Engine/Transmission (1998.5 - 2002) What size threads on the fuel return line to the tank at the engine T?

[rweis]

Fuel return comes out of the VP44 and goes to one leg of a T at the rear of the engine. One leg of that T goes to the injector return galley. One leg of that T goes to the return line back to the tank.



What is the fitting size that goes back to the tank? I am thinking of replumbing the return line to the tank but would like to know the fitting size in advance so I will have the right fitting before I take it appart.



Bob Weis

 

[Todd T]

dont know for sure but wanted to get back to you on the fuel return gallery on the head. I didnt post any pics of this while I had the head off last week. there just isnt anything to see other than a hole in the back of the head and plug on the front.

 

[rweis]

How big is the gallery? How much volume do you think there is at idle?, cruise speed?



Do you think it is under much pressure? Sort of like the VP44, maybe 14+ psi? Maybe much more comming off the injectors.



I am just looking for any information I can find on it. No one seems to know much about it.



Bob Weis

 

[DLausche]

How big is the gallery? How much volume do you think there is at idle?, cruise speed?



Do you think it is under much pressure? Sort of like the VP44, maybe 14+ psi? Maybe much more comming off the injectors.



I am just looking for any information I can find on it. No one seems to know much about it.



Bob Weis

I'm not sure of the line size. Pressure is zero (or VERY close to it). If there was pressure here, then your fuel tank would be pressurized and that would not be good.



-Deon

 

[rweis]

I think there is a little pressure because it will travel back to the tank and fill up a return cooler from the bottom and then empty into the tank filler line. I also think there is a little pressure because the VP44 bypass is set at 14 psi and (I think) whatever goes through the bypass is sent into the return line.



However the line off the engine T to the tank is sweged INSIDE the return line and the return line is smaller than the feed line so it can't be much pressure. The "pumps, lines, and whatnot" thread had an implication that up to 30 gph is returned at cruise rpm so there is some volume to it.



Just trying to get a better feel for how this thing works and what makes it work. My ultimate reason is I think it is one of the best ways to cool the return fuel and you don't mess with the feed to the lp (whatever lp you have) which I found out I did not do right (I had a hard time not getting some air in the feed to the lp).



I am going to try to build a cooler like Steve's and have a return cooler system of some sort. Those 100 degree OAT days are comming .



Bob Weis

 

[daveshoe]

dazed and confused

My understanding of this was that on a pre common rail truck[2002] that the lift pump fed the VP 44 fuel at low pressure and the excess amount of flow [bypass] went directly to the fuel tank from the VP 44. I am thinking about a Vulcan style setup and thought that I could bypass the old fuel lines and put a rubber return line directly back to the tank directly from the VP-44. My goal was to leave the stock feed lines in place on the truck rails until everything checked out.

Please ,someone explain what happens to the slug of fuel after it leaves the VP 44 --Does it all go into the cylinder,or is some left over and returned to the tank from the galley discussed above. If so,how does it get into the galley? I was under the impression that the injector put it right into the cylinder and just closed itself down once the pop-off pressure diminished to a certain point.

 

[Todd T]

There is a very tiny amount of fuel from each injection cycle used to lubricate the injector internals, that does not go into the cylinder. This tiny amount exits the injector from that little hole just above where the connector tube connects to the injector.



Think of it this way. The injector connector tubes are about 4 inches long. each presses onto the injector and seals using a press fit that the injection line nut holds tight. OK? Above that hole there is a smaller hole just above it in the injector body. This small "weep" hole is where the unused (lubrication) fuel exits the injector. the connector tube is about a 3/8 diameter. The tube is in a hole in the head about a 1/2 in diameter. looking at an injector tube you'll notice an O ring on the connector tubes. They are there to seal the outside area between the connector tube and it's hole into the head from the outside world making the return galley air tight for the "weeped" fuel to literally run back out, around the outside of the connector tube (about half way back) and out to the line borred galley in the head. It then runs down the galley shared with the other 5 injectors where it exits to the rear of the head through the line borred hole which runs front to rear in the cylinder head (where the fuel return line connects to the head).



So each cylinder has a connector tube, each connector tube is sealed (O ring) just past the galley so each cylinder can share this galley and drain off the fuel through the common line borred hole through the head. its a tube within a tube.



this is why most guys have a hard time finding leaks at the injector (damaged injector tubes or mal-allignment with injector). The fuel never makes it to the outside of the head where you can see it. it just runs back into the tank and you keep chaising your tail with a hard start problem.



Also guys tend to crank and crank after having air in the lines (injection pump replacement or new injectors). The trusty manual tells us to loosen 1,3 and 4 injector lines, crank it over and watch for fuel. Well, most of the time the seal opens at the injector and not at the line and the fuel goes right on out and you again, don't see it. Next time you are having a hard time cranking and waiting for fuel to leak out the head, just go ahead and crank them down and see what happens



clear as mud? :-laf

 

[Todd T]

Please ,someone explain what happens to the slug of fuel after it leaves the VP 44 --Does it all go into the cylinder,or is some left over and returned to the tank from the galley discussed above. If so,how does it get into the galley? I was under the impression that the injector put it right into the cylinder and just closed itself down once the pop-off pressure diminished to a certain point.

Dave, read above post. you have to provision for the return out of the head too.

 

[daveshoe]

Very informative post,Todd! I saw on another site how another guy used the Vulcan fittings and then ran a return line from the VP back to the tank--it had pictures and everything. He must have missed that gallery connection. My problem is that I wanted to use a bed mounted tank to ease installation of a single pusher pump and to also return all the fuel to this same tank. I was trying to get by without cutting any factory lines,but just by-passing them with new lines and fittings until I was sure that this was the system I wanted,and the next owner would have the option for a stock setup.

I guess we are back to this same question as the first post of the thread--What is the fitting size of the gallery exit from the head?

 

[rweis]

Just some FYI:



I went back and re-read the "pumps, lines, whatnot" and the 30GPH tank return was at 0 - 6 psi.



Still would like the fitting configuration to the T however.



Bob Weis