So I tried looking for pictures and doing a search without much success. I noticed that on my cylinder 4 (closest to firewall) injector has a rubber shielded line that is capped off with a screw. This is one line going in and the other that comes out being capped off. Is this line suppose to be capped off?

yes, unless you want fuel squirting all over your engine.

return fuel flows freely between the two nipples. you want it returning to the fuel filter mount, thats why you cap off at the other end.
you could also, use a T and extra hose, but there is no difference.

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1977 240D auto w/OM616.916 engine
2005 Matrix 5spd
2006 Prius

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Oh ok, thank you. Im gonna properly cap it off and remove that nasty screw

There is a "correct" cap for the end. I've had success sourcing the hose and cap at VW dealers if that is more convenient. Consider carrying injector return hose and end cap spares along with extra exhaust hangers...these are words born from experience...

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Never saw the point in buying caps myself. Nails have lasted years for me

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1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...many, many years in the making....
1977 280> 300D - 500,000km+ (to be sold...)
1984 240TD>300TD 121,000 miles, *gone*
1977 250 parts car
1988 Toyota Corona 2.0D *gone*
1975 FJ45>HJ45
1981 200>240D (to be sold...)
1999 Hyundai Lantra 1.6 *gone*
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)
2001 Holden Rodeo 4JB1T 2WD

I've always cut a little 1" section of the braided return line and stuffed an oversized ball bearing in there. It's not a high pressure line, so just like a nail or screw, the bearing won't shoot out. They get replaced about every year anyway, I don't want to spend more money on an item that should not cost anything more than a couple cents.

Phil Forrest

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1972 220D "Trudy," named by a friend.

"The 220D sounds good... I suspect it is the only car that you need a calendar for, rather than a stopwatch, when doing acceleration tests."
Tom Abrahamsson

It's very wise to carry a meter or two of this hose .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

The factory one looks like a nail, but a bit purtier when you take it out, though a stubby nail would work. I bought a roll of 1/8" Viton hose on ebay many years ago. From China, so I was suspicious, but appears to be true Viton. I use it for the short sections between injector returns and no more wet hoses there. I use the cloth-braided rubber hose for the longer run to the fuel filter since I still have a roll to use up. It starts seeping after 3 or 4 years, though less so at that location. It is stamped "Made In Germany", but who knows with all the fraud today. Once, I tried 1/8" silicone hose at an injector before I got the Viton hose. After a week, it had become swollen, as others have found for many alternate hose materials.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

a small rivet is smooth and works perfectly.

My injector return cap is leaking. Can the little brass plug in my OE cap be taken out and pressed into a short section of the hose?

Would a leaking return line cause injector noise? I can’t really see how, but it seems like it may be one option as culprit for my ticking sound. The return is leaking.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
2008 ML320 CDI (199k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)