Repairing a W111 Mercedes Fintail Fuel Pump


Returning home to Frontseat Driving HQ in the daily driver, we were surprised to see a puddle under our Fintail Mercedes that had been sitting idle overnight.  The bigger surprise was that it turned out to be fuel not coolant.

It isn’t often a vehicle is generous enough to break down in the driveway.   Perhaps it was her way of thanking us for the fun, and doting she’s enjoyed so far.

Driving a vintage car can be a unique experience, maintaining one a challenge, and finding parts to repair one nearly impossible.  So far we’ve been shocked at the availability of parts for the Fintail.  In fact Mercedes Classics carries nearly every part we’ve inquired about – nearly every part.  

While our immediate assumption was that a line had ruptured it quickly became evident that the pump itself was leaking from the weep holes, a sure sign that the diaphragm was damaged.

In normal operation the fuel enters and leaves from the same side of the pump.  The holes worn through the diaphragm however were allowing the fuel to spill out the backside of the pump casing and out the weep holes and onto the hot engine clearly a dangerous situation.

The entire pump splits into three pieces.  The part circled in red remains bolted to the engine block.

The part circled in blue is still attached to the fuel lines and is left hanging in the engine bay.

The diaphragm that needs to be repaired is found but not shown in the section of the pump highlighted in yellow. 


Mercedes exploded diagram of the pump.


Weep holes on backside of pump body (yellow in above diagram).


Pump housing (yellow in above diagram) showing diaphragm.


A hole through both layers of the diaphragm proves to be the culprit.

Okay so the problem had been found, now came the time to find the replacement diaphragm.  Unfortunately unlike many fuel pump diaphragms our unit doesn’t disassemble any further than pictured, so the spring, seal, rod, and diaphragm must be replaced as an entire unit.  Not only is it not offered by Mercedes Classics, it doesn’t even appear on the exploded Mercedes fuel pump diagram.  

While a new replacement pump is available it isn’t cheap, and it’s shiny.  Ideally we want this car to retain as much of it’s dirty original parts and patina as possible. 

A couple years ago, before the Fintail was even purchased we came across a forum post claiming that Fiat part number 0009918453N was a suitable replacement and that factoid was filed away for future reference.  Going with that vague info, we found and ordered the part from C. Obert & Company in Santa Cruz, California.

The Fiat diaphragm is a more typical example of a replacement part in that it disassembles to it’s component parts which can be individually changed out.  That said, the Fiat part is by no means a suitable replacement as can be seen in the following photos.  


Here the similarities of the Fiat part, on the left are obvious.


Here the differences, again Fiat part on the left are obvious.


Length of the Mercedes pump rod vs…


…the Fiat pump rod.


Here the diameter of the Mercedes pump rod is compared to…


…the diameter of the Fiat pump rod.

Initially it occurred to us that perhaps with a bit of minor surgery we could adapt the Fiat pump rod to the Mercedes.  It quickly became clear that the required alterations would be much more than minor surgery.  For now the idea of altering the rod has been shelved while we investigate an easier approach.  

Our revised approach involves disassembling the Fiat part, and using the diaphragm material on the Fintail pump.  At this point you might be saying, “But wait, didn’t you say the Mercedes part didn’t disassemble?”

Yes we did.  


The Fiat diaphragm disassembled.


Socket used to carefully cut the diaphragm.


Fiat diaphragm with hole carefully cut.

While the diaphragm material looked to be about the same size and the bolt hole pattern the same, it really was tough to tell.  We took one of the layers of the Fiat diaphragm and using a hobby knife and an appropriate sized socket as a cutting guide we cut a hole in the material big enough for it to fit around the Mercedes’ spring.

Fitting the newly cut Fiat diaphragm over the spring and between the two layers of the Mercedes diaphragms it proved to be a near perfect match.

