95 omc 5.8 runs cool without thermostat but eventually overheats with the thermostat

Lou C

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well here is the thing, these are open cooling systems that are not pressurized. As such, in order for them to keep the engine cool, there has to be a flow of a high volume of water in and a high volume of water out to carry away the heat. Either you're not getting enough in, enough out or there are deposits in the cooling passages of the engine that are interfering with heat transfer. Assuming that you do not have blown or leaking head gaskets. We had this on a '98 Subaru, 2x. When the head gaskets on these blow, the exhaust gas pressurizes the cooling system and it causes overheating at anything above low speeds. They did not get antifreeze in the cyls, or out the exhaust. The exhaust gas would push antifreeze out of the radiator and overflow the overflow tank. A dead give away was bubbles in the overflow tank and a black carbony like deposit showing up in the overflow. It did this both times the HGs failed.
 

VancouverBoat

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I understand. The only thing that does not make sense is is that I do not overheat without the thermostat. I can run all day without the thermostat. Gauge says 110"* and infrared says 140 at back of the engine after 30 minutes on a plane. Put in the thermostat and runs cool all day at low speeds but overheats on a plane after 20 minutes.
 

Lou C

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Well that suggests to me that with the stat the hot water exits fast enough at low speeds but not fast enough at high speeds. That points back to either not enough volume of cool water in or not enough volume of hot water out. When was the impeller last replaced and was it an OEM or aftermarket? Are you sure the exhaust is not restricted? Other causes often missed:
a crack in the plastic fitting for the drive to transom mount hose; it gets exposed when the boat is on plane and sucks air which can cause overheating
bad water tube gaskets in between the upper and lower gear housings same thing as above
the plastic fitting cracks because of incomplete winterizing and the water tube gaskets get distorted or melted by overheating
 

VancouverBoat

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Kenny, you were right. The engine circulating pump blades had sheared right off. There was no circulation from that pump. See picture. Despite all the forums that said the circulating pump will first leak water or squeal before it is bad, it obviously can happen. I took out the thermostat sender and tried to feel the blades. Could not feel anything. Of course I did not want to have to remove the the pump, but it had to be eliminated. Will let you know once replaced and I have tried with the thermostat in.
 

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Lou C

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Wow good to see you found a very obvious cause; I wonder if someone in the past replaced it with an auto pump. The first one I replaced lasted 22 years!
 

VancouverBoat

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Lou, thanks so much for your help. For me it was not that obvious. Most people I consulted, mechanics included, were reluctant to blame the water circulating pump. They pointed to the raw water pump, blockages, etc. Of course, I did not want it to be the pump either as it is a pain to remove it and then find out it is fine. As you mentioned, yours lasted 22 years. I think this was a marine pump with stainless blades but the shaft just failed. There was no noise, and it was hard to tell from the water flow with the clear hoses because the flow from the new raw water pump was so strong. I guess because there was no circulating pump pressure, the cold raw water was getting to the thermostat even when running hard. That maybe caused the overheat with it in. In any case, have ordered new pump so I will replace and test with thermostat in and hopefully all is well now.
 

kenny nunez

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I have to tell you I have never seen one with a shaft broken! I have seen impellers broken in pieces or slipping on the shaft. There was a radiator shop in the next building to my shop and sometimes we would help each other try to figure out problems.
 

Lou C

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a good lesson..."leave no stone untouched"
or "trust but verify"
lol.
 

kenny nunez

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If you are having a problem finding a replacement water pump, just match up an automotive one but be sure to re use the stainless backing plate. If you should find one with a plastic impeller would be a perfect replacement. With a little luck you may be able to have your original pump rebuilt. There are services listed in Hemming’s Motor News.
 

VancouverBoat

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Replaced the engine circulating pump, started engine and temperature went to normal 160 at the dock, then is at about 175 on a plane. All good!. Thanks guys. With the OMC thermostat housing, it is clear that low flow from the circulating pump does not provide enough pressure to prevent the raw water pump pressure to push cold water where it should not be going.
 

