Overheating issue.

[Kola16]

I have no idea how old the manifolds are.

Have you pressure tested these manifolds? You mentioned there was water in the exhaust part of the manifolds so that is something to suspect. Leaky manifolds can cause overheating. If they sat for that many years, there is a very good chance the water jacket is rusted through. Go to Autozone and rent a radiator pressure tester. They return all your money once you bring it back. You will also need a sheet of gasket material. Block off all the ports of your manifolds. You'll need the gasket you just picked up at Autozone to block off the ports between the manifolds and risers. When you have all the ports blocked off, you can then test if the manifolds hold pressure. I almost guarantee those manifolds were not freshly put on by the previous owner. More likely those manifolds are at least 15 years old. The only thing that this would cost you is a sheet of gasket. If they hold pressure, great one thing checked off your list, but with how old those manifolds are, they are due.

 

[cosmiccharlie1976]

Have you pressure tested these manifolds? You mentioned there was water in the exhaust part of the manifolds so that is something to suspect. Leaky manifolds can cause overheating. If they sat for that many years, there is a very good chance the water jacket is rusted through. Go to Autozone and rent a radiator pressure tester. They return all your money once you bring it back. You will also need a sheet of gasket material. Block off all the ports of your manifolds. You'll need the gasket you just picked up at Autozone to block off the ports between the manifolds and risers. When you have all the ports blocked off, you can then test if the manifolds hold pressure. I almost guarantee those manifolds were not freshly put on by the previous owner. More likely those manifolds are at least 15 years old. The only thing that this would cost you is a sheet of gasket. If they hold pressure, great one thing checked off your list, but with how old those manifolds are, they are due.

I agree that is a good way to test them with pressure. The water that was originally found in the manifold must have been from when I pulled off the riser, since there was no rust at all in the manifold.
If the manifolds are the issue I can run the boat with muffs and disconnect the hoses to the manifold and I should run cooler.

 

[Kola16]

If the manifolds are the issue I can run the boat with muffs and disconnect the hoses to the manifold and I should run cooler.

I would not advise that. I would think that that could overheat the manifolds causing them to crack, which would make replacing them mandatory.

 

[cosmiccharlie1976]

I would not advise that. I would think that that could overheat the manifolds causing them to crack, which would make replacing them mandatory.

How would I go about pressure testing the manifolds?
Another thing I'm thinking is, if they are not clogged and I have water coming out the exhaust how am I overheating because the water comes into the thermostat housing, and flows through the water pump into the motor, through the thermostat which is where the temp sensor is and out to the exhaust manifolds.
Is it more possible the temp sensor is faulty?
When I ground it out the gauge goes all the way to the top, I thought that means it works but is there a better way to test it?
I have a temp gun is there a place to aim it to get an accurate temp reading?

 

[Kola16]

You need to pressure test your manifolds regardless of an overheating issue. You test them with the free tool you get from Autozone, and the cheap sheet of gasket material that will go between the manifold and the riser to block off that port. You then put the pressure testing tool into the hose that goes to the manifold. You bought an old engine, and manifolds are parts that need periodic replacing. Rust happens the most where water is trapped and you can't see it. If your manifolds leak, water will leak into your cylinders. Since water is incompressible, it raises the temperature for one (PV=nRT), but it also causes far worse damage to your engine.

Yes it is very common that thermostats give a false reading. You can use an IR temp sensor ($25 or so) pointed right at the thermostat housing to see what it is reading. Your boat engine will also tell you that it is overheating. It will run rough and sound unhealthy.

 

[cosmiccharlie1976]

You need to pressure test your manifolds regardless of an overheating issue. You test them with the free tool you get from Autozone, and the cheap sheet of gasket material that will go between the manifold and the riser to block off that port. You then put the pressure testing tool into the hose that goes to the manifold. You bought an old engine, and manifolds are parts that need periodic replacing. Rust happens the most where water is trapped and you can't see it. If your manifolds leak, water will leak into your cylinders. Since water is incompressible, it raises the temperature for one (PV=nRT), but it also causes far worse damage to your engine.

Yes it is very common that thermostats give a false reading. You can use an IR temp sensor ($25 or so) pointed right at the thermostat housing to see what it is reading. Your boat engine will also tell you that it is overheating. It will run rough and sound unhealthy.

So I still need to pressure test the manifolds on monday. But I just idled the engine for 5 min. The gauge read 240 which it maxes out at. But my IR temp gun was pretty stable at 160 at the base of the thermostat housing. Even the manifolds were reading 160.
Now I'm wondering if I got a bad temp sensor when I replaced it.

 

[Kola16]

160 F on the outside of the T-stat housing seems a little high to me on a raw water-cooled engine at idle. The inside should be 160 F, but you might be okay. What temperature T-stat is in it? Your gauge is definitely off.

 

[cosmiccharlie1976]

160 F on the outside of the T-stat housing seems a little high to me on a raw water-cooled engine at idle. The inside should be 160 F, but you might be okay. What temperature T-stat is in it? Your gauge is definitely off.

160⁰ t stat.

 

[Kola16]

Okay so is the only indication that you are overheating your dash gauge? We can already conclude that your dash gauge is wrong if you shoot the temperature at your T-stat housing and your sender and it is 80 F difference. If you have a 160 F thermostat, and your thermostat housing is reading 160 F, then that is about right. Your engine will tell you that it is overheating. Excessive steam, loss of power, sounds different, etc.

 

[cosmiccharlie1976]

Okay so is the only indication that you are overheating your dash gauge? We can already conclude that your dash gauge is wrong if you shoot the temperature at your T-stat housing and your sender and it is 80 F difference. If you have a 160 F thermostat, and your thermostat housing is reading 160 F, then that is about right. Your engine will tell you that it is overheating. Excessive steam, loss of power, sounds different, etc.

Thanks I already order the quick silver temp sensor(admittedly I did go with a cheap one on amazon) so hopefully that works or I will replace the Gauge and see how that works.

 

[Scott06]

Thanks I already order the quick silver temp sensor(admittedly I did go with a cheap one on amazon) so hopefully that works or I will replace the Gauge and see how that works.

Also you replaced the thermostat and gasket. The oem merc thermostat housing gaskets have rivets in them to ensure continuity between the thermostat housing and engine. You may not have that if aftermarket gasket is used. More resistance to the gauge will indicate higher temps. if no rivet gasket was used try running a ground jumper to the housing or sensor body to test it

 

[cosmiccharlie1976]

I did get the oem gaskets with the copper rivets in them. And when i ground the sensor to the block the temp gauge jumps upto 240. As I believe it should. also I do not get an audible alarm for overheating.
When I first turn the key I get 2 beeps. So I would expect to hear an alarm when the gauge goes over 200.

 

[Scott06]

I did get the oem gaskets with the copper rivets in them. And when i ground the sensor to the block the temp gauge jumps upto 240. As I believe it should. also I do not get an audible alarm for overheating.
When I first turn the key I get 2 beeps. So I would expect to hear an alarm when the gauge goes over 200.

Good looks like a bunk sending unit

 

[cosmiccharlie1976]

Problem fixed the temp sending unit was bad. Thank you so much to everybody that chimed in.