What happened:
The return fuel lines were recently replaced on my 1984 300SD after the mechanic noted a diesel leak.

Post repair, the car was driven for about 10 miles (~25-45mph), before smoke was seen coming from under the hood and excessive temps were seen. The classic (as I now know from the internet) broken radiator neck was found. We replaced the radiator (DIY) but while flushing the hoses (so idling the car) noted leaking diesel fuel. We found the last return line popped off. We assumed we knocked it when messing with the heating block hoses and popped it back on. Took it for another test drive and 30 minutes another diesel leak again. This time the first line popped off, though we noted that several of the other lines had "backed up." As in the lines were going to eventually pop off they just hadn't gotten there yet.

Could this be because fuel filters need to be changed? Are there other potential causes? Is there something that happened from the overheating that is causing the lines to pop off?


Additional Information (possibly irrelevant)
The car went in to the mechanic originally for "groaning" when turning the steering wheel. They replaced the leaking steering gear box as well as the return fuel lines and a rear axle seal. They also noticed a leaking differential, but said we could wait on that repair since we had racked up a nice bill so far.

Welcome to the Forum.

The correct size Injector return line is 3.2 x 1.9mm, buy it by the meter.

The old part# 615-476-03-26 has been replaced by 605-078-05-81.

The fuel line just pushes on and held on by the barbs on the Injector Nipples. You don`t use any hose clamps.

When you remove the hoses, do not use a knife to cut the hose off the Nipples.
Nicking them could cause a fuel leak.

The last Nipple by the Fire Wall is just a short piece of hose with a metal plug. It can be removed from the old hose and used in the new piece of hose.

It is a bit difficult to just knock off the return lines, the are on pretty tight.
Maybe the mechanic used the wrong line?

A plugged Fuel Filter will not cause the lines to come off.

Differential could be the Pinion seal, and a good majority will leak a bit with high mileage. Usually not a problem unless it is really leaking a puddle

The rear Diff cover could be the problem also. If it is, it`s an easy DIY fix.
Put the rear on jack stands securely. Block the front wheels.
Make sure the fill plug will come out first, before draining the Diff.
Drain Diff.
then place a floor jack under the Diff.
Remove the 4 13mm bolts holding the Diff mount to the body.
Lower Diff down a bit.
Remove the 8 15mm bolts, and the cover will come off.
Degrease the cover good and the mating surface of the Diff.
Use RTV as the sealant.
Replace the cover and Diff mount bolts.
Fill the Diff with your favorite flavor of gear oil.

The plastic necks and the over flow nipple to the expansion tank are pretty brittle after many years of heat etc...
Charlie

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Thanks Charlie!, The lines are very easily removed, almost no pressure needed at all so I'm thinking that the mechanic did indeed use the wrong size. It's good to know that they should be a tight fit. I've order some of the correct stuff, but I'm also trying to see if the mechanic knows the size he used.

Time for a new mechanic...

you gotta' wonder what else he doesn't know.

....just sayin'

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Current Stable

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I use hemostats to replace injector line return hose.

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84 300SD
85 380SE
83 528e
95 318ic

Yup they said they used 3.5mm. The new lines come in Thursday, I'll post what happens after replacement.

I just ordered Viton hose for the injector returns ($19 for 10 ft on ebay, 1/8"ID I recall). The regular M-B woven hoses were weeping, probably due to using Diesel HPR (sort of bio) at maybe 3 yrs old. For sure don't use silicone hose. I tried on one section and within a week it had swollen and was seeping at the bottom. A search found most people running bio-diesel use Viton, and seems better even w/ D2, though Tygon works for some.

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

I slightly agree. There is a chance the Mechanic used the wrong hose for the Fuel Return Lines and is all so resoposiable for leaning on and cracking the plastic nipple on the Radiator.

But, the OP has not said anything that proves that for sure.

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84 300D, 82 Volvo 244Gl Diesel

Sorry for the late reply. I ordered the correct parts and sure enough the correct size tubing solved the issue. The difference in the tubing size was visible to the eye. Thanks for all the advice.

I'm 36 gallons into HPR with no seeping through the braided return lines. 3-4 and 4-5 are a few months old. The rest were done by the PO years ago.

The diff can also leak from the axle seals.

Sixto
83 300SD

Thanks for getting back to us.

It is not just the size of the hose that can be the issue. When I first got my Volvo Diesel (same sized return Hose) I had no idea where to get the return Hose and I used some rubber vacuum hose that had a good fit.

But, despite the good fit it only lasted a week before the diesel fuel ate through it. That was also back around 1994 before the couple of changes to the Diesel Fuel happend. At that time the Diesel Fuel was less likely to eat through hoses.

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84 300D, 82 Volvo 244Gl Diesel

Fyi... once fuel has been in the hose it will NOT go back on and stay!

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If gas rated hose is used it will not last. You need to use diesel rated hose. I tried the gas rated when I needed to put something on to get home and it only lasted a few days before turning soft and not staying.

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1983 300SD
200000miles