Air Leak In The Fuel System

On my 1983 C20 C-code every morning I find air bubbles in the clear return line I put on the front of the injection pump.

Yesterday, after sitting for 2 days I found there was NO FUEL in the return line at all, just air. Nonetheless, the truck starts fine on the first crank with zero hesitation or stumbling.

I can't find any evidence of fuel leaking (wetness) anywhere.

From the above evidence, what can I deduce about the general location of the leak?

Also, where the hell is that fuel going?

A possibly unrelated phenomenon: on a first cold start in the morning when there is both fuel and some air bubbles in the return line, I'll notice a few air bubbles Bob up from the IP into the return line when I turn the key into the"on" position. I have replicated this a number of times and without jarring or otherwise disturbing the truck in any way.

Strange.

It sounds like the leak is between the fuel lift pump inlet and the tank sender inlet (to include the primary fuel filter and related plumbing on your 83, which makes the filter itself suspect -not uncommon). The bubbles you see at key-ON is the fuel stop solenoid opening, allowing air into the IP, likely from a slight vacuum from the leak-down from sitting. During colder temps, after running, the fuel system cools and condenses, creating a vacuum condition in the system. Loosening the filler cap at shut-down relieves this vacuum condition, which may indicate the cap vent has failed (just replace it -they're cheap), which will exploit any leak opportunity that would not leak otherwise. Loosening the cap may also allow an external leak that you can identify. If a loose cap test reveals no air as before, and no external leaks, then the issue was caused by the cap itself. Unlike gasser caps, the Diesel caps vent both ways, and should not allow any significant pressure or vacuum condition, so make sure the cap is, in fact, a Diesel cap. Another thing to check is any clamp connection in the fuel lines, which are more likely to leak when cold, but may seal up when warm due to thermal expansion of fuel lines at the connections. Simply, follow all the fuel lines and tighten the clamps, and replace any clamps that appear damaged or worn (out of round). Also, loosen and re-tighten the primary fuel filter, or R/R it and make sure the seal and head mate properly. Some grease on the gasket and threads helps to seal it and prevent leaks.

Thanks!

I loosened the caps on both fuel tanks today. We'll see what happens in the morning.

Forgot about the dual tanks. The tank selector valve is another usual suspect. Check it for operation, and any fuel wetness. They can be a real PITA.

Just ONE tiny bubble in the return line today, and only a few passed through right after starting. This is a tiny fraction of what had been doing and is a substantial improvement.

Thanks again!

It's the return line. Are you sure it's not just draining back into the tank?