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#1
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Bubbles not welcome -odd air in fuel system
Help.
I've pored over my fuel system the past two months in effort to reduce what feels like air in the system after prolonged shut-downs. To date, and from low to high, I've replaced the following: -Tank strainer (and gasket) -Line from tank strainer. -Both rear fuel lines -Lines to and from primary fuel filter -Primer pump -Rebuilt lift pump -Banjo bolt Orings (2) on secondary fuel filter -Injector return lines -Overflow valve spring stretched to 27mm -Cigar hose First thing in the morning (every morning), I see this: Clue #1 Upon engine shutdown, standing above that line -return line from IP- a few tiny bubbles work their way back up to the T (I'm deducing that the leak is not coming from the T). Clue #2 This occurs at no other clear lines. While the engine is running, all clear lines remain clear -no tiny bubbles. Clue #3/possible red herring While the engine is running, if I pump the primer pump, the clear line from the lift pump grows turbulent and a stream of tiny bubbles appear. This is only when I'm pumping, and I recall a technical term that might explain this phenomenon... something about a sudden drop of pressure releasing entrained air. What on earth am I missing?
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1981 240D 4sp manual. Ivory White. Last edited by Shern; 05-23-2018 at 04:20 PM. Reason: additional info |
#2
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OM606 fuel supply issues
The above makes interesting reading. I once tried to get my w124 pre filter devoid of air but eventually gave up. I presume it starts up ok with no issues after sitting overnight or for longer periods ?. |
#3
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Do you have the old style primer handle? If so, replace it and see if your problems go away. They're known to leak air as they age.
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#4
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I knew I forgot to include something... yes, I have updated to the black bosch pump.
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1981 240D 4sp manual. Ivory White. |
#5
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Quote:
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1981 240D 4sp manual. Ivory White. |
#6
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Which version of the secondary fuel filter do you have? Is it the one with the 1 O-ring and crush washer, or 2 O-rings on the bolt? Have you replaced the O-rings on the bolt?
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#7
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Check tank vent is clear, mine caused huge problems with pressure build up forcing fuel out over the diff housing.
Removed valve which appeared to be working but stuck, no problems since.
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David 1996 Mercedes S124 E300TD - 129k - rolling restoration project - 1998 Mercedes W210 300TD - 118k (assimilated into above vehicle) |
#8
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Quote:
I have the second version with two o rings (no crush washer) -have replaced the o -rings.
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1981 240D 4sp manual. Ivory White. |
#9
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Quote:
That sounds fine but might be worth checking to eliminate altogether, on my 124 the valve had a small rubber bot which made it look like a water drain. Looking at various online diagrams you could find it vents into a breather system, US versions are probably a different layout I always remember seeing this photo, incredible how the engine kept on running. edit, there's a description of vent location from above pic http://www.benzworld.org/forums/w123-e-ce-d-cd-td/2096337-fuel-issues-300d-1984-a.html
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David 1996 Mercedes S124 E300TD - 129k - rolling restoration project - 1998 Mercedes W210 300TD - 118k (assimilated into above vehicle) |
#10
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Quote:
Quote:
Quote:
I haven't dealt with the clear lines but I haven't seen many (if any) instances of leaking, in the archives.
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1981 240D 4sp manual. Ivory White. |
#11
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Anyone else? Odd, often overlooked places?
It’s possible my hose clamps are biting slightly into the line, but I’ve checked them pretty well. It’s also possible that my crush washers on the overflow valve are not sealing properly. Seems unlikely I have any microcracks in the clear lines, so I suppose that’s also possible. One thought that did occur, if any of my delivery valve crush washers within the injection pump are no longer seating properly, is it possible air would be getting into the IP from there? (I am not getting any fuel leaks from the IP)
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1981 240D 4sp manual. Ivory White. |
#12
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1) Pinhole leaks in the metal fuel lines (typically from chaffing under the line clamps, due to deteriorated rubber isolators).
They may not drip fuel, but they may let air in (venturi effect). Test by running fuel from an under the hood mini tank and see if you still have bubbles in the lines in the morning. 2) The clear lines also tend to fail at the junction with the barbed metal fittings. Often not visible until you remove the entire line to inspect it. 3) Any recently opened fuel connections are also suspect, especially if any crush washers, o-rings, and hoses are re-used instead of being replaced.
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78 W116 300SD 'Desert Rose' new as of 01/26/2014 79 W116 300SD 'Stormcloud' RIP 04/11/2022 Last edited by Alec300SD; 05-24-2018 at 11:45 PM. |
#13
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Ah, very useful thank you. It’s taken awhile to eliminate all other possible causes. When I prime (15-20pumps) before firing up in the morning, it starts without any shaking or missing. It’s a minor slow leak, but enough to cause one to two misses before the lines are cleared.
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1981 240D 4sp manual. Ivory White. |
#14
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cracked fuel filter mount.
not easily visible. easy to replace. |
#15
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Now there’s an interesting one...
However, in my case the bubbles are coming up from the IP upon shut down.
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1981 240D 4sp manual. Ivory White. |
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