Short Electric Fuel Pump O-Ring Question

[Tomnistuff]

I'm rebuilding (big word for a little job, mostly cleaning) my short electric fuel pump that sat long enough to dry out and smell really bad.

I'm not using the two small O-Rings in the kit for the electrical wire connectors because they have never leaked, and I don't want to de-solder the terminals to add the new O-Rings.  This is a "cleaning" job, not a problem resolution job. 

My question is about the "third" small O-Ring that, on some fuel pumps goes between the motor/brush housing and the vane housing.  In the photo below, it's the small O-Ring groove on the brush housing below the motor shaft and above the almost obscured wire terminals, and it would mate to the hole in the shiny flat spot on the side of the vane housing opposite the vane near the fuel outlet.  I think it's a fuel bypass to cool the motor, but I don't think there was an O-Ring there when I took the pump apart.  I just hope that I did not "lose" the O-Ring that would go there.  I think the fuel is supposed to fill the entire cavity.  There should be no reason to try to seal the fuel out of that cavity, since there is normally fuel on both sides of it - the motor side and the vane side.

I'm about 50% confident that this model pump does not need an O-Ring there, but I don't want to put it back together until I know for sure.

Can anyone advise me?

Tom Kizer
Levis, Quebec, Canada

[Tomnistuff]

I think I found the answer myself.  I bought the MercedesSource on-demand video entitled, "1963 to 1973 Short MFI Electrical Fuel Pump Lower Section Resealing Instructions".

Although Kent Bergsma does not mention the O-Ring in his On-demand Video, the video clearly shows during disassembly that the O-Ring is not there.  I must assume that if there was supposed to be one there, he would have either mentioned it or would have installed one during reassembly.

Anyway, now that it's been disassembled and reassembled once, I'm confident I can do it again if necessary.  I may get another chance anyway if it leaks when I test it.  That's typically my luck.

Tom Kizer

[Rick]

I dont claim to be a short pump expert, but I have rebuilt several and I do not remember an o-ring a that location.

[mbzse]

.../... question is about the "third" small O-Ring.../...Can anyone advise me?

In this step-by-step guide, the O-ring is certainly there...   See:
http://www.mercedes-scheune.de/index.php/vollrestauration-w111-coupe/step-5-motor-achsen/benzinpumpe-restaurieren#g_1_4

Vendor Vintage Euro parts has in their kit, too. No 5 in parts list
http://www.ebay.com/itm/SHORT-STYLE-FUEL-PUMP-REPAIR-KIT-PAGODA-280-SE-SL-BOSCH-MERCEDES-W108-W111-W113-/263111373335?vxp=mtr&hash=item3d42a8f217

[Tomnistuff]

With the Vintage Euro Parts kit comes an list of parts and instructions.  Difficult to read but there never the less is a note that says about #5 that it is used on some applications.

Apparently DB doesn't believe in providing too much information.  After all, information is power and power is money.

Tom Kizer
Levis, Quebec, Canada

[BaronYoungman]

It depends on the year make of the pump, I have seen both, I would say 90% of then DO NOT have them.  I always check as I disassemble.   You can look in the piece it fits up against if it has a filled in hole(the hole only goes  2mm down and then solid aluminum) it does not need one, if the hole feeds down then YES you need it, otherwise it disrupts the flow when you are priming the pump, and may effect the flow rate when running.  Sorry took me so long to answer.  If you need just that one gasket I can mail you one.
Bob
And I really would take the time to replace the two o rings behind the electrical tabs.  I have found after doing 50+ pumps that it comes back to haunt you , better to do it once and be done with it.

[dirkbalter]

I was worried reading the post and the chance that I forgot the o-ring when rebuilding the pump. I looked at some of the photos  I took and also believe mine didn't have the o-ring in question. The photo shows the pumps insight right when opened and no sign of ring .

Dirk

[Tomnistuff]

I think I understand how the pump flows, but the relative amounts of flow are beyond my understanding because they are dependent on the dimensions and pressures.

