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

[VancouverBoat]

I have a 95 OMC 5.8 (351 ford) that is raw water cooled. Replaced raw water pump and flushed hoses after raw water impeller broke down. Removed thermostat as previous owner had put auto stat in wrong place. Great water flow from raw water pump and all the way through the system to the exhaust. Can run no problem even under load without overheat but temp is too cool all the time (never more than 110 degrees). Installed new thermostat housing with new marine thermostat thinking that I would get a higher/better operating temperature. At the dock, the engine would still not heat up to more than 100. After running on the ocean for about 25 minutes under load the temperature suddenly started to to climb and I shut down when it would not stop at 220. I removed the thermostat and temp immediately came down and I ran the rest of the day at 110. Can anybody give me any ideas - I am stumped. Manifolds and risers are not blocked, temperature gauge is working.

 

[kenny nunez]

Add a short length of clear hose to the incoming water supply to the thermostat housing. You want to look for bubbles running with the thermostat installed. Also add some clear hose between the thermostat housing outlet to the exhaust manifolds. If bubbles appear before the thermostat housing then there is a supply leak or after the thermostat housing a head gasket leak. Also with a severe head gasket leak the gas pressure will be so great that it will actually stop the incoming water supply and the clear hose that supplies the thermostat housing will have no water in it.

 

[VancouverBoat]

Kenner, thank you. Will try that. Would the bubbling issue not also create the same problem without the thermostat? There is no water leak before the housing and there is no water in the oil at all.

 

[kenny nunez]

Yes, on the incoming side. Just take it for a cruise with the clear hoses attached and watch if you see any bubbles. Have you checked that the thermostat is working by putting it in water on a stove? Is the thermostat held in place with an “O” ring inside the housing? Try to post a picture of the housing,

 

[VancouverBoat]

I tried with clear hoses and no bubbling. Yes, I tested the thermostat in how water and it opens as it should. I attached pic of thermostat housing. Yes, there is an o ring holding the thermostat. Housing, thermostat and o ring are brand new.. Engine runs cool all day with out the Thermostat - just not with it.

 

[kenny nunez]

You really got me scratching my head. Usually whenever there is problem causing an over heat it happens quick. I still think that there is a head gasket leak. With the thermostat in place idling at the dock the engine should reach 160*. The fact that it stays so low makes me wonder why. What is the water temperature? I guess where you are it is pretty cold and that may be the reason it does not heat up and the by-pass port may be the reason.
Does this boat stay in the water? Is the sea water pump belt driven or is it in the drive?

 

[VancouverBoat]

Yes it is strange. Water is 66 degrees. Right now boat is in marina slip yes. The raw water pump is belt driven on engine, and there is a separate engine circulating pump of course.

 

[kenny nunez]

When the raw water pump impeller was replaced was all the impeller blades accounted for? I know that the Mercruiser Bravo series have been known to get pieces of marine growth or small sea shells in the hose that joins the drive to the gimbal housing and the only way to know is to pull the drive and remove the hose.
I had a similar problem with the Donzi and re located the intake through the bottom of the boat and it worked out perfect.
If you do that you can join the drive supply hose with the through the bottom suction with a “Y” pipe which is what I did.
I figure that the P O may have overheated the engine in the past and could have even caused a head to crack. What causes it to happen is when the engine gets hot and by slowing down sometimes allows water to push out the steam bubble and it shocks the inside of the engine.

 

[VancouverBoat]

Thank you for help. Yes, I got all the blades. After clearing that, I purged the raw water supply line from drive to to the housing by having it run into the bilge for a while. the flow is super strong and clear. I might try your clear hose idea on the hose from housing to engine circulating pump to see if there is flow when the thermostat is closed. This may also tie in with your idea about the pressure lock. It just does not make sense that it would run at dock at low temp for so long with the thermostat closed.

 

[kenny nunez]

There is one other thing that may be the problem. The circulating pump impeller could be slipping on the shaft. I have seen that happen before and the only way to check is to pull the pump off. If that is the case that would explain why the engine temperature stays low at the dock. What may be happening is the area around the temperature sender is cold but the rear of the engine may be hot. Try to get a hand held external temperature sensor and check different areas of the engine.

 

[jc turtle]

Hi Vancouver Boat, my son and I had the same bad dream with his 350 3 weeks ago. It turned out to be a 1/4 inch hole in the pipe between the lower and upper halves of the outdrive. Also, with a infrared temp gun, the gauge said 200 but the temp gun said 162. Get a gun and see what you really have for temp. I bought the gun for $25. on sale at Harbor freight.
This might be another thing to check. We did all the things you did plus a few more. Good luck. John

 

[VancouverBoat]

I checked all areas with infrared temp gun. At the dock, the only way I can get it up to temp is run it for while, turn it off and then once the residual heat has permeated to the engine, I can turn it back on at 150, but it soon goes down. The back of the engine does not go over 140. There is no leak on the supply line to the housing.

Kenny, I put a clear hose on the hose from the housing to the engine circulating pump and I was not sure how much flow I should see. It did not look that strong. I could see air at the top of the tube where is meets the housing and crud in the water looked like it did not move that fast. I took a video (see link below) You can see a piece of crud float by. As you suggested, does this mean the circulating pump may be bad? You said I may have to pull it off. Can I pull off the hose to the pump and the belts and check the rotation of the impeller inside by rotating the pulley wheel and touching the impeller wheel inside with a screwdriver or something?

However, I still hesitate about blaming the circulating pump. If the temperatures at the back of the engine are not going up, how is the water circulating if the pump is bad?. Is it the pressure from the raw water pump? I could see that the pressure from the raw water pump might be enough to keep water circulating even without the engine circulating pump at lower speed , and then not being enough at high speed when the thermostat is installed. Without the thermostat, I guess the raw water pump could be enough to run the cool water through the system at any speed. Is that what you mean?

