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Old 18-03-2010, 20:13   #1
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Overheating Volvo

1980 md7a 2 cylinder volvo diesel. Raw water cooled engine. Since I have owned the boat (8 months) It has always run very cool. Less then 1/4 of the guage at all times I normaly cruise at about 2000 rpms . Last weekend while coming in fighting a 25kt wind on the nose and a falling tide, engine at 2250. It sudddeny and instantly pegged hot and shut down. Dropped anchor and checked the basics. Impeller = ok water to impreller ok . Replaced impeller just beacuse I was there. Motor cooled off started right up took the boat home at slow pace. Pulled the exhaust manifold off and found some crud in the passages, found one small passage in the head blocked it appears to be the bypass for the water from the cylinders around thethermostat to the exhaust .Cleaned the passages in the head and the manifold. Really wasnt much blockage except the one passage. After cleaning water from the hose flows freely though the manifold and and the head.
I did find that there was no thermostat installed. I installed a thermostat and the cleaned mainfold today and was supprised to find that at idle at the dock it ran up the temperature to the line between green and red.
Theres 2-3 times the amount of water coming out the exhuast as there was before my cleaning. I cant come up with any reason I could have good flow, good volume and still run hot.
Heres what I figure my options are.

New thermostat bad, could happen but I dont belive it
or the wrong temp sensor in the manifold.

Am I missing anything?
The tstat housing is warm but not hot to the touch and the exhuast water is warm not hot.

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Old 18-03-2010, 20:33   #2
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If the sending unit is after the TS housing and you can hold your hand on TS housing and the gage is reading hot you have a bad gauge or sending unit. If it is before the TS housing place you hand right there where its at or on the head/top of block temp measured with an infrared sensor runs 190 to 210 depending on load you will not be holding your hand very long there.

Had you disconnected any lines in the cooling system right before this happened to service anything?...Im thinking air in the system here if you did is why I ask.

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Old 19-03-2010, 08:12   #3
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My old Volvo would run just fine at moderate rpm but overheat at high rpm. Eventually discovered the hose from the raw water intake was not stiff enough and would collapse at high rpm. Changed hose, problem solved.
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Old 19-03-2010, 20:56   #4
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Check tubing upsream and downstream form the raw water pump...impeller pieces (from the past) can and usually do lodge in the first sharp bend or restriction.....sometimes low speed is not a higher speed, you starve for water.
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