At this moment, I do not believe that is the case. The copper washer does ALL of the sealing that is required for the inner chambers / plunger and barrel.

Why do you suspect that additional sealing is required outside of the copper washer? Forgive me, you have seen much more than I have.

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The Del. Valve holder has Two Seals, for Two Different purposes!

the Inner Copper seal is for the High Pressure outlet from Plunger to del. valve holder....

The Outer Rubber 'O' ring seals the Gallery (Low, Lift-Pump) pressure....

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Alastair AKA H.C.II South Wales, U.K. based member

W123, 1985 300TD Wagon, 256K,
-Most recent M.B. purchase, Cost-a-plenty, Gulps BioDiesel extravagantly, and I love it like an old dog.

W114, 1975 280E Custard Yellow,
-Great above decks needs chassis welding--Really will do it this year....

thank you for clearing that up. I was hoping I was missing something.


SAS,

I totally understand what you are saying. Nice job. Lets hope you are on your way to a successful marriage, ... now that the pre-nups are signed.

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It is the gallery pressure that somehow at an undetermined point in time becomes negative just long enough to slurp some air that has bypassed a badly worn o-ring. So, jt20 is right in his assumption that the copper seal was the culprit (if I am postulating correctly) due to it being worn enough to allow bubbles of air (from a worn o-ring seal in the LP galley side of the system) to get by on the down-stroke of the plunger.

The nature of my air-bound IP condition was such that it would take several days to accumulate enough air to start the nailing. I got to where I could hear a change in the injector sound, and predict a nailing episode for the next day.

Thanks to the both of you for the insight. However, it is still a huge mystery to me where the negative pressure came from if not from the worn copper seal.

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327K on 1986 W201, 602.911, 722.414 2.5 190D ("The Red Baron")
139K on 1993 W124, 104.942, 722.433 2.8 300E ("Queen")

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Do not worry about your difficulties in Mathematics. I can assure you mine are still greater.
- Albert Einstein

take a walk down memory lane...

I think you were right when you mentioned 'frothy mixture'. Air gets past the o-ring in very small amounts with the vast pressure fluctuations. All the fluctuations excite the fuel/ air mixture = froth.


It may gather in the fuel filter since the filter sits higher than the helix, but I don't think that is the source of the air thats messing w/ your pump. I would imagine that its the frothy mixture around the barrel and that your keen understanding or rubber o-rings is correct....

air in, no fuel out.

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I was wrong in assuming that. If, in fact, the gallery (fuel supply) for the helix to the barrel and plunger is in that interstitial space b/w the DV carrier and where the DV holder threads into the pump body, then the frothy mixture is finding its way into the barrel via the helix under normal operating conditions. And the o-ring is the fail point.

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Personally, I believe the froth gathers in the pump gallery. The air gets in as there's loads of pressure fluctuations in the gallery during running. The L.P. itself will supply fluctuating pressures just to add to the issue, a leaky 'O' ring then allows a little air in...

the fuel Inlet and the Outlet are below the top level of the gallery void inside the pump. There is therefore a large void the length of the pump to allow air/foam to accumulate...

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Alastair AKA H.C.II South Wales, U.K. based member

W123, 1985 300TD Wagon, 256K,
-Most recent M.B. purchase, Cost-a-plenty, Gulps BioDiesel extravagantly, and I love it like an old dog.

W114, 1975 280E Custard Yellow,
-Great above decks needs chassis welding--Really will do it this year....

To positively identify the 'O Ring' as the culprit paste grease externally around the screw on fittings in the malfunctioning IP for test period. This would seal the source of the air.

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Dionysius

I am looking to see if any of you think this idea is valid or not.

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Dionysius

it may be effective, but certainly temporary

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For me, the act of replacing the o-rings and the subsequent boost in performance/quality was an indicator of problems in the pump/atmosphere barrier. However, I did simultaneously replace the copper seals as well, so the grease method might work, as long as the grease remained congealed enough to seal the air gap (heat would thin grease).

I am still not completely sure how the air got in. My pump shop's "pump specialist" swears up and down that there is no way for air to get in the gallery unless it comes from the lift pump because of constant positive pressure in the gallery. But, as we have discussed here, the possibility for air infiltration as a result of some negative pressure in the gallery created by the dynamic pressure shifts in the system due to acceleration, shift patterns, slow-down, etc, as well as sitting over several nights with less than 1/2 tank of fuel and the negative siphon pressure possibility therein, still looms in my mind. By that logic, every seal that presents a barrier to gallery pressure/atmosphere should be inspected and replaced.

In my case, the pump sat without injector lines connected to the DV holders for about 5 months while we painstakingly rebuilt the head. Also, there was 246,000 miles on this pump with 60 or 70K of those miles using B99. After touring a biodiesel plant as a research project, I know that there is a crapload of lye and methanol in B99 and both substances are known to deteriorate EPDM - the stuff that o-rings are made of.

__________________
327K on 1986 W201, 602.911, 722.414 2.5 190D ("The Red Baron")
139K on 1993 W124, 104.942, 722.433 2.8 300E ("Queen")

http://farm5.static.flickr.com/4081/...0bb92d3c_m.jpg http://i370.photobucket.com/albums/o...g?t=1325284354

Do not worry about your difficulties in Mathematics. I can assure you mine are still greater.
- Albert Einstein

take a walk down memory lane...

I just love it when these threads get all scientific and stuff..

good info

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Ye guys are so great. Thank you Sasq for your dedication and thoroughness.

JT.....Very true. The grease method is only suggested as a diagnostic tool to prove/disprove the O Ring issue. One could use another compound (years ago in oil-well tools we used Dow Corning DC3 on O Rings going down into the earth) to take the temp. Now there is probably a better compound around.

Now I have another question. Wouldn't it be totally possible for air to enter the same area (top of sec filter) from the soft daisy chained lo pressure braided fuel return lines from the injectors? Those little hoses can stiffen up and certainly could allow air ingress at the hot metal barbed connections on the injector body. If so then could that also cause the very same symptoms???
The reason I ask this is that I never see this listed as a possible cause of nailing.....

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Dionysius

Yes, thanks again SAS, a thorough thread that is updated with conclusions is always enlightening.



'D' - it was previously implied, I believe.

but the system is self-priming, and to aide this benefit, the filter is located ABOVE the IP, in a relative sense. The air rises quicker than the fuel can be sucked into the IP.

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jt....I am confused. I just posted and I do not know if you are answering my question or talking to sasq.

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Dionysius