 

[VancouverBoat]

Video link to the circulating pump with clear hose is https://photos.app.goo.gl/NfYgCLPqgAmYqSem7

 

[kenny nunez]

Try to do with the circulating pump with something to probe the impeller. The sea water pump definitely has enough volume to keep the engine cool without the circulating pump without a thermostat. “V” and jet drive boats usually route the water through the exhaust manifolds then to the engine and usually maintain 140* without a thermostat.

 

[Lou C]

In that video is the thermostat installed or not installed? Normally what you would see there is the thermostat cycling open to allow water out of the engine when the temp of the water in the intake manifold reaches the set point of the thermostat which is 160*. So if the engine is running at at temp below that, and the thermostat is installed you won't see much flow there. If the engine is running on the water hose with no load on it, the thermostat may take a while to start to open and not stay open continuously as it would if the engine was moving the boat under load. I've never done one on a small block Ford but changing the circulating pump on the small block Chevrolet engines is a very easy job, they are widely available, just make sure to use a marine quality pump if you do replace it. I have found that the bearings start to get noisy first, then maybe a small leak by the weep hole, I have never had one that stopped pumping though.

this is a mysterious problem....
no thermostat you run cold but do not overheat
thermostat you still run somewhat cold but overheat
manifolds/elbows not blocked, are you sure the exhaust flappers are in place and are not blocking the exhaust at the bottom of the Y pipe?
still the possibilities are:
circulating pump impeller slipping
leaky head gasket putting exhaust gas into the cooling water
possible blockage in the exhaust due to melted flappers

is this boat in fresh water or salt?
It is also possible you have a build up of sand silt etc in the block and heads that is slowing down heat transfer. It happens in raw water cooled engines whether they are inboards or outboards. might need to really flush the block out, I might do that if you wind up changing the circulating pump. stick the end of a water hose in the opening on one side of the block that the pump would normally bolt to and blast it out.

one last thing, this boat is kept in the water...is there any marine growth...because I had a chronic problem with marine growth on that plastic screen in the water intake in the Cobra lower unit. I had barnacles growing on that thing no matter what I painted it with. Finally I split the drive removed the water passage cover and cleaned it out, removed the screen and never replaced it. Seven years later and no cooling problems!

 

[VancouverBoat]

Thanks so much for the for the help guys. Kenny, I will try probing the impeller. Lou, the video is with the thermostat installed. Here is video without thermostat https://photos.app.goo.gl/deo8drooEV7zL2Wv5 I am pretty sure flappers are not melted/ he boat was just put in the water and there is no growth. It was flushed with fresh on the hard. So that level of low flow with the thermostat installed and closed is normal? The issue I think I am having is that the thermostat never opens because there is always enough cold water flow near the thermostat. Using the infrared thermometer, I can only get the TS housing up to to 160 (enought to open) if I run for a while and then shut off to let heat buildup. I am ready to just give up and just use the boat for the rest of the summer without the thermostat and hope for the best...

 

[VancouverBoat]

More video with thermostat https://photos.app.goo.gl/hVa4VtH1PQmyrCBt7

 

[Lou C]

Thanks so much for the for the help guys. Kenny, I will try probing the impeller. Lou, the video is with the thermostat installed. Here is video without thermostat https://photos.app.goo.gl/deo8drooEV7zL2Wv5 I am pretty sure flappers are not melted/ he boat was just put in the water and there is no growth. It was flushed with fresh on the hard. So that level of low flow with the thermostat installed and closed is normal? The issue I think I am having is that the thermostat never opens because there is always enough cold water flow near the thermostat. Using the infrared thermometer, I can only get the TS housing up to to 160 (enough to open) if I run for a while and then shut off to let heat buildup. I am ready to just give up and just use the boat for the rest of the summer without the thermostat and hope for the best...

The thermostat not opening if the engine is at idle, is really normal an idling engine with no load does not build enough heat to heat the water to the point where the stat will open and let hot water out of the block. This is also why you should never use the suck up the Antifreeze kits, because the stat won't open enough to let all the raw water out of the block while idling on a cool day. The stat will for sure open all the way with the engine running on plane. Did you test the stat and watch how much it opens? It must start opening at 157-163 or so and should open to a minimum of 5/32nds" or 3.7 mm at 182* according to my OMC shop manual.
With raw water cooling I have found that these stats get sticky after a few seasons and I now replace them at the start of each season. Usually the problem I've had is they don't close all the way and the engine runs way too cool unless up on plane.

 

[Lou C]

Watching the vid with no stat installed I see a lot of bubbles, is that what you saw? It could be head gaskets leaking and putting exhaust gas in the cooling water. Any sign of water in the cyls or in the motor oil? I'd pull the plugs and look at them any sign of corrosion on the electrodes? Then disable the ignition and crank the engine over with the plugs out. Any water expelled by the plug holes?
I had this exact problem when my engine blew both head gaskets, water in a cyl and slightly elevated temps but not overheating + water in the oil. I rigged up another test to check for exhaust gas in the cooling water, watch:

https://www.dropbox.com/s/8lz4p4e7pd...skets.MOV?dl=0

see the bubbles coming up in each funnel at the end when I shut it off, the starboard side was the one with the water in a cyl (#2) on the port side there was just a spray of water in #1.

this test has to be done fast because you might not be able to run as much cooling water to the exhaust (need 2 hoses) and I also pulled out the stat for this test to allow max water flow.

 

[VancouverBoat]

Wow interesting video. For me it is a brand new stat and I tested in in a pot. It works. There is no oil in the water or water in the oil. Plugs look good. It runs super smooth, starts right up.