Diesel overflow

I have been wondering about something for a while, so I guess I'll just ask. I have three diesel machines, a JD4300, a New Holland TL100A and a Kubota X1100C. The overflow tube from the injector system returns to the fuel tank. I have looked at the manuals but I have not seen a reference as to where full is on a fuel tank. Do you leave the fuel level below where the overflow line comes into the tank or can it be above the overflow line? I have kept it below on the JD and the NH but I don't even know where it is on the Kubota. Just curious, does it make a difference?

Doesn't matter. Fill 'er up, Mac.

I have been driving and filling diesel tractors my whole life. I have always filed the tanks without consideration of the return line entry to the tank. After all the fuel has to come the tank before it can return to the tank. Therefore the tank wont overflow and since the return is under pressure, the fuel will return to the tank no matter if the return is above or below the fuel line.

If it mattered, it would be designed into the system so you would not need to worry/think about it.

The return is always returning less fuel than is being pulled out of the tank so there's no worry of overflowing the tank. It's also under pressure so there's no worry of "backwash" not that that would even matter.

Cars and trucks have been returning fuel to the tank for many years and no one has had a problem, so yes, just fill-er-up.

No. As already stated, it makes no difference. Return flows usually go in near the top of the tank. On Ford backhoes the return goes into the filler neck. So if you go to put fuel in the tank while the engine is running, you can get a little back splash interference there! And the little rubber line which connects right there can get chafed by the hood, and cause a big leak. On 'thermostart' systems the return flow fills the reservoir for the next start.

Return lines are simply there for injector bypass leakage and return flow from the transfer pump in some cases. Fuel level in the tank has no bearing on that as it's just circulation flow.
What you might consider is that many tanks have an air gap to allow for expansion. That's usually above the neck but not always. If you fill from cold fuel in a building or inground tank and then park the machine out in the heat, the fuel will expand... sometimes beyond that capacity of the tank if you filled it to the lid... so then you may see overflow or leaking at the weakest point on the tank (fuel sender).

Rod

Long story here and some of it may be talking through my hat as they say. But, some gasoline start diesel engine tractors IH sold ran the fuel return into top of tank to a pipe that extended down to near bottom of tank inside tank.

I guess this was to reduce foaming so I was told. It also was inconvenient when working on pump when you removed fuel return line down at pump, you had the suction effect if any fuel was still in the return line so fuel would keep running. Had to blow back into tank to eleminate the effect, after removing cap and covering with rag of course.

First experience I had with that was a 450 Farmall diesel and no power. Running on dyno you could see air bubbles coming up through the water trap drain. Well, everyone says, sucking air is your problem. Realize at that time my experience with Diesels was near zero. Never even stood along side one until I started working for IH dealer.

I called Diesel repair shop, talked about the issue, and he suggested maybe the return line inside tank had fallen off and was aerating the fuel. That didn't sound reasonable to me. I had to put on my think hat, not hot air hat, and I removed fuel supply line from tank and found very little fuel would flow from tank. Stuck and object in through outlet and a piece of tin was covering outlet.

The baffle inside of tank had come loose covering outlet. I cut a groove in the stand pipe of the outlet fitting, put her back together, horse power is now just fine and now I'm the diesel (expert) .

Absolutely no problem on the KUBOTA fill it to about two inches below the neck. Very little fuel goes back down the drip line and it will force its way back into the tank.

be no different than pumping your tidy tank pump with the hose stuck in the other tank inlet. its just fuel circulation for cooling injectors. also the fuel in the tractor tank will warm up also.

Thanks everyone for the feedback. I have always left the level down @ 3 inches......I'll fill it up next time. I don't know what I'd do without this resource, you folks are great. Thanks again. Jim