Peterbilt 379 fuel tanks not using fuel equally

Do your fuel levels equalize after you turn truck off? If after a few hours your fuel levels are unequal the splitter valve on the return is probably not the problem. The splitter valve on the return lines should only affect fuel level while the truck is running. When engine is off, gravity should cause fuel levels to equalize. Also, a blocked vent would prevent equalizing. You could have algae growing in your fuel tanks that can cause a restriction. Or other stuff in your tanks.

My '03 379 has the same problem. I never had a problem until I had all the fuel lines replaced. But I also changed the splitter because it was leaking. I changed the splitter again because I thought the new one might be defective .But the truck usually wont equalize while off. I have also used about $500 worth of algae killer and still have some algae . I once switched my supply lines from one side to the other to see if there was a change. So, i have had this issue for about 2 years. It used to draw perfectly evenly. Nowadays it doesn't pull from the right tank unless i shut off the left. And it will run just fine off the right when i make it do it. I recently thought about taking it to the shop and letting them figure it out. I have checked the vents. I am starting to think it might be that right fuel line. And maybe the algae just comes along and confuses the situation every now and then.

 

Opening a filter is a good idea. Also, when my fuel is low, I shine light down in the tank and you can see the bottom. Algae looks black and slimey. My truck does the same as yours. If you have algae, you might try killem . It is a biocide that seems to have some effect on the algae. But I haven't gotten rid of it. I used Lucas biomedic and couldn't see any improvement.

I may go to a shop and let them figure it out. You have to be careful to not let a tank go empty. I have had to stop on the side of the highway to shut off a tank before it goes dry. Very, very inconvenient and can be dangerous. Good luck .

 

I didn't have much success with Killem by FPPF either. What you need to get is Fuel Prep by Penray. Its costs anywhere between $7-$10 per quart. You need 3 quarts total. 1 in each tank at next fill up. Then split the last quart on the follow up fill. I now add 1 quart per month, just for maintenance and in case I got a bad batch of fuel. BTW, algae in the fuel tends to decrease fuel economy too.

 

Supernova, did you finally get this problem sorted out? I'm having the same issue with a 2004 379. We changed the crossover valve, removed both tank vents and replaced them with diff vents (no float in diff vents eliminates the chances of that being a problem), swapped covers from side to side, and it's still drawing randomly from either left or right. The driver says he can shut off the tank that is getting low, and the truck will draw form the other side without problem. I never thought of algae being the culprit.

 

Shine a flashlight inside your tanks. If they look black, you may have algae.
Pull your fuel filters and dump the fuel into a clear or white container. If the fuel is black or you find black slime in the filters...you have a contaminated system.

 

This is a very frustrating problem isn't it. You can be in danger of running out of fuel and air locking the system, yet you have a full to the brim tank of fuel. I finally got mad and put shut off valves on both the suction and return lines to both tanks, as well as I put on an old-fashioned cross over line between the tanks like trucks used to have in the old days. End of problems.

 

Thanks all, for the quick replies.

I only heard about algae for the first time a couple years ago, and honestly thought it was a myth. The last time the truck was in the shop it was low on fuel, and I did see some black stuff floating around the bottom of the tank, but I just assumed it was dirt and that it wouldn't be an issue since it was below the pickup line. I never thought of it being algae. So, now I have my parts guy looking for an algae killer. He's pretty good and he's been around for a while, and I think I'm the first customer he has had ask for something like this. I'll see what he found on Monday morning when he stops in.

The old fashioned crossover line was the first thing I wanted to do too, but the problem is this truck gets towed through mud and rocks on occasion, so keeping the fuel lines up as high as possible is a must. I'd rather run the truck on one tank permanently than pay the clean up bill for a fuel spill.

Supernova, I don't doubt what you're saying about one line being partially collapsed, but I don't understand why the problem would randomly switch from side to side, if it's a line on just one tank. But then again, there's a lot of things I don't understand. This truck only has taps on the pickup line of each tank. It might be worth putting them on both returns, as well.