So I'm having a slight "stumble" during a cold idle after I replaced my glow plugs. I've noticed small air bubbles traveling through the lines at idle. After the car sits for a while I have large bubble in the line to the pre filter right where it makes the connection to the top of the housing. I know this is a very common issue with the plastic lines, but I have zero external fuel leakage. How is that I'm introducing air into my fuel without any leakage either during operation or at rest? Is the pump drawing air in through a bad line/seal? I've searched the forum but all related issues are accompanied by fuel leaks.

Picture of the monster bubble

It's much easier to draw air through a hose clamp than to leak fuel past one in the opposite direction.

Any restriction between the pump and the tank will cause the pump to draw air past the hose clamps. They are usually insufficient to prevent air ingestion.

Most likely from o-rings on the plastic lines or at the pre-filter.

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It took me a while to figure out that the picture is 90 deg turned to the left and I'm looking at the line running from the fuel heater to the filter assembly.

On my car, there's always a bubble like this when the engine is off. On start up, it disappears.

I believe that the bubble "lives" in the top of the filter housing where it can't move on through the system. Then it reverses slightly into the inlet line when there's no more draw from the lift pump.

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-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250

I have the same bubble on my '97 E300 D. I just changed my plastic lines. I assume it was there before but the old lines were too dark to see through. It seems strange that the pre-filter feeds from top to bottom. If it had been designed to go from bottom to top, the air would self purge. If you are seeing small bubbles in that line with the engine running, you have an air leak between the prefilter and the tank, as this hose is under suction.

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'97 E 300 D

If someone has not mentioned it; depending on where you live there is about 14.5 psi of atmospheric pressure pushing on everything.

Evidently that is enough to hold none pressurized liquids that have some viscosity to them inside of hoses with small cracks.

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84 300D, 82 Volvo 244Gl Diesel

1/2 of the system is under suction as per the attached see attached fuel flow photo

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1999 Mercedes E300TD daily driver sold at 238K miles 106K miles were mine, rust worm got it :-(
2006 Mercedes CDI new daily driver! 56,000 miles May 2016 now 85,625 Apr 2018 and Apr 2019 101,000 miles Apr 2020 109,875. March 2024 135,250

Thanks for the information. Fuel lines have been on my to do list for a while. I actually just posted the car for sale here in the classifieds. I told my wife I'm spending $100 on fuel lines after $200 on glow plug stuff, she gave me the green light on a w211! Any advice on sweet talking her into an E55 over the E350..

Link for my car if anyone is interested: http://www.peachparts.com/shopforum/mercedes-cars-sale/332805-1998-e300-turbo-diesel.html

All I can tell you about the E350 is that something is funky about the cam timing mechanism or the balance shaft. All I really remember was that it was an intermittent problem that was going to cost around $4000 to repair.

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250

I've never heard of that, but I don't doubt it one bit.