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Help Me Understand Yanmar Thermostarts

Most Yanmar series use Thermostart “Glow Plugs.”


Thermostarts are extremely efficient glow plugs that help your tractor engine start in the coldest weather. With winter here, you do not want to leave “starting” to chance.

Yanmar Thermostarts drip a small amount of diesel fuel (fed from a small plastic container mounted just above it) onto the red-hot heating coil. This produces lots of heat, thick smoke, and occasionally small flames. They produce a lot of heat.

This generates far more heat than typical glow plugs that simply rely on heating coils alone.

How Thermostarts Work

The fuel line is connected to the thermostart through the fitting. The fuel is not allowed to leak out because the set screw is screwed up into the fitting and is tightened up against the check ball - forcing it against the ball seat deep inside the fitting.

When you are ready to start the tractor the glow plug gets energized through the terminal and heats up the element. This causes the hollow tube at the bottom to expand. When the tube expands it also pulls the set screw away from the check ball because the set screw is threaded into the tube that is expanding. This allows a very small amount of fuel to drip down onto the heating element which starts to smoke and burn.


Eventually a thermostart, just like any glow plug, needs to be replaced. This happens when the heating element (like a very heavy-duty light bulb), burns out. The check valve can also start to leak. You can also damage a thermostart by running it for too long (over 10-15 seconds) at one time.

Priming a New Thermostart

You may need to bleed the air out of a new thermostart for it to start producing smoke and fire. First, you must install the new thermostart. Next, fill the reservoir bottle with fuel. Then cycle the thermostart on for 10 seconds --- then off for 30 seconds. NOTE: It may take 10-20 cycles to bleed the air out of the line and thermostart.

If you need any further help or have any questions about thermostarts, tractors, or anything else, please contact your dealer, local mechanic, or call us at 602-734-9944. Please ask about our current new and used tractor supply.

Team Tractor Ranch #1 Tractor Dealer in Arizona. We sell and service most major brands of tractors including Yanmar, Kubota, John Deere, TYM, Mahindra, Kioti, Case, New Holland, Massey Ferguson, Ford, Deutz, Case IH, Farmall, International Harvester, Branson Tractors, LS, Shibura, Claas Tractor, McCormick Tractors, Valtra, Solis, YTO, Montana, and Nortrac.

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