in there that could hold a piece of impeller.
I assume you don't have to winterize where you keep the boat, so maybe you haven't needed to drain your block.
It would probably be smart to pull the block drains which we use in cold places each winter. Don't do that in the water though because they are below the water line and you could sink your boat.
The drains are low on each side, they are little T shaped petcocks. Remove them completely from the block and poke in the holes with a coat hanger end or something. There will be a skim of rust in there that you need to break through. That could possibly release some crud that holds back cooling.
You could also remove the hoses and check for obstructions. If the hoses haven't been off in a long while you'll want a hose remover pick to free them from the metal fittings they slide onto. You just stick the end of the pick between the metal of the hard part and the rubber of the hose. Then slide the pick around inside the hose end as much as you can reach. Otherwise they can be a bear to take off because they stick to the metal.
Also when you have the thermostat housing off make sure nothing is lodged in it either. They have a few passages that could catch debris.
If that doesn't fix it come back and give us a report!

Thanks a lot for the advises, I will try them this weekend on my boat and will reply with the results.

 

[lvecch]

Re: Mercruiser 350 MPI Overheating Problems

:welcome: to iboats

I'm not sure you have a problem. The MPI should have a 160 degree thermostat, and having it go to 165 is not a problem when running on plane. The warm water of your area may cause it to come up a bit more which would not concern me. If you started getting up to 170 then I would look into it further. Your engine must reach 160 before it will run correctly.

Yes, but I am afraid if I push a little bit more the throttle it would go up to 170 or more.

 

[lvecch]

Re: Mercruiser 350 MPI Overheating Problems

I would start by putting the thermostat back in. I tried to run mine without the thermostat and the guardian system started going off about poor water flow. I think its needed to build pressure in the system so water circulates proper. I know you said that you have been running without the thermostat without issue but I would get the system back to its default configuration before I started ripping things apart.

Thanks for your answer, I will try that too.

 

[lvecch]

Re: Mercruiser 350 MPI Overheating Problems

if you do not have a thermostat, you will at a minimum need a restriction orifice.

I would put a new thermostat in, flush out the motor, lines, oil coolers, and check your raw water impeller

Thanks for your answer, I do not really understand why is needed a restriction in that path, there is a hose that comes directly from the water distribution box to the elbows, with a restrction in the engine more fresh water will go directly to the elbows, not passing through the engine. But, I will try all and see.

 

[alldodge]

Re: Mercruiser 350 MPI Overheating Problems

Yes, but I am afraid if I push a little bit more the throttle it would go up to 170 or more.

If it goes to 170 then check the gauge to make sure it is reading correctly. Use an infrared temp gun to determine. As others have mentioned the thermostat is needed to keep the block full of water

 

[Scott Danforth]

Re: Mercruiser 350 MPI Overheating Problems

Thanks for your answer, I do not really understand why is needed a restriction in that path, there is a hose that comes directly from the water distribution box to the elbows, with a restrction in the engine more fresh water will go directly to the elbows, not passing through the engine. But, I will try all and see.

A thermostat restricts the flow to a specific velocity within the block. without a thermostat, the water moves too fast to do any cooling. Also, with the MPI engines, you need to have a thermostat to regulate the temp as AllDodge pointed out, or the motor runs rough.

 

[crazy charlie]

Re: Mercruiser 350 MPI Overheating Problems

As stated,put the tstat in.Without it,the faster you run the engine,the faster the water passes through the system.The faster it passes through the system,the less it cools the engine.When you slow the water inside the engine you give sufficient time for the heat to be displaced/transferred into the water.Charlie

 

[lvecch]

After many things done, I had to take off the engine due to another problem, an oil leaking. And discovered that the water hose (sea water) that comes from the leg through the transom plate, the point where it ends, the aluminum cast that rounds it, was corroded and swollen and shrinking the end of the hose. To discover it I had to remove the plastic connector that is there. I repaired it and now the engine runs fresh as time ago. Thanks you all for your help.