As I understand it, flow enters from the tank into the pump and fills the filter area A due to gravity when the pump is off and dry.  With time, the pump will self prime with cavities B and C also filling up with fuel due to internal leakage.  When it is started, the VANE in cavity B will prime the pump from cavity A if not already primed and will pump fuel from cavity A through cavity B to cavity C at a higher pressure.

So long as the pump is running, fuel flows from the outlet to the engine compartment fuel filter and to the injection pump to start and run the engine.

Without the cavity D to cavity A O-ring seal:

While the pump is running, the fuel pressure is highest in cavity C and lowest in cavity A.  Without the O-ring seal between cavity D and cavity A, fuel can flow from the high pressure volume of cavity C through the gap created by the missing O-ring and into cavity A, resulting in some reduction of fuel flow to the engine.  Because there is no O-ring seal around the motor/pump shaft between cavity C and cavity D, there is some small amount of “leakage” fuel flow from cavity C into cavity D due to the lower pressure in cavity D caused by the suction of the VANE through cavity A.  How much fuel flow into the motor cavity D is available to cool the motor is questionable.

With the cavity D to cavity A O-ring seal:

If as in the second sketch, the O-ring is positioned between cavity D and cavity A, then there is no fuel flow from cavity C to cavity A.  Any fuel flow that is sucked into cavity A from cavity C must come through cavity D and must pass from cavity C to cavity D via fuel flow leakage around the shaft between cavity C and cavity D.

The presence of the O-ring between cavity D and cavity A should have two effects.

1) it results in a slightly higher fuel flow to the engine because of the reduction of leakage from cavity C directly to cavity A, and

2) the fuel flow that is pulled from cavity C through cavity D and into cavity A serves to cool the motor.  The cavity C to cavity A leakage without the O-ring serves no obvious (to me) purpose other than possibly to cause faster pump-off initial gravity priming.

If this analysis is reasonably correct, then, had I done it before reassembly, I would have installed the small O-ring between cavity D and cavity A.

I am satisfied that the pump performs adequately without the O-ring, since it has not been there for approximately 50 years.  Only five years of storage prompted me to clean the pump and replace the O-rings, with the exception of the cavity D to cavity A O-ring.

My pump when primed and tested after disassembly, varnish removal and seal replacement, primed almost instantly with diesel fuel and flowed one liter per 11 seconds.  The target is one liter per 15 seconds or less.

These results were without the O-ring seal.

Tom Kizer

[BaronYoungman]

Yes but after Bosch determined that they did not need that (A-D) flow through so all the later pumps it is just molded closed.  I will post a picture of the cavity so you can see what I am talking about, but 90% of the ones I work on do not have a hole that goes through just a indent (cavity) of where one use to be, so they require no o ring.  My guess they at some point Bosch just changed the die and filled in where the flow through was.
Bob

[Tomnistuff]

Thanks, Baron, for the explanation.  I guess that means that in the case of the pumps without a passage to cavity A, there is no pressure differential that causes flow through the motor cavity for cooling.

That motor cavity is a little like an upside down glass pushed into a bucket of water.  The air in the glass compresses up into the glass to a pressure that matches the pressure of the liquid but the air can't escape so what little liquid gets into the bottom of the cavity (glass) as the air compresses and makes room for it just gets thrown around a little bit by the bottom of the armature.

I can't see any way for the motor cavity to get filled with fuel, but it must, or there would be a risk of explosion if the air/fuel vapor ratio was right in the presence of a spark from the brushes.

Undoubtedly, I'm missing something and I just have to trust 50 years of Bosch Pump History.

I really wish I understood the difference in the three "systems":
   1)  the drilled hole into cavity A without an O-ring to isolate cavity C,
   2)  the drilled hole into cavity A with an O-ring to isolate cavity C and force flow through cavity D, and
   3)  the plugged or un-drilled hole to cavity A allowing only leakage from cavity C to cavity D, but no forced flow.

I wonder if there is a European patent that describes how it's supposed to work.

Tom Kizer

[BaronYoungman]

Here is a picture of the "typical" bottom piece that matches up to where the o ring in question mounts (or doesn't) as you can see from 1st picture it looks like it has a hole but with more light in the second picture you find it is filled in.  I have 5 pumps that I am currently working on (waiting for rewound armatures) and NONE of them have a flow though.