hi all,

i have an air leak somewhere before the primary filter, making the idle really rough. i already changed the rubber line that runs from the metal line to the primary filter. also replaced the primary filter with a different brand because i thought maybe it was the source of the air leak. at idle the bubbles are really tiny, when i rev it up the bubbles get much larger. i'm using new high quality german made hose clamps on the rubber line (both at the primary filter and at the one on the bottom of the tank) i even put some RTV sealant on the ends of the rubber lines just to make sure air wasn't getting sucked in. now i am thinking that the air is getting in somewhere on the metal fuel line (there are no fuel leaks whatsoever, but i know a line can suck air without leaking fuel) my next step is to run a vacuum on the metal line to see if i can find a leak. so, just wondering if anyone had an air leak from the metal line or if it can suck in air somewhere else (the threaded line from the bottom of the tank maybe?). also, i was thinking if there is a way to use some sort of colored smoke and run it around the lines and then have someone look in the primary filter to pinpoint the air leak (just a crazy thought, but in theory i think it would work)...any input will be greatly appreciated

I had the same problem with my SDL. Sucking air into primary filter when engine was revved. I thought I was low on fuel and I put more diesel but to no avail. I was 60 miles from home and I managed to get home by driving slowly. What happened was that my tank filter was clogged real bad and the lift pump was sucking air from the weakest point, o-ring of tank strainer, hose clamp etc. The lift pump is more powerful than I think possible.

You can run a bottle of fuel just before the primary filter, then just underneath the tank, after the hard line, to see where the air is coming from. You should be able to isolate the problem.

Good luck.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

If I am not mistaken, the lift pump has crushed a fuel tank or two. Saw pics once. So yes, pretty strong!

And I thought of this when I read the title

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

Don't let them drive you crazy because you will never stop them.

Air is constantly coming through from the tank in the form of tiny air bubbles due to foaming of the fuel slopping around in the tank. They collect in the pre filter as a bubble and are then flushed through into the main spin-on filter at high RPM. The main filter is designed to deal with it.

Once the low pressure circuit has been fully bled it is impossible for air entering the main filter to get into the low pressure chamber of the Fuel Pump.

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Beagle

thanks for the input guys; i'll try running straight from a bottle to see if the rough idle goes away (the bubbles wouldn't bother me if there wasn't a rough idle...lol) i replaced the threaded line from the bottom of the tank about 6 months ago and remember looking into the strainer from the bottom-it looked clean. also, the car runs really good and fast for an SD; it's got no problem getting up to speed or hesitation of any kind.