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Curtis B.
Curtis B., Technician
Category: Agriculture and Farm Equipment
Satisfied Customers: 28093
Experience:  Technican turned service manager on multiple lines of equipment and rental equipment.
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Kubota L3400 tractor. Able to work the tractor for about 30

Customer Question

Kubota L3400 tractor. Able to work the tractor for about 30 minutes then will loose throttle response and quit running. Let sit about an hour will restart but not run very well. Realized the fuel transfer pump was not working. Drained fuel system, installed new fuel filter and fuel transfer pump, put in clean fuel. The tractor does not run any different. Temporarily connected clear fuel hose from fuel filter to transfer pump to injection pump also from injector return to fuel tank to visibly check fuel flow or air infiltration. Good fuel no air to the injection pump. Bleed system through jet start valve. When the tractor begins to run rough and loose power I can open the jet start valve get a flood of air through the injector return line, close the valve and it run for a few more minutes and then loose power again. Looking second opinion. Unless I am missing something I believe the injection pump may have been damaged from engine running with a bad fuel transfer pump.
Submitted: 13 days ago.
Category: Agriculture and Farm Equipment
Expert:  Curtis B. replied 13 days ago.

Hi,let's see if we can work together to get to the root of your problem! will try my best to help!

Expert:  Curtis B. replied 13 days ago.

There is a return line in the top of the fuel filter, that runs directly back to the tank. For test ,pinch this tube off and see if engine runs correctly.

Customer: replied 13 days ago.
Please explain what is being accomplished by closing off the return line. What should I anticipate happening. The tractor is at job site I will have to try that tomorrow. What else can I look for after that this test.
Customer: replied 13 days ago.
Please explain what is being accomplished by closing off the return line. What should I anticipate happening? The tractor is at job site I will have to try that tomorrow. What else can I look for after that this test?
Expert:  Curtis B. replied 13 days ago.

Tractor has two return lines, one is the one that comes off the top of the injector nozzles, and is the one you find on all the tractors. The return off the filter, back to tank, is only on a few of the Kubotas. I don't know why it is there or needed, but I do know it can cause fuel supply problems, as it bypasses back to the tank, instead of going through the injection pump. If engine runs, then we know where the fuel is going!

Customer: replied 12 days ago.
Hello, I ran the tractor blocking off the fuel filter return line and it made no difference. Do you know what the pressure specification is for the transfer fuel pump?
Expert:  Curtis B. replied 12 days ago.

4 to 6 psi.

Expert:  Curtis B. replied 12 days ago.

Engine has a stop solenoid, could it be keeping engine from running at full rpm? Back t off and test. See below.

Customer: replied 12 days ago.
Fuel pump pressure is just over 4.5psi at idle and nearly 6psi at 1500 rpm. I have a clear hose on the injector return line going to the tank. It has minimal return flow. After the engine has warmed up and begins to loose power, I notice large air bubbles working there way thru the fuel that is in the line. Yesterday I had the engine running with the stop solenoid removed and capped off. It did not make a difference, the engine would still loose power. It would only idle. After it sits for several hours it will start, run normal until it reaches operating temp, then start to power again.
Expert:  Curtis B. replied 12 days ago.

The air has to come from somewhere , and everything after the transfer pump is under pressure. Seems like the injection pump would not be damaged by lack of fuel from transfer pump.

Expert:  Curtis B. replied 6 days ago.

Fuel tank is vented, not drawing a vacuum?