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Discussion Starter #1
I have a problem with over heating on a Volvo MD6A diesel.
When the engine is out of gear and running the temperature stays in the proper range. Lots of water out the exhaust. New thermostat, new impeller on raw water pump, and clear intake. However when the engine is in gear and under medium load the temperature gage eventually rises to high temp zone and then the needle drops back to normal for a few seconds then back up near to the red limit. It keeps repeating this and will near the red zone if the throttle is not reduced. If the throttle is reduced to a little above idle the temperature drops back to normal range while under light load. Any comments would help:hothead
 

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Just a SWAG from insight gained by reading posts about troubles; but, it sounds like a mixer/elbow clog,ta me. just enuff flow w/o load; but not enuff when powered up. Figgers, AS it's (one of) the most expensive and. Difficult tosource part onna Volvo, it seems.
I luvVolvo autos of; but by gosh; the marinized do seem to have problems with parts.. Sorta like expensive anything..IF ya haft a ask the price, Ya can't afford IT (the maintainance) ;)
 

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Discussion Starter #4
The heat sensor is wired off a plug temperature sensor on the exhaust manifold. I changed the wire terminal with a fresh one.

I will check to see if it is a semi-plugged water exhaust elbow. Very easy to do.
For the past 26 years I have had my own water exhaust elbow design made up in third world machine shops all over the South Pacific and Micronesia. Gone through 4 so far. Probably 1/16 the cost of Volvo's price.
 

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correct thermostat?

You say it is a new thermostat. Did you have the same behaviour with the previous thermostat? The fact that the temperature cycles indicates the thermostat is opening and closing, the fact that it gets down to normal temperature indicates the cooling capacity is ok. I am thinking the temperature setting of the thermostat is too high.
 

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Discussion Starter #7
The old thermostat was doing the same up down swings that's why I replaced it only to see the same results. The Volvo replacement thermostat was the same part number had the same temp rating. I will next check if the water exhaust elbow is clogged. Currently I'm thinking about taking off the exhaust manifold to check if the cooling ports on the engine block are plugged with crud. Also thinking about using a chemical de-scaler.
 

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Yes its almost certainly the thermostat cycling. You "just" have to figure out why. A restriction in the cooling circuit (like the exhaust elbow) is a good place to start.
 

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Discussion Starter #11
Checked for blockage in the water cooled elbow. No problem there. So now it is off with exhaust manifold to see if the water passages are scaled and blocking water flow at higher RPMs.
 

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Discussion Starter #12
OK, the last step was to descale the manifold and engine. I used a ice maker descaler by Manitowac . I let it soak and work over night.... some fizzing and bubbling and then drained and flushed the manifold. Lots of small black chunks and calcium buildup. Voila no more over heating and the temperature gage stays in the proper zone when engine is in gear.
 
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