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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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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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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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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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