Why Is My Engine Overheating?

Yanmar diesel engines are extremely simple and there are very few possible causes of overheating.

1. Clogged radiator fins. This is an EXTERNAL clog of the fins meaning that air can not blow through the radiator and cool the water. Use compressed air to blow out all of the grass and dirt clogging the fins. DO NOT use a pressure washer or you can (will) damage the delicate radiator fins. 
2. Clogged radiator passages. This is an INTERNAL clog preventing the coolant from getting to the fins in order to cool the water. When minerals build up in the tiny passages of the radiator they can block enough of the water flow that the radiator can no longer cool the water. A radiator shop can SOMETIMES boil out these minerals but we have not had good results. This problem can often require a replacement radiator. To test for this type of clog, get the tractor up to temperature and then run your hand along the front side of the radiator (not the side with the fan!) to check for spots that are noticeably hotter than others. The radiator should be a fairly consistent temperature across the entire surface. If you feel a colder than normal spot then that area is not getting water circulation.
3. Working the tractor too hard. If the tractor is constantly being bogged down it could just be that you are overworking the poor little fella. The harder the engine is worked - the more fuel the governor sends to the engine and the hotter the engine gets.  
4. Collapsed hose. It can happen that the inside of a radiator hose can collapse and block the flow of coolant. 
5. Stuck thermostat. Very few 'YM' series Yanmar tractors actually have thermostats. Most models were designed by Yanmar to operate correctly without one. If you do have one of the models that has a thermostat (*most are in the list below) then it could be possible that your thermostat is stuck closed. 
6. Special Note: If you have a Yanmar model without a water pump (YM135, 155, 1500, 1700 and some others) then you need to be sure you are not using more than 25% antifreeze during the summer. A 50/50 mix in those tractors that use a thermosiphon system is too thick and will not circulate correctly. 

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 * This is a list of most of the Yanmar models that have a thermostat YM140, YM142, YM146, YM147, YM169, YM180, YM187, YM226, YM276, YM336, YMG1800, YM1802, YM1810, YMG2000, YM2002, YM2010, YM2310, YM2620, YM2820, YM3220, YM4220, YM4300, most F series models.