Thermostart - theory of operation

ggb3

Member
Hi Y'all,
I have wondered about the thermostart theory of operation. In particular the fuel "manifold device" that has the fuel line going to the
intake manifold (mounted on bulkhead just in-front of the fuel tank). I see no electrical connections to it so how/when does it send fuel
by way to the intake manifold? Is it always sending some fuel and the thermostart heater is just not energized unless the procedure is
followed for a cold start? Anyway, just curious as mine has never worked and being in south Mississippi, I have used other methods for
the rare need for cold starts. I am doing a major "overhaul" of the tractor and will buy a new thermostart glow plug and fuel line to get
it operational, so knowing will be beneficial to the process.

Have a good day y'all,
George
 

If you see no wire going to it... It will NOT work. There should be a wire from the ignition switch that activates the thermostart when the ig switch is in a special position. the electrical current slowly heats up a heat coil.. as the coil gets hot, a spring loaded ball valve will slowly back off and release fuel onto the heat coil. Often the thermostart is gooked up from non use and the sping/ball valve will not be able to move and release fuel.. sometimes the fuel line back to the return line is gooped up and needs flushing as well. the fuel line connects to a tee on the return line that is higher up.. on the return line, so it will put a little fuel into the thermostart line. Early models even had a little reservoir at the tee to hold additional fuel to feed the thermostart.
 
On how it works, we went through the operation on this forum (within the last couple of weeks as I recall).

On my '65 3000, in the neck of the tank is mounted a small (⅛ to ?" ID) tube that drops down to a "T" on top of the Thermostart Reservoir. This line returns "leak off" fuel (Soundguy mentioned) from the injectors to the fuel tank which comes in to the side of this T fitting and is routed back to the tank.

Fuel is supplied to the injectors at a given rpm that may not be squirted into the cylinders because of a light load condition on the engine.......you have to oversupply the injectors for times when the governor says gimme some more fuel to feed this loaded condition. This over supplied fuel is referred to as "Leak Off" fuel.

The other outlet of the T fitting goes directly into the Thermostart Reservoir which holds a quantity of fuel for the Thermostart system prior to engine roll over. The bottom of this little 2" dia x3" (roughly) closed container is plumbed over to the inlet of the Thermostart plug in the front end of the intake manifold. Engine roll over will activate the injector pump, pumping fuel to the injectors, the excess of which will be diverted back to the TS reservoir and when full will move on up the inlet pipe to the neck of the fuel tank returning the excess there.

Obviously, in the design of this system, engineers realized that you have to satisfy the fuel requirement of the TS system when the engine isn't running. In the engine start process, TS is activated for 30 seconds, give or take depending on ambient temp, before rolling the engine. Then you have to refresh this source for the next time the engine is started requiring TS assistance and Leak Off fuel was a likely candidate.......keeps it separate from the main fuel supply.

Take this and what was discussed recently and you should know how it works.

Good luck.
 
"Fuel is supplied to the injectors at a given rpm that may not be squirted into the cylinders because of a light load condition on the engine.......you have to oversupply the injectors for times when the governor says gimme some more fuel to feed this loaded condition. This over supplied fuel is referred to as "Leak Off" fuel."

That's total HOGWASH!

The fuel coming from the leakoff ports on the injectors is just simply fuel that has sneaked past the tight tolerances between moving parts inside the injector and has NOTHING to with "an oversupply at light loads".

Matter of fact I'm pretty sure it is just the opposite, at light loads injection time (when the internal leakage occurs) is going to be SHORT compared to a longer/larger quantity injection cycle at full load.
 
A thermostart with NO electrical connection? I've never seen one. CAV/Lucas/Delphi makes both 12 and 24 volt models, in low, medium, and high flow capacity. There are two coils in each one, one coil starts the flame, one opens to allow fuel to enter. What ruins them usually is lack of fuel at the inlet connection, the fuel inlet coil overheats, and can't close the fuel flow off when it cools like it's supposed to. Older tractors did have a small storage supply reservoir that was fed by the return lines, and any excess then went back to the main fuel tank. My MF TO-35 and 85 both have that small reservoir. Long, MF, Ford, and David Brown were all the most common tractors with thermostart units.
 
The actual glow plug part of the thermostart system of course has a wire to it, the part I am talking about is a small tank of sorts with three fuel lines to it and has some operational involvement in supplying fuel to the intake manifold. This unit has no electrical connection and that is what I am wondering about, how does it "know" to send fuel to the intake manifold when needed.
 
(quoted from post at 12:02:41 11/17/16) "Fuel is supplied to the injectors at a given rpm that may not be squirted into the cylinders because of a light load condition on the engine.......you have to oversupply the injectors for times when the governor says gimme some more fuel to feed this loaded condition. This over supplied fuel is referred to as "Leak Off" fuel."

That's total HOGWASH!

The fuel coming from the leakoff ports on the injectors is just simply fuel that has sneaked past the tight tolerances between moving parts inside the injector and has NOTHING to with "an oversupply at light loads".

Matter of fact I'm pretty sure it is just the opposite, at light loads injection time (when the internal leakage occurs) is going to be SHORT compared to a longer/larger quantity injection cycle at full load.





WWWWWhen the pressure to the injector drops to a set level, the needle starts closing and then exposes the return line port during the closing cycle. This is due the pump cycle, and then the injector opening and then the resultant pressure dropping as the pump is no longer supplying pressure.. And as the needle closes... This momentary venting to the return line allows any fuel pressure left, to leak off or drain off and eventually end up going back to the tank. AAAAAAND the closing of the injector is set at various pressures pending on the tractor and system.. guessing at 1000 lbs or higher,, so yes... even though the amount is very very small, its still at pressure and will pump a couple of drops until the pressure drops a bit as the needle closes the rest of the way.

older injectors had a opening pressure of 1600 lbs and closing at 1400 or so. it varies by the design of the injector needle, nozzles, and spring tension. Newer system have even higher pressure to get even a more finely atomized shot of diesel, and the pizo injectors and even to pre- injections and long and short duration injection cycles during a single firing cycle.
 
This is straight from CAV, the company that made the thermostart for Ford (and others).
a242974.jpg
 
I covered that in my response. It is a reservoir filled by returning oil from the injectors. When it gets full the oil goes on up the top tube and back into the fuel tank neck. On the outlet of the reservoir, when the thermostart coil gets hot it opens an internal valve and lets the fuel dribble out over the glowing coil and ignite. Once the engine is rolling over, the injector pump is pumping, feeding fuel to the injectors and the surplus is refilling the reservoir. Purely a passive system and needs no operator intervention.
 
In the diagram that Bern provided, it's referred to as the "Header Tank". The Ford manual calls it what I called it. Notice no electrical connections, and the difference between the diagram and my 3000 is that the return goes into the neck of the fuel tank, not the tank proper. No biggie unless you are following the tube out the top of the reservoir and get confused because it goes to the neck, not the tank proper.
 
No electrical connections? The diagram I provided clearly shows a "+" and a "-". The negative side is case ground, the positive side comes from the key switch.
 
Comment directed at ggb3's original post where he is talking about the "header tank" located in front of the fuel tank "has no electrical connections", which it doesn't, as witnessed in your diagram. Obviously the TS device has to have power to heat.
 

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