Can't keep fuel equal in tanks.

[hhow55]

I recall an article in Landline Magazine by Paul Abelson, stating the life of a fuel line is 8 year's.

[TerryB]

Well, I continue to fight the problem with the fuel level in the two fuel tanks not being equal. More fuel ends up in the right hand tank, and quite a bit more. I've replaced all hoses had both tanks off had the suction pipes out, find no problems. I thought I may have found something with one of the return hoses but turns out it didn't fix my problem. Here is my question, the fuel suction obviously has a stand pipe that goes to the bottom of the tank, as does the return line from the engine. There is a return line that comes from the electric pump. Should this return line have a standpipe also? It does not. As I stated earlier the previous owner replaced the factory Cummins fuel transfer pump with a Fasse pump. Does the Cummins OEM pump have a return line going to the tanks from the pump? I'm trying to figure out if the return line from this Fasse pump was added when the pump was added. I'm wondering if the fact that that return line doesn't have the standpipe going down into the fuel could be my problem.

[Rawze]

all fuel that returns to the tanks has to go thru a return line splitter y-pipe for it to send it evenly.

[TerryB]

(02-05-2023 )Rawze Wrote:  all fuel that returns to the tanks has to go thru a return line splitter y-pipe for it to send it evenly.

It does, it does go thru a splitter. Goes from the pump to the splitter and to the tanks. The suction and return fitting in the tank coming from the engine have a stand pipe on them. The return fittings from the pump do not, the fuel just splashed in from the top. In theory, it's the static pressure in from the fuel in the stand pipes that keep the tanks level. If one tank it 3/4 full and the other 1/4, there would be more fuel returned to the 1/4 full tank because it takes less pressure to overcome the stand pipe that has 1/4 tank of fuel in it, until they come level or close to it. On the suction side it's easier to suck out of the 3/4 full tank, and so on. With the engine shut off, a siphoning effect takes place and will level the tanks(of close to it) with the engine shut off. Does the fact the return fuel from pump does not return thru stand pipe mess with the leveling capabilities? If the OEM pump did have a return line going to each tank and it didn't utilize a stand pipe, then I would say it doesn't matter. If the OEM pump did not have a return to the tank from pump and the return was added with the Fasse system, then maybe this is my problem.

[TerryB]

(02-06-2023 )TerryB Wrote:  

(02-05-2023 )Rawze Wrote:  all fuel that returns to the tanks has to go thru a return line splitter y-pipe for it to send it evenly.

It does, it does go thru a splitter. Goes from the pump to the splitter and to the tanks. The suction and return fitting in the tank coming from the engine have a stand pipe on them. The return fittings from the pump do not, the fuel just splashed in from the top. In theory, it's the static pressure in from the fuel in the stand pipes that keep the tanks level. If one tank it 3/4 full and the other 1/4, there would be more fuel returned to the 1/4 full tank because it takes less pressure to overcome the stand pipe that has 1/4 tank of fuel in it, until they come level or close to it. On the suction side it's easier to suck out of the 3/4 full tank, and so on. With the engine shut off, a siphoning effect takes place and will level the tanks(of close to it) with the engine shut off. Does the fact the return fuel from pump does not return thru stand pipe mess with the leveling capabilities? If the OEM pump did have a return line going to each tank and it didn't utilize a stand pipe, then I would say it doesn't matter. If the OEM pump did not have a return to the tank from pump and the return was added with the Fasse system, then maybe this is my problem.

Looking at the OEM parts build for this trucks serial number, I only see 2 fuel lines going to the tanks. Just the suction and return from the engine. Both of them go thru a splitter before that tanks. I don't see a 2nd return line. That leads me to believe there was no return from the pump, the pump return was added with the Fasse system. I'm assuming if there was return fuel from the OEM pump was plumbed in with the return from engine. I could be wrong on that but seems like that's what they should have done with this Fasse system.

[DVT873]

The return line should NOT be going into a standpipe. On either side. If one is and one isn't that is likely the problem. The FASS return needs split also.

Only other thing is the line pairs need to the same height. So both returns have to be as close to the same height as possible. If one return is close to the floor of the cab and the other one is on the top of the frame making them say, 6 inches apart in height, there will be a problem. Same for the draws.

[TerryB]

Return fuel from the engine definitely goes thru a stand pipe, both sides do, no question about that. I actually think it has to for the fuel leveling. I have another truck of the same year and model, only difference is it has 100 gallon tanks, this one has 150. The other truck has standpipes on return fuel also. Plus the leveling function actually works on that truck. I was able to find the installation instructions for the Fass pump that it has. The instructions state the return fuel from the pump does not need to be separate and the proper installation is to plump pump return in with engine return. That's not how it is now. After I read that I looked closer at the fuel tanks and now a can see the return port in the tank that the pump return goes to was added by someone. It's noticeable the port was welded in the tank. They did a fine job doing it, but it doesn't look the same as the suction and the engine return ports do. So, I'm changing that. I'm going to plumb the pump return if with the engine return, before the splitter and cap off the existing ports. All it can do is not work....I've already replaced all the hoses and splitters, this will be a cheap experiment.

[DVT873]

If all of the FASS return was going to one tank that will be a problem. Interesting about the returns having standpipes. Mostly I see medium duty trucks, smaller tanks and those don't have standpipes. If they are on both sides and the same length it would be fine. These dual draw dual return systems can be a PIA when they act up.

A lot of newer stuff use a fuel pump to move fuel from the "storage tank" to the run tank. Pretty reliable but when they act up it can be a problem if somebody doesn't know the system. Hope splitting the FASS return fixes your problem.

[TerryB]

No, the Fass return was going to both tanks also. Went from pump to splitter to tanks. The splitter that was used looked just like the splitter used on return line. I'm gonna change all that and see what happens.