Fuel Tank Overflow Issue

Check the vent on the other tank. If it can't return to both tanks it'll over fill one for a while after you fuel up.

 

Fuel lines from the splitter valve to the tanks have to be exact length and diameter, including fittings. 2 different brands of hoses and fittings will be different in size by just tenths to hundredths of inches. Use the same brand all around if you have different brands. I went thru figuring this problem myself. I had a fuel line rupture from the valve to the tank and had a truckstop replace it, but with a different brand of hose and fitting. But it only overflowed on one side when the tanks were 3/4 to full. I don't know if that would be the case if there is too much pressure in your driver's side tank because it's not venting correctly.

 

If the right tank is significantly hotter like you mentioned, it's probably getting more of the hot fuel from the return.

So either an issue with the left side return line , vent, routing, etc, or an issue with the splitter. Maybe it's tilted to the right? I know you said you replaced it

 

I've heard of problems with Paccar using splitter valves that have some flow return balance design that malfunction. I don't remember the internal design-how they work. The valve self adjusts to direct return flow to the tank with least fuel. But if yours is new, I would not think that's the problem. I think I remember Bruce Mallinson of Pittsburgh Power saying on the radio they like to use plain t-fittings. The problem has to be one of 4 things: 1) too much pressure in one tank because of clogged vent valve; 2) fuel restriction because of collapsed gose to or from the tank to the splitter valve; 3) splitter valve; 4) or hoses not identically matched.

 

I forgot: you could have something restricting fuel flow thru the supply side fitting on your driver's side tank like a plastic seal from a bottle of fuel additive.

 

Do the fuel caps match? Is one vented and the other is not? Cap gasket seals good? The caps will act as a bigger vent or pressure holder than the vents themselves.