I have a mini excavator (Terex HR16). I am trying to understand how the fuel tank assembly works.

First some context:

I was experiencing intermittent engine sputtering, sudden loss of power, and sometimes the engine dying. A mechanic told me it was very probably the fuel system. The fuel filter was changed very recently, so he said there was a good chance it was due to a clog in the tank or fuel line from that. After a bit of experimentation with the easily-ish accessible parts of the fuel line, I decided to remove the tank and clean it out. I did manage to get the engine running again, but I don't fully understand how.

Refer the image below for points A through E. The actual question is in the next section.

  • A: Goes to fuel fill pipe. Obviously, this is for pouring diesel into the tank. (yes, this plastic strip should probably be replaced with a hose clamp - not the point here, though)
  • B: Attaches to the side of the fuel fill pipe near the top. I am guessing this is serves as an airvent, to allow air to escape when filling the tank.
  • C: Has a metal diptube inserted into the tank. The line goes to the water separator at the bottom of the machine after which the fuel pump is located. Tank is marked "OUT" at this port. Obviously this is where fuel flows from tank to engine system.
  • D: Wires coming out. Large diameter pipe going into tank. I'm guessing this is the level meter.
  • E: Thin bendy plastic tube going into tank. Tube is simply pressfit through a gasket-like plug. Not sure where the other end leads. Tank marked with "IN" at this port. From comment: This is probably a feedback line from the injector.

After cleaning and reassembly the engine didn't run immediately. I could see that the water separator wasn't filling, even though I let the fuel pump run for a bit. I would have thought that the fuel pump would create the required suction in C and pull fuel from the tank. It did not.

I then re-removed the fuel line from the water separator – which is located lower than the fuel tank – and blew into it. I could hear bubbling from the tank.

Fuel immediately started flowing unaided by any pump. Now drenched in diesel, I quickly reconnected the tube to the water separator.

After this, and a few more minutes of fuel pump action to fill the other end of the line, engine sprang to life, no problem.


I don't understand at all why the fuel started flowing. To my mind, it seems I would have emptied the dip-tube and hose with the blowing. Having my end open to the atmosphere, I would simply think the fuel would settle at the tank level within the diptube. In stead, it rises to the top of the tube and starts flowing like a siphon.

What initiates this siphon effect?

EDIT A comment suggests that by blowing into the tank, I created an overpressure that forced fuel up the dip tube and started the siphon-effect.

This makes sense to a point: A good blow would have enough displacement to fill the diptube and start the siphon effect. However if the system was sealed (with the fuel cap on) this would also mean that underpressure would build up in tank as fuel was drawn, which should stop the flow at some point. So this really only makes sense if either the system is only semi-closed, so that any pressure difference can be relieved, but only slowly, or – perhaps – E also functions as a vent, but perhaps only when the engine is running.

Fuel tank assembly, Terex HR16

  • 1
    Is it possible that the tank was full to the top of the fill pipe? This would give the extra height to start a siphon. Another possibility is that if the tank is relatively well sealed (including the fill pipe) you may have pressurized it when you blew into the hose. This might have been enough to start the siphon. – BobT Jul 18 at 17:09
  • 1
    Pipe E could serve as the feedback line, routes fuel back from the injector return line to the tank – Martin Jul 18 at 17:51
  • Tank was definitely not full. Having just emptied it, I know it had about 10L in it and its nominal volume is 30L. – AdamAL Jul 19 at 6:00
  • Interesting thought about the pressurization. The fuel inlet cap feels quite loose, so I would be surprised if actually held pressure. On the other hand it may just be the plastic cover of the cap that sits loose on the actual cap. – AdamAL Jul 19 at 6:07
  • @BobT, I added a paragraph with thoughts on the pressure build-up theory. – AdamAL Jul 19 at 7:23

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