TC35 fuel filter problem SOLVED!

[jedjoe]

I got these check valves from Amazon. Here is the link to the correct product.

https://www.amazon.com/dp/B087PFHNTZ?psc=1&ref=ppx_yo2ov_dt_b_product_details

There are metal valves that would probably be more durable, but I went with these for two reasons: 1) It was an experiment that I didn't know would work and thus didn't want to waste money if ineffective; 2) these were the only valves where I could clearly determine a small opening pressure. I didn't want a valve that would require too much pressure to open. These valves have a documented opening pressure of 0.5 kpa or 0.07 psi. There were some nice aluminum alloy valves, but their opening pressure was 2.9 psi (or 20 kpa) and I was afraid that would be too high a pressure to operate on these low pressure return lines.

 

[jedjoe]

Have now used the tractor for high throttle work for more than 15 hours with no loss of fuel height in the filter bowl and no compromise to the workings of the tractor. I think I can now recommend that anyone out there with this same problem could use my fix; namely, two low pressure check valves with one preventing suction flow from the return line to the tank, and the other permitting only outward flow from the filter assembly out to the return line.

 

[jedjoe]

This is just an update on the solution above. It has now been over three months and a couple hundred hours of tractor use. I've had no further problems with the fuel lines and filter bowl assembly since installing those two small check valves. While I still cannot say definitively WHY the problem occurred (and has occurred apparently with my other TC owners), the above fix really did FIX the problem. Happy holidays all!

 

[Steve in ATL]

Attached is a diagram of the fuel system and a description of how air is bled from it.View attachment 761130

I ran across this thread because I have / had basically the same issue. Here's something interesting: I have a 2000 model year TC 35D. In the repair manual for the above section (Fuel Flow Chart), MY manual shows the Lift pump *ahead* of the fuel filter (with everything otherwise the same plumbing-wise).

This makes me think that the original design intent was to have the fuel bowl / filter pressurized. Which would make a return line to the tank necessary.

As the tractor's fuel system is currently constituted (as per your diagram, with the lift pump *after* the fuel filter, there is no need for a return to tank line from the filter bowl to the tank - the main supply line would provide all the equalization of pressure needed. It's ONLY purpose would be to provide a path out of the filter bowl for AIR, not for fuel (which should never be needed).

Thoughts?

 

[jedjoe]

What you say makes complete sense. In my case, however, air was definitely ENTERING, rather than leaving, the filter bowl assembly. The check valve solved that direction of flow, while still permitting either air or fuel to leave it. What makes this so weird is that the original arrangement (no check valves at all) worked perfectly fine from 2004 until 2022, when the symptom first appeared. A real head scratcher! My “fix” is still working, so I’ll take what I can get!

 

[Steve in ATL]

My thought is that as the main fuel supply line ages and degrades, restriction goes up and flow goes down. When that results in there not being enough fuel supply to the bowl, the suction from the lift pump draws air down the relief / return to tank line as you originally posited.

 

[jedjoe]

Your theory of increased friction in a degrading line makes perfect sense to me. If the lift pump produces a constant suction at the intake and pressure on the output side, then that friction in the main line would create resistance and the pump would seek a source of fluid or air to compensate. Hence, air being pulled down into the filter bowl from that return line extension. Once I installed the one way valve on that line, the lift pump had no other source for the suction than to pull more forcefully from the main line. If the friction isn't too high, it could do it. That's probably what is happening!

 

[BlueMachine]

Thanks for this very comprehensive analysis.
I'm having a similar problem except that in addition to a nearly empty filter bowl, mine loses power while mowing on slopes but does not stall unless I refuse to push in the clutch.
You've given me some new ideas for things to check.

 

[Harry in Ky]

Thanks for this very comprehensive analysis.
I'm having a similar problem except that in addition to a nearly empty filter bowl, mine loses power while mowing on slopes but does not stall unless I refuse to push in the clutch.
You've given me some new ideas for things to check.

I still maintain that the check valves are just overly complicating the situation. If you have good supply from tank to filter base, then eliminate the vent hose entirely. Cap both ports it's now connected to, and go back to work. The issue will go away. It has in every case I've been involved with.