Funola was kind enough to put on a clear hose on my fuel line. Here's a video of all the bubbles. 1984 300TD Air in fuel lines - YouTube

The first one is from the the return line. There are lots of bubbles... The harder to see one is the other line. There's a just a dribble coming from that. Is that normal or should the line be full of fuel?

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1984 300td euro turbo

"I just removed it (fuel tank vent valve) from my car and I can blow on either end with little to no effort. I'm guessing that's a problem?

The vent valve may not be opening at the correct pressure but we know it is not clogged. With the fuel cap open, blow into the tube that the vent valve is connected to, if you can blow through it, then you do not have a tank vent problem.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I'll try and do that. I don't think I fit under the car without lifting it.

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1984 300td euro turbo

Just watched the video. You should not get the occasional bubble in the supply line. Crawl under the car and check the supply line for any signs of wetness. Look under the fuel line clamps (take them off) and chances are there's a pin hole leak under there. take a section of 5/16 fuel hose, slip over the leak and hose clamp it for an instant fix.

I had such a pinhole in the supply line under a supply line clamp that I found (finally) after much troubleshooting. It was a very small pinhole that did not dribble fuel on the ground, yet caused constant bubbles in the return line.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Get a 3 ft x 1/4" i.d. vinyl hose (Home Depot). Dip the end in a cup of microwaved boiling water to soften it and slip it over the steel vent line. No clamps needed

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I've got my 300td on stands right now to address this. The fuel lines are rusted out, producing the air leak. There's a particularly bad spot at a clamp above the subframe, so the subframe and differential have to drop to get at it. Nasty job.

Ok great! I will go check that as well!

Ok here's a dumb question... Look at attachments... I have 2 caps that came with the car. The black one is the one that's currently in use. Should I be using the silver one? I tried using it but It always gets stuck when I turn it. Makes it impossible to get it off without using pliers. It's not pressure holding it. It just get's stuck.

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1984 300td euro turbo

I have an easy solution for you. I can sell you a length of 5/16 o.d clear nylon fuel line to replace the steel line. Cut away the steel line and fish the nylon line in place and snaps in the holders. Makes a nasty job a breeze. Plus now you have a clear line so you can see bubbles through. Win/win.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The silver one is the Mercedes one I'd use that. Put a smear of grease on the rubber sea if it's hard to turn.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I would say that this looks suspicious... It might just be oil. I'll go test.

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1984 300td euro turbo

Be very careful when you go in there (if the line is rusted). Any disturbance may enlarge the pin hole and increase the size of the leak and make the car not drivable (and drain your tank slowly). My advice, leave it alone for now, come by next weekend and I will replace the line with a clear nylon line.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Ok Ok. I'll wait till next weekend.

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1984 300td euro turbo

Thanks Funola, but I'm beyond that -- I just fitted the fuel lines over the rear axle this afternoon. I'm also looking at the brake line while I'm there -- it shows some external corrosion.

Looks like I'll need a new subframe bolt to button things back up. It's seized to the aluminum sleeve inside the subframe bushing. Fortunately, I have a spare subframe bushing somewhere.

The issue with air in the supply line can also be caused by a restricted primary. I suffered with this on the SDL once and it cost a pretty penny to finally resolve.

Can a restricted primary filter cause air in the supply line (output from the tank, input to the primary)? We can change it in your honor just for the hell of it. It's cheap and easy enough.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked