You'd think this would be a no-brainer! I've got clear fuel lines so I can see where air is coming from, and yet...

1987 300D. Fuel going into my secondary filter has no air, but the hose coming out of the filter is only 1/2 full of fuel. I can see the fuel cascading down the empty tube and starting to form a little resevoir at the bottom before it heads into the temp sensor (?) then the lift pump. This is the first time I've had a clear hose on this part of the system, so I'm not sure if this is normal. It seems like the fuel system should not have ANY air pockets, except sometimes a little bubble at the top of the prefilter. My configuration is a bit different since I have a greasecar system, but it's always worked fine up until recently.

The problem is really frustrating because I will work on it, and run the car that night, everything is fine, but then the next morning when it's time to go to work, as it warms up there is more and more air, until it's ready to stall.

I've checked all hose connections for tightness, I've replaced the filter, I replaced the crush washer on the banjo bolt that holds the filter. The only thing on the filter I haven't messed with is those bolts on the side of the housing that hold on the hose fittings, other than to check that they are not loose. What might go wrong with those? Could they be causing the problem?

What else am i missing? Could my lift pump be going out? Not pulling enough fuel to keep the filter full? That being the case, though, why would the line leading up to it be full of fuel with no air?

Arrrrgggg! Help!

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1987 Mercedes 300D ~200K (Greasecar & Biodiesel)
1993 Ford F-250 7.3 IDI diesel 165K (Biodiesel)
1996 Thomas/International Bus with DT466 engine

1. Any chance that the clear line you put on is bigger than stock & the pump can''t keep it full?

2. Black hose will squish under the clamp & seal very tightly ..... the clear hose is harder -- perhaps a little larger in diameter -- perchance the clamp can't make a good seal ??

Try going back to stock hose .................

Those kind of fuel line leaks drive me crazy too. When I was having this problem it took numerous tries to correct it as it was leaking in several places. First the primer pump leaked (your car wouldn't have one but the older ones seem to develop this problem a lot) and then the line going to the prefilter....which didn't appear to be leaking from visual inspection, but my mechanic pressurized the system and found the leak. I had given up...

What Estod says makes sense....sometimes you can't "see" the leaks but if the clamp isn't getting it tight enough or it is leaking around the edges right where the fuel line terminates into a fitting (such as the filter clamps), there could be an air leak. Good luck!

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1976 240D "Katja"

I couldn't visualize the different hoses you described, so I'm not sure if my experience is relevant, but it may be.....

I will spare you the details, but I had a very frustrating air-entry problem, which cost me a lot to figure out and remedy (mechanics costs, rental car while my car was in shop, etc....). In the end, the cause was an old rubber fuel line (probably original) going from the hard metal line from the tank, to the pre-filter, was the problem. I did tighten the band clamp to make sure it was tight, but I eventually found that the air was entering through cracks in this hose on the inside diameter of the hose which would distort and open when I tightened the clamp. I cut a half inch off this hose and re-attached, and it was fixed!!!!! The next day I replaced the hose with a piece of goodyear rubber fuel line from a local auto parts store for about a buck and a half.

So, the moral of the story...replace all your old rubber hoses in the fuel system.

Good Luck,

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)

Well, I've tried so many things, sometimes more than one fix at a time, that I'm not sure what fixed it! The most recent attempt was to check the tank screen. When I drained the tank, I saw that the tank screen was fine, but the bottom of the tank had some fuel that had become "cloudy" from a recent freeze/gelling incident. So far the car has been running without any problems. I think I was just clogging filters repeatedly with the cloudy fuel. I did also seal some house nozzle mountings with thread sealant, and replace hoses, etc. But I don't think it was the hoses. The final fix was the sealant and the tank draining, which also included a new secondary filter. I really think it was the fuel clogging up my secondary filter though. I changed it a couple times during my troubleshooting. The car would run fine the night of a given fix attempt, and then the next morning would start sucking air again. I think the filter was just getting full of gelled fuel, which would further emulsify with the colder overnight temperatures.

Drove to work today no problem, so let's see what happens...

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1987 Mercedes 300D ~200K (Greasecar & Biodiesel)
1993 Ford F-250 7.3 IDI diesel 165K (Biodiesel)
1996 Thomas/International Bus with DT466 engine

My 300sd is having similar problems. I have replaced both filters and checked the tank screen. Can the primer pump be sucking air and pressurizing the system? If so, do I need to replace the entire pump assembly, or just the plunger?

The best thing I've done to aid in troubleshooting fuel issues is to replace all my rubber fuel lines with translucent polyurethane hose. You can get any size you want at www.mcmaster.com. The translucent hose allows me to track down air leaks easily by simply looking for the bubbles in the fuel lines.

I swapped out the hoses on both my Mercedes and my Ford F-250. Helped tremendously in both cases.

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1987 Mercedes 300D ~200K (Greasecar & Biodiesel)
1993 Ford F-250 7.3 IDI diesel 165K (Biodiesel)
1996 Thomas/International Bus with DT466 engine