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Discussion Starter · #1 ·
I am still having an overheat issue with the Isuzu DH100 main engine. I will recap what has been done so far.
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The heat exchanger was overhauled 2 years ago while taking the boat from Seattle to the USVI and the engine has always run about 190 degree's at 1500 rpm. 6 weeks ago I replaced the raw water impellor as routine maintenance prior to a sail over to the BVI. The main engine overheated while in the BVI when the oil cooler sprung a leak. We sailed back only using the engine to dock and then sent the oil cooler and water pump to MER to be overhauled as they don't have those parts available anymore. We put on the newly rebuilt parts and the engine seemed to run normal at idle and at the dock. When we took the boat out the first time after the repair, we ran the engine at 1500 rpm the engine overheated after about 15 min. I thought maybe the raw water through hull didn't get opened all the way after changing a hose into the raw water strainer. That was not the case. We next suspected the thermostat and took that out which appeared to be closed. We put the system back together after double checking the raw water impellor and filled the fresh water system with about 6 gallons. I thought that the last time the engine overheated the freash water might have boiled out the six gallons I had to add. There is no apparent water in the oil so I don't think it's a head gasket or other major engine issue.
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I was told to check all the hoses and it is possible that a hose could be collapsed and causing restriction. I was also told that the engine could overheat with the thermostat out.
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We will preasure test the system today and check the hoses. Please tell me your thoughts and what I can do to figure this problem.
 

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Some engines will overheat with the thermostat out (mine will). It depends on the installation. You can check the thermostat by putting in a pot of water on the stove. Turn the heat on and see if it opens as the water apporoaches a boil.
 

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Is it really overheating?

Two tests in one---

Remove BOTH the thermostat and the temp sender--

Boil a pot of water and take to the engine compartment--

Insert a meat thermometer, the thermostat and the temp sender(hooked to the wires)--

Now with the key on you should see the thermostat open wide, the same temp on the gauge in the cockpit as the one in the pot and watch each operate as the water cools.

If you have an overheat alarm you will know when it comes on or off.

A quick and easy way to check the control and reporting units of your cooling system.
 

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Discussion Starter · #4 ·
I am sure the engine is over heating, but not sure about the thermostat opening up. I was just told I could have a blown head gasket or a cracked head and to fill the water back up and idle the engine looking for air bubbles. Or it could be the heat exchanger gone bad.
 

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Could also be that the inside of the hear exchanger is gonked up and partly blocked. That was the problem I had and I used this stuff call barnacle buster which cleaned it out. I had no overheating last year.
 

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Six gallons of liquid seems like a lot to loose with out it comming out somewhere.

Do you have a hot water heater?

Air in the cooling system could be your problem.
 

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Discussion Starter · #7 ·
Yes I have a hot water heater, but that seems to be working fine. I am told now to put some dye or food color into the fresh water and run the engine and see if any of the dye comes out the exhaust and that would tell me my heat exchanger is the problem. I will also presure test the system to see if the head gasket is blown or a cracked head
 
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