John Deere 755 Yanmar 3TNA72UK Issues After Running a Couple Minutes

MoparMuscl · Mar 10, 2018

I have a 1998 John Deere 755 with a Yanmar 3TNA72UK engine. It ran fine until last year when it started to act like it was out of fuel occasionally. It made it through the summer until last fall when it wouldn't run well after it was running for 5-10 minutes. The past couple weeks I have tried to use it, but it is doing the same thing. It acts like it isn't getting fuel, then stumbles at a low rpm, catches up slowly, and runs wide open again for a second or two before the process starts again. I put a new electric fuel pump on it last week, but it didn't help. I put some clear lines on the pump to filter, the filter to injector pump, and the return line from the front injector to the tank. There is no air getting in the system before the high pressure injector pump. As it starts to stumble, air bubbles start to pass in the return fuel line and go back to the tank. After several air bubbles pass and it is solid fuel in the return line, it runs good for a few seconds until the process starts again. Once I let it cool, it runs fine again for a little bit.

Has anyone run across this problem, or have any suggestions I need to check? Thanks in advance!

thepumpguysc · Mar 10, 2018

Loosen or remove the fuel tank cap.. sounds like a clogged vent creating a vacuum..

MoparMuscl · Mar 10, 2018

I've tried that a couple weeks ago...no luck. The cap has a vent as well as the tank at the cap threads.

Peter 315 · Mar 10, 2018

Did you look in the tank for any debris

MoparMuscl · Mar 10, 2018

Peter 315 said:
Did you look in the tank for any debris

No, but I don't think that would be the issue. If it were debris, it would act that was as soon as you start it up. It seems to be more heat related.

thepumpguysc · Mar 11, 2018

It would NOT act that way at start up.. IF it was a "floater"..
THATS WHY its OK when you let it rest.. the debris, floats off..
You said you installed an electric pump.. what psi does it pump?
That system only needs about 8-10psi to function properly.. You DO NOT want to put a 20-30psi pump on that system..
& on the other hand.. if its not pumping sufficient psi, say 3-4.. you'll get the same result..
IF your pumping/pushing more than that.. you might be sucking down the feed line?? just something more to check..
Regardless of what psi your using.. check the condition of the fuel lines.. they might just be collapsing internally.??

MoparMuscl · Mar 11, 2018

thepumpguysc said:
It would NOT act that way at start up.. IF it was a "floater"..
THATS WHY its OK when you let it rest.. the debris, floats off..
You said you installed an electric pump.. what psi does it pump?
That system only needs about 8-10psi to function properly.. You DO NOT want to put a 20-30psi pump on that system..
& on the other hand.. if its not pumping sufficient psi, say 3-4.. you'll get the same result..
IF your pumping/pushing more than that.. you might be sucking down the feed line?? just something more to check..
Regardless of what psi your using.. check the condition of the fuel lines.. they might just be collapsing internally.??

When it has the issue, I have disconnected the factory electric fuel pump on the outlet side and there is good flow. I didn't put an aftermarket pump on it, it is the factory John Deere replacement pump....I didn't check the psi. I replaced all fuel lines last year and there is no change.

TractorGuy · Mar 12, 2018

Check the fuel shutoff solenoid and make sure it is pulling all the way open. Mine sticks sometimes. Not sure why it makes a difference but it seems to work better if I push the throttle on full at start up then pull it back as the engine comes alive. When it's messing up pull the right side engine cover and pull back on the solenoid rod with you hand or a pair of pliers.