what is the function of the injector pump return line? why wouldn't you put the clear hose on the supply line?
Not an expert but as I understand it:
In the couple diesel engines I've worked on the fuel is constantly being sent to the injectors, but not at enough pressure to 'pop' them and inject the fuel. When the IP needs a cylinder to combust it sends a pressure pulse to that injector, once the pressure rises to the set value of the injector it 'pops' a small amount of fuel in to the precombustion chamber. The fuel is always flowing from the IP to the injectors but they don't inject without that pulse. The system sends a lot of quantity constantly but not pressure... so if its going to flow it has to return or go somewhere or pressure would spike and it would back up and make the fuel pump work harder. Also by sending back to the tank, it helps move the air out of the lines and gives contaminates a path out of the system as well. So the injectors have fuel return lines that link together and then route to one return line that dumps back into the tank.
On a Gm 6.2 there is a return valve that controls how much is let back to the return side (it also ties to the vacuum advance as well if I'm not mistaken). I understand as well on initial start, with the throttle not pushed... its supposed to be wide open so it bleeds air out for the start, but if its too open you may not be getting enough fuel to start. I think that is why if the initial start doesn't 'take' the back of the visor says to depress the throttle about 1/2 way and try again (ie it should close the valve and send a longer pulse to the injectors to inject more fuel). But I'm not 100% on that and need to dig through the TM and let the more experienced folks here chime in.
Anyways, If you take the hose off the top of the IP that links to the return system and replace it with clear hose you can watch for bubbles and return function etc and trouble shoot the return valve/return system/IP. Remember bubles tend to form on the return side from small air leaks that come with old trucks, old lines, loose clamps etc. Air entry allows the fuel to drain out of the injectors on leaky systems so that they have to charge back up with fuel before they reach the pressure where they 'pop'. With a mechanical pump this may take some cranking... similar to the TM method of priming a new fuel filter. That is why I put in an electric pump near the tank... it -should- pressurize the system and purge any air that leaked in out before I hit the starter, and it makes fuel filter changes etc a breeze. My CUCV leaps to life compared to when I had it on the mech pump. But it didn't help my civi J code 6.2 (which is my wife's daily driver so I really baby it... 'if momma aint happy, nobody is happy').
I've been wanting to swap the clear tube in but I've been concerned about heat and diesel compatibility. So I go slow and read alot.... The clear PVC you can get at the big box hardware store isn't rated for heat much, and its definitely not listed as Diesel compatible from what I read... but I'm frustrated and may swap it in for a short while in order to trouble shoot.
hope it helps.
Tom
