Oil & Fuel Fuel Supply Line Vented ???

[rbargeron]

My (new to me) L5450 has its fuel tank up behind the seat like many Kubotas. The line from the bottom of the tank to the filter has a tee in it with a short run of hose back to a fitting high on the tank. What useful purpose does venting the supply line like this serve?

I can report one effect it has that isn't useful - if there is even a slight restriction getting the fuel through the pipe at the bottom of the tank, the lift pump draws air through the vent line instead and the engine starves for fuel.

On my rig the balance of pressures was close enough that with a full tank, everything was ok but at 1/3 full or less, air was getting pulled in whenever the rpm was above idle. There is some visible deformation of the outlet pipe (bent too tight - slightly kinked - maybe since new) which is apparently just enough restriction to tip the balance. I did what I could to reduce the kink and put it back together without the vent and it runs perfectly.


Yeah, yeah I know... green wouldn't have this problem. But except for this little anomaly, the 5450 is the best tractor on the face of the earth. By quite a lot. Maybe I'll figure out how to post a wav file so you can hear it run - sounds like it has pistons the size of paint pails. Power is good.

 

[kubmech]

I'm not sure, but sounds like that may have been "modified" for some reason(???) there should be an injector return line going back to the fitting high on the tank.

 

[rbargeron]

There are two high-point fittings on the tank - one is the injector excess air-fuel return line as you noted - the other is the vent from the supply line. It is definitely NOT a modification - the vent line, tee, and all its clamps are clearly shown in the parts manual. It's bone stock.

I'm wondering what the theory is behind this feature. I've asked Kubota-Columbus to see if they can get an answer from Engineering.

 

[kubmech]

Looked at one today. Yup, bone stock. Here's what I think,
Normally the fuel filter is vented back to tank, but on yours the fuel filter vent is going to a shutoff valve that if opened will bleed air out of the filter bowl and on to the ground. In the mechanism section of the service manual they show a different routing for the fuel lines. Shows feed line uninterrupted to the filter and the vent line on top of the filter housing going back to tank, actually teed into the injector ruturn hose. I think what they might be trying to do is avoid running another line back to the tank. By teeing into the feed line on the "gravity" side of the filter. They allow any trapped air to escape back to the tank through the feed line, the bowl fills and the suction portion of the fuel line takes the available fuel that's in the bowl and provides the head pressure to the injection pump by way of the lift pump. Never really took notice of this before on these machines, so I was kinda scratchin' my head too. There all set up that way, so maybe your problem was just a kinked line or an obstruction.
Just my thoughts, looked pretty weird to me too. Let me know what you find out.

 

[rbargeron]

Kubmech - Thanks for the thought and comments. Haven't heard from Kubota yet. For the time being, I have eliminated the tee and am running the supply line direct to the filter. I mowed for a couple of hours and there was plenty of power. But once in a while there's also some uneven firing (vibration) so maybe there's still some air somewhere. Is the air-bleed valve always effective? Or is it sometimes necessary to individually bleed the injectors or lines?

 

[JATO_RaT]

Dick, as you know I have the L4850. Same setup as you, just slightly smaller cans. I do not have air/vent problems, but I do have a restriction problem going into the fuel filter from the bottom of the tank. Seems a bunch of mud dobber wasps decided to make the diesel container their home and inadvertantly, one of the guys dumped the diesel fuel into the Kubota, dirt nests and all. I will have to remove the tank sooner or later, but for now, I remove the supply line and blow the obstruction clear with some air. Kubota Mechanic is correct, the little knurled knob does bleed off the air in the bowel/filter. Try doing that and see just how quickly the bowl fills. If it does not fill fast, you may be obstructed as well. The opening at the bottom of the tank is small and it does not take much to block it. The 5 cylinder diesel on these tractors is a great motor, perhaps their best. Easy on the fuel, powerful, and instant starting all the time, anytime of the year. Rat....

 

[rbargeron]

When I had the tank off I hung a light inside - at the end where the sump cup is, they weld in a square plate over it. This makes a nifty small baffled fuel reservoir in case you are up on the leftside wheels and still want fuel to get to the engine. Unfortunately it blocks the view of the outlet pipe - which is indeed small. I blew out the lines with air and carb cleaner - tank is clean like brand new. I have the parts book for the 4350,4850,5450 and it shows two high fittings on the right end of the tank with one connected to a tee about 4" away from the tank outlet, the other connected to the injector return hose. If your 4850 doesn't have this feature let me know. There's too much evidence of designer-involvement for Kubota not to know how it got that way.

 

[Anonymous Poster]

The line you describe is for venting trapped air in the line when you have changed the fuel filter. At least that's what the Cat mechanic down the street says.

Kevin

 

[kubmech]

You should never have to bleed your injector lines (I know never is a pretty strong word) I wonder if your having problems with your lift pump (diaphram leak) Have you noticed if the engine oil level is increasing with a hint of diesel smell to it? Notice any exhaust smoke changes during the uneven firing?

 

[rbargeron]

Kubmech - Thanks again for the input. The diaphragm lift pump is fine (actually new) . When the starving problem first surfaced, it seemed worse going uphill but ok backing up. Thinking it was the lift pump, I put on a new one. But it made no difference. I noticed that on older Kubotas there is an "orifice valve" which controls the flow of excess fuel from the lift pump. This would seem like a way of keeping a higher inlet pressure at the injector pump - and would also mean a lower flow rate in the return line. But that valve isn't used on later L-3 models. Without this valve, the lift pump runs at full volume. The more volume the lift pump tries to circulate, the more suction at the tank outlet, and the quicker the supply line venting causes trouble. Anyway, that's my theory. I think they should have a bigger supply line if they're circulating that much fuel.

I'm running without a vented supply - mowed 3 hrs again last night - seemed fine. Fact is, the 5450 has so much power that even running a 90" finish mower through 2-foot grass it's loafing - and if there is still a small fuel issue it could be masked. Doesn't seem to misfire or weakfire - maybe the air is finally all out.

Dick Bargeron