Inside A Yanmar Thermostart

What is a Thermostart?

A Thermostart is a very efficient glow plug used in most of the Yanmar tractors. Instead of relying on just a heating coil like other glow plugs- this unit also drips a small amount of diesel fuel (fed from a small plastic container mounted just above it) onto the red hot coil. This produces thick smoke, lots of heat, and occasionally small flames. All of these things help the engine to start in the coldest weather.

So how does it work?

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

When you are ready to start the tractor the glow plug gets energized through the terminal (D) and heats up the element (F). This causes the hollow tube at the bottom of (A) to expand. When the tube expands it also pulls the set screw away from the check ball (B) 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.

Why do they go bad?

Over time the thermostarts, just like any glow plug can go bad. This is typically because the heating element, which is sort of like a very heavy duty light bulb, burns out. The check valve can also start to leak over time. Running the thermostart for too long (over 10-15 seconds) at one time can also damage it.

Priming A new One.

It may be necessary to bleed the air out of a new thermostart in order for it to start producing smoke and fire. This is done by installing the new thermostart & filling the reservoir bottle with fuel. Then cycle the thermostart on 10 seconds then off 30 seconds. It may take 10-20 cycles to bleed the air out of the line & thermostart.