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  #1  
Old 05-23-2018, 03:43 PM
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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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Last edited by Shern; 05-23-2018 at 04:20 PM. Reason: additional info
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  #2  
Old 05-23-2018, 03:52 PM
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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 ?.
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Old 05-23-2018, 04:03 PM
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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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Old 05-23-2018, 04:05 PM
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Quote:
Originally Posted by Diseasel300 View Post
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.
I knew I forgot to include something... yes, I have updated to the black bosch pump.
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Old 05-23-2018, 04:06 PM
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Quote:
Originally Posted by Murkybenz View Post
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 ?.
Bucks around, misses once or twice, knocks a little, then clears up. Smooth idle, smooth drive. Only happens in the morning. The prefilter isn't the issue, I know the bubble in there is normal. It's the bubble in the Ip return line that concerns me.
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Old 05-23-2018, 04:09 PM
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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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Old 05-23-2018, 04:16 PM
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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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Old 05-23-2018, 04:21 PM
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Quote:
Originally Posted by spock505 View Post
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.
Interesting idea... how far beyond the bell housing did you have to go? I never get any suction when removing my fuel cap so haven't paid it much mind.

Quote:
Originally Posted by Diseasel300 View Post
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?
I have the second version with two o rings (no crush washer) -have replaced the o -rings.
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Old 05-23-2018, 04:33 PM
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Quote:
Originally Posted by Shern View Post
Interesting idea... how far beyond the bell housing did you have to go? I never get any suction when removing my fuel cap so haven't paid it much mind.



I have the second version with two o rings (no crush washer) -have replaced the o -rings.

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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1996 Mercedes S124 E300TD - 129k - rolling restoration project -

1998 Mercedes W210 300TD - 118k (assimilated into above vehicle)
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Old 05-23-2018, 05:30 PM
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Quote:
Originally Posted by Diseasel300 View Post
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?
Quote:
Originally Posted by spock505 View Post
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.
Quote:
Originally Posted by spock505 View Post
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

fuel issues 300D 1984 - Mercedes-Benz Forum
Yes, that is definitely not a fun prospect.

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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  #11  
Old 05-24-2018, 01:28 PM
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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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Old 05-24-2018, 10:17 PM
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Quote:
Originally Posted by Shern View Post
Anyone else? Odd, often overlooked places?
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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Last edited by Alec300SD; 05-24-2018 at 11:45 PM.
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  #13  
Old 05-24-2018, 10:47 PM
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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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  #14  
Old 05-25-2018, 12:54 AM
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cracked fuel filter mount.

not easily visible. easy to replace.
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Old 05-25-2018, 01:37 AM
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Quote:
Originally Posted by jt20 View Post
cracked fuel filter mount.

not easily visible. easy to replace.
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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