Fuel system

[JSF01]

Well after like two weeks of trying to figure out what is wrong with my glow plug system in my M1009 causing it not to start when cold, I was looking through the forums and learned it could be a small air leak into the fuel line. Using some clear tubing off the IP like some people suggested in other threads I found that after a few hours of sitting there are a lot of air bubbles in the line when trying to start. I figure that confirms my problem but I do have a question. I have never worked on a fuel system before and therefore do not know much about how the system works. I always thought the process was simply the fuel pump drew fuel from the tank through the fuel filter and pumped it to the injectors which regulated fuel injected into the cylinders. If it was that simple of course why would the IP have a fuel drain back pipe? It looks almost like it is sucking fuel out of the IP. While the solution to my problem is replace the rubber fuel hoses and check other fittings, I was would appreciate it if someone could give me a good description of how the fuel system actually works, as I generally like to know how the things i am working on work.

 

[cucv1833]

Well after like two weeks of trying to figure out what is wrong with my glow plug system in my M1009 causing it not to start when cold, I was looking through the forums and learned it could be a small air leak into the fuel line. Using some clear tubing off the IP like some people suggested in other threads I found that after a few hours of sitting there are a lot of air bubbles in the line when trying to start. I figure that confirms my problem but I do have a question. I have never worked on a fuel system before and therefore do not know much about how the system works. I always thought the process was simply the fuel pump drew fuel from the tank through the fuel filter and pumped it to the injectors which regulated fuel injected into the cylinders. If it was that simple of course why would the IP have a fuel drain back pipe? It looks almost like it is sucking fuel out of the IP. While the solution to my problem is replace the rubber fuel hoses and check other fittings, I was would appreciate it if someone could give me a good description of how the fuel system actually works, as I generally like to know how the things i am working on work.

Welcome to the site

Download the TM's and go the the TM-20 figure 1-17 its shows a good figure. Then start changing rubber fuel lines. The stock filter housings are also known to suck air.
JATONKA's CUCV TM download page

Good luck

1-17. FUEL SYSTEM.​

a. Combines a low-pressure and a high-pressure fuel delivery system.
(1) Low-pressure fuel delivery system consists of:
(a)​

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Fuel Tank.​

(b)​

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Fuel (Lift) Pump. Driven by camshaft. Pulls fuel from fuel tank and sends it
through fuel filter to injector pump.
(c) Fuel Filter. Mounted on bulkhead. Removes sediments and water from fuel
before it reaches injector pump.
(d) Fuel Lines. Connects fuel tank to injector pump.
(2) High-pressure fuel delivery system consists of:
(a) Stanadyne Model DB2 Fuel Injector Pump. Delivers high-pressure fuel
from injector pump to nozzles.
(b) Nozzles. Provide high-pressure fuel to each cylinder.
b. Fuel return system delivers excess fuel from the injector pump and injector nozzles​

back to the truck’s fuel tank. Fuel lines provide fuel to and from the fuel tank and injector pump.

 

[chicagodiesel]

If there was no return there would be build up of pressure in the lines to the ip and the ip itself if not all the fuel pumped was delivered to the injectors for combustion. This way, the fuel sending unit, lift pump, and injection pump don't have to be perfectly calibrated with each other at all moments. I'm sure there are other reasons too that someone will certainly point out.

 

[JSF01]

So what would happen if there was a buildup of pressure on the IP, and why not just build an IP that could take the pressure.