Curious about the fuel line and return line from the fuel tank?

harleyman_1000 · December 28, 2014, 08:02:17 AM

I have a GM 4104, and I'm trying to figure out where the main and return fuel lines run( to and from what) I see 1 line that goes to the fuel tank and connect to one of the fuel filters( the fuel primer pump also connects to this). I'm trying to figure out where the return line is? I'm still having problems getting fuel, and want to check these 2 lines for air leaks first to eliminate this as a problem.

bevans6 · December 28, 2014, 08:11:10 AM

The return line is going to originate at the front or back of one of the cylinder heads. Probably at the rear of the engine (towards the front of the bus since you have a tee-drive). There will be a brass fitting threaded into a port on the cylinder head with a hose connected to it, the hose leading to the return fuel line that goes towards the fuel tank. The brass fitting will have a restriction drilling, around .060" ish.

The cylinder heads each have two drillings that go front to back for fuel. One is the supply passage, one is the return passage. There are multiple ports drilled, threaded 1/4 pipe (from memory) and most of them are plugged. The head castings have "in" and "out" cast next to them. This is so that you can access the supply and return passages from many different places so that you can have a lot of flexibility in how the engine is configured.

Brian

harleyman_1000 · December 28, 2014, 09:30:17 AM

Does the return line run the same route back to the fuel tank as the main fuel line from the tank?

bevans6 · December 28, 2014, 10:15:29 AM

I'm going to say "usually" but a GM guy would have to say where yours was before it got the new engine install. On my MCI they are right side by each down the center trough of the bus, roof of the bays, just under the floor.

Brian

luvrbus · December 28, 2014, 10:22:47 AM

It should be easy to identify the supply should be 1/2 inch and the return line should be 5/16th

harleyman_1000 · December 28, 2014, 12:10:26 PM

Thanks guys. What is the easiest way to check these 2 for leaks? Can I simply use a air compressor and a rag over the line or tank where you put the fuel in to see if it hold pressure, and also to see if fuel comes out the line to the fuel filter? Geoff told me how to do it on the phone but after my coma I forget everything unless it is written down so I can reread it.

luvrbus · December 28, 2014, 01:56:03 PM

No need to check the return line for leaks it will show up

harleyman_1000 · December 28, 2014, 02:03:35 PM

Quote from: luvrbus on December 28, 2014, 01:56:03 PM
No need to check the return line for leaks it will show up

Please explain what you mean "It will show up"

daddyoften · December 28, 2014, 02:06:14 PM

Your return line will be under a positive pressure and fuel will leak out at a leak. Where as the supply line will have a negative pressure and can suck in air from a leak and sometimes will not show up as a wet spot. Harder to find the air leak
Eric

Sent from my DROID RAZR HD using Tapatalk

luvrbus · December 28, 2014, 02:26:40 PM

Scott,just go O'Rilley or Auto Zone and they have the Mighty Vac pump they lend out,disconnect the suction line at the filter and (the big line) pump it a few times if the line is good you will get a steady stream of fuel with each pump if it has a leak you get nothing

Beesme · December 28, 2014, 03:24:04 PM

I installed an inline pump and the prob showed right up.. Broken under bulkhead where it goes into luggage bays to the fuel tank... Had same prob no fuel ... Thought fuel pump and anything else it could have been ... Now I know look at easy stuff first ... You could always run brand new fuel line from tank to filter and that would eliminate your fuel line then check return line ...

OneLapper · December 28, 2014, 07:43:12 PM

Scott,

Take a look at this post of mine from 2009. I had the same symptoms and found the supply at the tank was plugged.

http://www.busconversions.com/bbs/index.php?topic=11011.0