Unequal Tank Volumes

This post deals with an overfull tank, not minor differences when using dual fuel gauges. An over full tank can have fuel exiting out of the vent line, or just always be nearly full while the other tank continually empties.

With two tanks, the fuel system will be a dual draw and return, unless the customer has ordered or modified to single draw/return utilizing a crossover line. Yes, crossover lines are still legal, though undesirable in most cases. If in use, make sure you have breakaway double check valves on the line to avoid a hazmat spill.

The return line splitter is typically just an in-line tee. This fitting ensures both tanks receive equal amounts of hot return fuel. There is no mechanical feature that ensures fuel is level in both tank, atmospheric pressure and gravity take care of this naturally.

If you have a customer who is experiencing over full tanks:

First check for in-tank contamination which wreaks havoc with any system. This is the most common culprit, but typically the last resort due to effort.

Then, make sure the line lengths (both intake and return) are the same length (within 6” or so). It is easier to draw fluid from a shorter line so unequal lengths will bias fuel use from that tank. If tanks are not placed symmetrically the line will need to be coiled to accommodate.

Next check for any defect, kinks, rubber flappers, etc. that could cause a restriction. Also check the vent lines for plugging. Venting is required to maintain atmospheric pressure.

If your customer wants to make sure the hot fuel is distributed equally, they at times replace the tee with the GT splitter, but gut feel is this will not help an on-going issue.

You can reposition tanks on the vehicle, just make certain the lines are routed similarly, your line lengths are with 6 inches of each other, and that you do not introduce a defect or contamination.