In another thread someone suggested that the source to a problem I am experiencing might be because the supply & return fuel lines on my '87 300D may have been swapped at the metal lines on the chassis. My question is which line is the supply and which line is the return line. I did notice that one of the metal lines has a dot of red paint on the line. My original question was prompted because the clear in-line fuel filter is only about 50% full of fuel. It appears that air is getting into the fuel system. For some reason the filter is never more that about 60% full of fuel. My other diesel always has the filter almost full with just a small air bubble. I would like to rule out the question about the lines being switched. How do I determine which line is which? Any other thoughts?

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• 2001 E320

Well, if its anything like a 123, the return is on top, the fuel feed is on the bottom. From the main fuel filter housting, trace a "cigar hose" out to the rear of the compartment, this is the return, but again, I don't know if its similar to a 123. Hope this helps.

Crowe,

It doesn't matter if there is air in the primary (prefilter). That's just an air bubble in the top of the filter. The fuel goes thru just as well with air in the filter. If any part of that bubble passes thru to the primary filter, it will go out the bleed hole in the secondary (can) filter and return to the fuel tank.

I doubt you engine would run if the lines were switched as the return line goes into the top of the fuel tank and would not draw fuel. If you only worry is the air in the primary fuel filter, you have no problem. Next time get a primary fuel filter that is not transparent (available from Fastlane) and you will not be able to see the bubble and all your worries will be gone.

It ain't broke so don't try to fix it.

P E H

I would certainly agree that "if it ain't broke don't fix it". The reason for the question was because the car has failed to start a couple of times and it appeared to be fuel related. I had a small fuel leak from the injector pump which was fixed when I tightened up the screws on the side plate on the injector pump. Another member posted their opinion that the lines may have been switched. Since this vehicle is new to me, I can not say for sure if the lines are correct. Hence, the question of how to distinguish between the two metal lines. Is the return line always on top? Does the red dot of paint on one of the lines indicate anything?

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• 2001 E320

Crowe,

If you still think your fuel lines might be reversed (I still think its impossible for the engine to run if lines are reversed) get under the car and trace the lines. The supply line comes out the bottom of the fuel tank so just follow it and see if it goes to the primary (in line) filter. The return line runs from the "cigar" hose parallel to the supply line to the rear of the car and disappears someplace where it goes into the top of the fuel tank.

I have no idea what the red dot means.

P E H