Need help with Kubota KX040 FUEL PROBLEM

   / Need help with Kubota KX040 FUEL PROBLEM #1  


Elite Member
Apr 21, 2012
Cascade Mtns of WA state
Kubota B-series & Mini Excavator
It won't run at full-power.
It stumbles and dies periodically (often).
I am able to work it lightly at about 1/3 throttle, but whenever the control system pours the coal to it, it stumbles and often quits.
When not stumbling, the engine is perfectly smooth.
I'm not a diesel mechanic but I may be on the path to becoming one (just for this machine). ;)
Fuel filter looks recent. Poured it out into a pan and no sediment appeared.
Air filter is recent (looks new too).
This problem is on-going for years, Kubota dealer unable to fix it.
Kubota dealer replaced the injectors at about 400 hours which did not eliminate the problem, which has become much more frequent.
I think possibly they have a harder time identifying the problem because the Kubota dealer can't dig with it.

2016 KX040-4, 1400 hrs, 9800 lbs

Not my machine. Owner is letting me borrow it to dig at my place in hopes I might find the problem and he knows I'm a persistent sob.
He needs to lend it to a genuine diesel mechanic, but that opportunity has not arisen.
But I'm happy to have it here even if I have to run it at 1/3 throttle.


There's an air bleeder valve at the top of the fuel filter block.
Opening it often ejects some air.
I put a clear hose onto it and eject into a fuel can, and eject air, until it's fully clear of air.
But later the problem returns and I check the ejector valve and there's air bubbles in the fuel stream.
It acts (to me) like there's a candy wrapper in the tank that periodically gets sucked onto the inlet. Then it stumbles, the machine lurches, and the wrapper washes away, clearing the inlet.

What I've done:
Clean water separator. It shows no air bubbles in the visible part. It has a new o-ring.
Removed and cleaned the fuel tank. Removed the fuel level sensor, so I could LOOK inside the tank. There was only a very tiny amount of sediment/water in the bottom. I made it so clean you could drink out of it. Tank is frickin' clean.

I blew out the line from the water separator to the tank with compressed air.
There is a vent line to the tank that has a one-way valve. I sucked on it and it flows one way (to the tank) cannot blow out to it.

On the fuel separator (which is under suction from the fuel lift pump) one of the nipples is 'ever so slightly' loose. I can wiggle it. It seems like it can let in air, but can't be much. This nipple fitting is like a glued-in/crimped in fitting, a permanent nipple on the separator.

One question I have, is, does a modern Kubota have an air ejection process capable to eject air? The air I'm ejecting from the fuel filter (manually) ..... can the system do this on it's own?
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   / Need help with Kubota KX040 FUEL PROBLEM #2  
You found the problem, the loose nipple on the water separator outlet. If there are no visible bubbles in the clear part of the separator the problem has to be between it and the fuel filter where the air is found. Exactly where the loose nipple is. Diesels will not tolerate the steady introduction of air into the fuel supply.
   / Need help with Kubota KX040 FUEL PROBLEM
  • Thread Starter
thanks for your attention RickB!


Here's the system from the Messick's site.
Dang it doesn't have the injector pump, I'll go get that pic.


I took the separator off. The wiggly outlet holds vacuum, it cannot be the leak. There must be an O-ring in there. Nevertheless I eliminated the whole water separator with the copper tube -- and I'll go dig again but I'm not optimistic.....

OK so you are saying the system cannot have ANY bubbles in it, that the system does not automatically eject air?

Here's my idea: Put a long clear hose between the filter and the injector pump and route the hose past the operator seat.
Where I can watch for bubbles passing by to the injector pump.


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   / Need help with Kubota KX040 FUEL PROBLEM #4  
Diesels will not tolerate the steady introduction of air into the fuel supply.

That is what I said.
You might think there is an o-ring sealing that nipple. I think there is not. One of us is mistaken. Your test results will be interesting.
   / Need help with Kubota KX040 FUEL PROBLEM
  • Thread Starter
Dunno. It holds vacuum. Maybe it takes little sips of air under vibration.
But it's outta there, replaced by a copper tube for now.

I was moving forward on my project to watch (while operating) for bubbles going by (thru a clear tube) to the injection pump.
I started the lift pump to pre-fill the tube and noticed the flow is almost nothing out of the filter. So thats a big clue of course when a machine could be starving for fuel.
:thumbsup: Clues are good. :thumbsup:
Filter looks "new", so we didn't suspect it, but could be plugged with algae or something?

I ran the lift pump for awhile (ahead of the filter) and it was pretty slow but then started belching out air and the flow increased. The air depleted and it doesn't seep like any more air is entering the system. So the pump was cavitating, not building flow or pressure.

I took the filter block off and inspected it carefully. No apparent problems.
But where do I get another filter on a Sunday....??

I'll try putting it back on and see if the flow out of the filter increases.

OK theres some Sunday reading for ya. Hope you all are having a good weekend!


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   / Need help with Kubota KX040 FUEL PROBLEM
  • Thread Starter
I bypassed the filter block.
I sealed the wiggling nipple on the water separator.
No change. It still burps air out of the bleeder.

So I proceeded ahead with my plan to add a clear line to observe the fuel that goes directly into the injector pump.


That hose is the last hose, directly to the injector pump
All that air is going into the injector pump.
It seems to run OK, I tracked it up a steep hill but I didn't push a pile of dirt with the dozer blade.
I'm gonna dig with it tomorrow and see if it can put out work with that much air in the fuel.
But even so, this CANT be right.
There is no reason Kubota would allow so much air into the fuel, there HAS to be a leak somewhere.


The fuel system is complicated, there's a fuel cooler, and couple check valves, and maybe the worst thing is, you have to remove the counterweight ( 1069 lbs! ) to get to the (fuel) lift pump.

So I bought the danged shop manual.
And as usual, I should have bought it last week.

This just keeps getting deeper and deeper.
But just the 4-hour roundtrip to the Kubota dealer (2 times), and the machine owner's lack of faith that they can fix it when they failed before, he wants me to keep trying.
What I will get out of this is he'll likely lend gthe machine to me anytime, so this is good.:thumbsup:

So what I need help with now is, how can I find a suction air leak in the fuel system?
If I could pressurize it and spray soap on the hoses,
but if it's only a suction leak not a pressure leak that may not reveal it, but it may.

Are there any other tricks?
   / Need help with Kubota KX040 FUEL PROBLEM
  • Thread Starter
The owner took it to the Kubota dealer.
Kubota tech said there are lots of other things it could be in addition to air, such as the regen acting up.
Which I never even considered.
   / Need help with Kubota KX040 FUEL PROBLEM #8  
On the 24 valve Dodge Cummins diesels, they went with a electric lift pump mounted on the left side of the motor. [ no cam lobe to run a mechanical ] The VP44 injection pump required positive fuel pressure, and this lift pump pulled fuel from the frame mounted tank. Some of the trucks developed leaks in the flex line going from the frame mounted steel fuel line, up to the motor. It would suck in very small amounts of air which a lot of folks pulled their hair out finding. Mounting the lift pump onto the frame, along with replacing the line going to the motor, generally cured the problem [ all though the lift pump itself was a pain in the arse ] So any flexible line could be suspect. So any line/device in the area where the fuel is being ' sucked ' is where I would look. [ good luck and stay safe ... !!!!!!!!!!!!! ]
   / Need help with Kubota KX040 FUEL PROBLEM #9  
Did the owner ever get to the bottom of this? We're having identical issues with a kx040
   / Need help with Kubota KX040 FUEL PROBLEM
  • Thread Starter
I ran the fuel out of the water separator, thru a long clear vinyl hose past the operator seat so I could see the bubbles going into the injector pump. I conclude that the Kubota injector pump can tolerate or eject some air. I don’t know how much - but it tolerates much more than I expected. Long strings of bubbles.

I found several possible leaks on the suction side. Fixing them didn’t solve the problem.
1) water separator had no o-ring. I was certain this was the culprit. Nope!
2) Elbow fittings at the water separator “wiggled”. I pulled them out, sealed very carefully with Permatex gray and put them back in. Tested with vacuum and it was tight. Still there was air entering the system somewhere.
3) Fuel “wetness” at the lift pump suction hose nipple. This is very difficult to access behind the counterweight. I fussed with it as best I could, moving the clamp to a “more proper” location on the nipple bead. And still it had bubbles passing by (in the clear hose) to the injection pump.

Dealer#2 told the owner that the machine had the “wrong fuel filter”, that the 040 has a “specific required fuel filter dependent upon the _machine serial number_” and said the whole problem was this. And said not to let non-qualified people mess with his eqpt. Well Dealer#1, 4 hours away who put the wrong filter on, and replaced the injectors, holding the machine for weeks, might be non-qualified too I say.

OK good luck! Simplest thing to try is the serial# specific fuel filter. And please let me know if this solves it. I’m curious if Dealer#2 was blowing a little smoke, perhaps not at me but at Dealer#1 (a notorious Kubota sales-price undercutter 4 hours away that other dealers hate on). My impression of Dealer#2 was the tech was very good. I think they like to promote their services, but even so, he was the real thing.


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