View Full Version : Why do Diesels Have Return Lines?

03-31-2008, 04:04 PM

I am learning a LOT here! Thanks!! I have another quick question.

1. Why are there return lines coming out of the injectors?
2. Why does one end have a little stub?
3. What is a banjo bolt?


03-31-2008, 04:29 PM
to purge air from the fuel system

03-31-2008, 05:22 PM
Disclaimer. Some of what is to follow is probably wrong. If I knew which parts, well, you wouldn't be reading this disclaimer. Perhaps some of what I say will be useful, and not betray my smarts as the book kind instead of the more valuable, practical kind.

1: Not all diesels have return lines. The ford PSD engine does not, at least that's what I learned on the streetcorner. A high pressure common rail system may not either. A diesel fuel system has to provide injection pressure to the injector for a portion of the power stroke- the ideal diesel cycle is expansion (power stroke) at constant pressure- combustion has to take place over much of the power stroke to keep the pressure up. How much? I donno.

In contrast, the ideal otto (gas engine) cycle generates all the combution heat & pressure at once, in a constant volume event- the charge cools & pressure decreases through the power stroke.

The benz injection pumps use a piston (1 per injector) with a variable stroke volume to squeeze too much fuel into a tight space (the injector body) at 1300 or so psi, the injector pops open and squirts fuel into the prechamber. How much? enough to relieve the pressure. The more you squeeze into the injector, the higher the pressure, the more it squirts. Fuel that doesn't pop the injector overflows out the return lines, and I bet it takes trapped air with it.

2. The stub is just a return hose that doesn't go anywhere. That way you can make all injectors the same, instead of a special one for the end of the row. Saves on tooling.

3. Who doesn't like 'Deliverance'? Squeal like a pig now!

03-31-2008, 06:02 PM
A banjo bolt is just a hollow bolt that allows fluid to flow through.

03-31-2008, 06:19 PM
1. Why are there return lines coming out of the injectors?

Some injection systems send a pulse of fuel from a piston pump in the IP to the injector via a hard line, this pressure wave overcomes the force of the spring in the injector and lifts the pintle allowing fuel to escape into the combustion chamber, the fuel in the pressure wave below the pressure required for injection does not get injected but is used to lubricate the internal workings of the injector so it does not seize up, this fuel escapes through the Bypass Lines and is returned to the fuel tank via the return line.

2. Why does one end have a little stub?

Well since all the injectors are producing bypass (excess fuel) and we want to return it to the tank, they are daisy chained together, one end is plugged and the other end of the chain connected to the return. This way only one line is needed rather than one for each injector. I suppose you could do away with the stubbed line if you made a special injector with only one fitting but it would not be as cost effective as a little stubby rubber hose.

3. What is a banjo bolt?

In a normal Mercedes the fuel filter is under pressure since it is mounted between the lift pump and the IP, Bosch put a tiny hole in the top of the filter housing to allow any air in the fuel to accumulate and be released before the injector pump, Like when you change a filter. Since we have a return line and an injector bypass line and this little air bleed and all of them need to return to the fuel tank, Bosch decided to thread the little bleed hole and make a special hollow bolt to go into it, this polt passes through a banjo shaped fitting with 4 connections on it, the little hole that connects through the hollow center, the return line from the IP, the injector bypass line, and the line running back to the fuel tank (Also called the cigar line because it looks like a cigar and is designed to flex absorbing pulses in the fuel system so the people in tha back seat done have to hear "Hiss, hiss, hiss, hiss").

We move the filter to the suction side of the pump and plug the center of the banjo bolt or use a solid one to block the air bleed in the filter which, when placed on the suction side will allow air to be sucked from the fuel tanks return line and prevent the vehicle from running at all. By just blocking the center we don't have to come up with 3 new fittings to control returning fuel.

03-31-2008, 07:38 PM
john_galt, moon161, Hotbox, and cgoodwin,

you guys are what make the internet great! I never thought I'd get such a GREAT response! I'm much smarter thanks to you guys!