help fuel leak

i have a 2001 gmc 3500 crew cab 6.6diesel. i have fuel dripping from a line by the i think is called the fuel cooler under the gas tank, the leaking line seems to be right at the gray connetor that holds both the lines together.
i think its the return line that is leaking not the feed because the truck only leaks when running,, is it recommened that you splice in with a rubber hose. or do you have to replace with a gm new line, i herd to put the new line in you have to take the bed off.. is there a easier way.
any help would be great

Welcome aboard!

The fuel system is suction from the tank to the filter/pump, and only very slight pressure from the injection system return to the tank. For this reason, significant fuel leaks are very rare. One of the most common causes of a leak, or several other fuel system problems, is the fuel filler cap. If it's original, it's probably due for replacement. If it's been replaced, it may still be failing. If it doesn't vent properly, excessive fuel tank vacuum or pressure may be occurring, which will exploit any weakness in the system. The caps are inexpensive. Just replace them if there's every any doubt.

If the leak continues, it may be more simple than it appears. The return line coupler is sealed with an O-ring. Disconnect the coupler and inspect for damage. If it appears undamaged, replace the O-ring, reconnect and test. If it continues to leak, I would splice it with a section of fuel hose. Use constant tension clamps to prevent future leakage.

thanks for the quick response d max. never thought of the filler cap, i will check that out and your other suggestion and report back .. thanks again,, rooferman

well the leak ended up being the fuel cooler return line. it was leaking right were the plastic holder holds the lines to the frame, were all the sault can gather and rust the line out.. ended up cutting the bad part of the line out and replacing with high pressure injector fuel line and dubble clamping, seems to be holding up well...