Lou C

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great outcome! Here's what mine runs at....160 at idle, most it goes to is about 170 after coming off plane....
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VancouverBoat

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Guys, I wish I was not back here but I am getting some creeping overheat when I am on plane at higher rpms. I now have 140* thermostat and engine heats up and runs all day at about 150 - 170 on anything under 3200 rpms. If i run too long over 3200 i can see it start to creep towards 190 - 195. I then back down a bit and she drops down quickly to 150 -160 again. Seems to have slowly gotten worse. I am stumped as I have done all those things you should do:
- no intake blockage on drive,
- new raw water pump. good flow in.
- no blockage in power steering cooler (flushed and cleaned)
- new thermosat housing and thermostat.
- New engine circulating pump as per original issue.
- flushed engine.
- risers and manifolds checked for blockage (they are almost new but I removed risers anyway to check).

It seems worse with a 140* thermostat than with a 160* one. I changed to 140* to run at lower temps.

Anything?
 

kenny nunez

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I am beginning to think that there is a head gasket problem caused by the previous overheating cycles. Combustion gasses gradually build up in the water jackets and overcome the incoming water pressure. Try the clear hose from the thermostat housing again and watch for bubbles.
 

VancouverBoat

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Kenny, thanks. Do you mean the clear hose from the large outlet on thermo housing to the engine circulating pump?
 

Lou C

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I would try replacing the hoses that go from the thermostat housing to the exhaust manifolds with clear hose, with the boat in the water run it until you start to get that elevation in temperatures and then check for bubbles. That suggests leaking head gaskets allowing exhaust gas into the cooling water. Same problem I had but I also had water in a cyl.
 

kenny nunez

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Lou and I are on the same page and the clear hose method should tell the tale one way or the other. On post #8 I described how I solved a heating problem on my Donzi. If you have the boat hauled that would be a good time to add a through hull pick up with a sea **** and if there is room a marine water strainer.
 

Lou C

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Here's what my blown head gaskets lead to...not too bad of a job...just keep all the parts organized...keep dirt out of the engine and take your time...a few tips...use a thread chaser to clean out cyl head bolt threads so you get accurate torque on the head bolts...follow torque specs...use sealer on threads if required....a click torque wrench makes it much easier to accurately torque the bolts....
 

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VancouverBoat

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Ok, here is video of the hose to manifold at idle and at 3000 rpm. In both cases, it looks like the flow does not fully full the tube so of course air takes up the rest.The system is not pressurized so would that not be normal?. There is no overheat creep until I get up to about 3200 plus rpm. If I am on a plane and drop rpm to 3000 it drops right down even though the boat barely stays on a plane and the engine is struggling harder to keep the boat on a plane. I would have thought it would get hotter if it is working harder. At 3200, the boat is planing easy with what appears to be much less load on the engine.
Idle https://photos.app.goo.gl/MQoMJehmJEGb1AtRA
3000 rpm https://photos.app.goo.gl/r6wT4yWxPAkDuKmz6
 

Lou C

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I think that's a lot of bubbles, even though these systems are not pressurized like a closed system is, there is still pressure created by the impeller. What I'd do is a compression test, and look for signs of water in the cyls, rust on the spark plug electrodes, or any sign of water in the motor oil.
I recall doing this test years ago when I had a running warm problem (related to marine growth on that plastic water screen in the lower unit) and and did not see anything like that for the hoses from the stat housing to the manifolds.
One thing you can do is get an accurate torque wrench (click type is much easier to use) and make sure all the cyl head bolts are torqued properly, follow the exact sequence listed by Ford.
 

kenny nunez

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Is it possible to add some clear hose on the incoming side like I posted on #2 ? Also in the video was that taken with the boat on a plane? I have to agree about the load on the engine when the boat is just under planing speed. As Lou had also posted about having a restriction on the drive. Since the boat is in the water I wonder if the hose between the pivot housing and transom plate is leaking from a crack.
 
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