There is a metal tube on the rear side of the engine bay filter that attaches via a banjo bolt. Above the banjo bolt the line heads to the rear of the head, and below the bolt it transitions to a ⅜ hose. I have to wash the engine and look again, but that banjo bolt or the line just below it is leaking badly when the engine is running. I came home tonight with a trail of fuel behind me up to where I park.

What is that line, what does it do and why does it continue to the back of the head when the other end appears to head down the driver's frame rail? I think the Mopar replacement part number is 05093088AA. Mopar calls it a drain tube.

This isn't my picture, but representative of what I am looking at...

 

The common rail fuel system is confusing. There are 3 fuel return lines, all going to the fuel filter cannister.
The fuel return passages in the fuel filter cannister are chambered and separate from the fuel filter and fuel feed passage and feed line to the Cp3.

There is a 1 return from the CP3. 1 return from the injectors at the back of the cylinder head and 1 return from the fuel rail pressure relief valve.
All 3 connect to the fuel filter cannister and combine into 1 fuel return line back to the fuel tank.

 

I certainly wouldn't tell you that line isn't leaking, you are there and I'm not. However, these engines have a very common issue with the #4 injector line cracking and leaking. If your's hasn't cracked, it probably will. While you are in there working anyway, replace this line first (easier and cheaper then drain line). Afterwards, test for leaks. Caution: keep your fingers away from leaking injector lines while under pressure. The high pressure can directly inject fuel into your fingers/hand.
Back to morning brew . . .

 

So, the one I am referring to is a combined return from the rear of the head, connects to the filter bowl, and continues as a return to the tank? Now, the next questions I have is why is there a return line coming out the rear of the head? In a gas engine with multi-port or sequential-port injection, the injector sprays fuel in the intake runner that gets pulled into the cylinder. If diesel is injected directly into the cylinder at the right time to produce ignition, what is exiting the rear of the head, and how does it get there? Isn't all the fuel the injectors are squirting go into their cylinders for combustion?



I replaced all of my injector lines almost two years ago as preventative maintenance. I see nothing wet above the banjo bolt in the rear of the filter housing. I am going to soak the engine in degreaser and hose it down tomorrow so I can crawl under this weekend and get a better view of where it's coming from with the engine running. I should have just done these lines anyway while I did the manifold-to-injector lines.

Thanks for replying with information guys.

 

There is a internal return passage in the head that is common to all the injectors. This injector return passage ends at the hole at the back of the head where the injector return line is fitted.

Sorry but I can't recall exactly why the return fuel line from the injectors is required. Excess fuel? Purging fuel when the key is on and the engine has not yet started? Failing injectors?

I do know that on my engine at idle, there is very little fuel retuning from the CP3, zero from the pressure relief valve (as it should be) and zero from the injector return line.

In an older gas engine with port fuel injectors the rail fuel pressure is about 40PSI versus 5000-25000 PSI with the 3rd Gen Cummins common rail direct injection.
Nearly all current gas engines now have direct injection and are running 5000-15000 PSI. I wonder if those gas engines have a injector return passage and line. I don't think so.