By now the secondary plan of attack is probably becoming clear.   While the new layer of diaphragm fit well, it alone wouldn’t be capable of preventing fuel from travelling to the other side of the pump because it isn’t sealed along the inner diameter.  The next step then was to try gluing the old diaphragm to the new diaphragm.  While the diaphragms looked like butyl rubber and smelled like it too, were they?  An experiment with a couple of scraps, seemed to confirm it.


Near perfect match.


Sticking together but will fuel dissolve the glue?

The result, as suspected that the glue did not hold up to the effects of gasoline.  Tomorrow, we will try a glue known to be gasoline resistant.  We will update this post as soon as we have results.  Rather than wait to post this blog entry until the project was completed (successfully or not) we thought we’d publish it now to solicit some input.  Perhaps with the above details you might imagine a good approach.  Maybe you have repaired one yourself, or better yet you have a lead on a replacement.  Feel free to comment below with any ideas, or questions, and stay tuned for the update!

 


This cement is fuel resistant, and currently curing.


Both surfaces were scored.


Glued and clamped

After a couple days of glue, clamp, and repeat the finicky job of gluing the two diaphragms together was complete.   It looks promising folks!  That said, we have a new mechanical pump in transit.  While we aren’t thrilled with the replacement (more on that when it shows up) we found it much cheaper than that we first sourced.  If nothing else the replacement will serve as good research. 

 

Time now to reassemble the OEM pump;  

 

  • the spring plate slips into the actuator lever.
  • the open spring end fits over a post in the pump body.
  • the pump seal is fit into place on the pump body.
  • the spring is compressed while the pump rod is extended.
  • the rod mates to the actuator simultaneously. 

 


From left to right the parts are assembled.


The yellow circle shows the placement of the spring.


In the center of the pump the fork of the actuator lever is visible.


This shows how the diaphragm rod mates to the actuator fork.

 

The F-Bomb Studio Presents


Adding a Fire Extinguisher and Driving Lights to the Mercedes Fintail

You have entered the F-Bomb Studio, where the craftsmanship applies as much to unique combinations of curses, as it does the cars we work on.  We recommend all children and sensitive types don ear protection until work completion.

Truthfully there wasn’t a lot of swearing on this small collection of projects, some very straight forward brackets to mount a fire extinguisher and driving lights to a Mercedes Fintail.

The Mercedes W111 Fintails have a rich history in Rally competition especially going back to the early 60s. The intention with this car was always to present it as a survivor car of some local rally competitor that modeled their car after those of the big boys in Europe that they read about in “Canada Track & Traffic”.  Probably some proud owner with one car, that drove it to work during the week and flogged it in competition on the weekends.  Perhaps it was retired when, finally it just wasn’t competitive any longer, or perhaps a new addition to the family showed up and racing weekends just weren’t practical any longer.  Either way our story sees the car relegated to more pedestrian use maybe even right up to the point that the ‘new addition’ inherited it as their first car on their 16th birthday.  Eventually as happens the car is finally carefully stored away in the garage, left to slumber until we stumbled upon it decades later.

“Fintails have a rich history in rally competition especially going back to the early 60s.”

The car itself is pretty solid, and running well, so most of the effort is being directed at adding period rally equipment that will still be functional and serve our modern campaigning needs.

While we invested grueling months researching the actual competition cars and searching for the correct vintage equipment, the actual garage time is thankfully, pretty simple and rewarding. 

We searched long and hard for the right car when shopping for the Fintail, and an important trait of this particular car is its solid but bumped and bruised appearance adding to that aura of a life of competition. With that in mind the last thing we want is to bolt on a bunch of fresh shiny parts.

 

While authentic vintage fire extinguishers can be found easily enough, no respectable shop will refill them as their safety expiry will have long passed.  While we could have opted for a vintage extinguisher for the car shows, and a modern one for regular use, the thought of suffering a fire with the wrong extinguisher at hand was sobering. Instead we opted for an extinguisher with a vintage chrome appearance and modern effectiveness.  The extinguisher and extinguishing agent were carefully chosen but we will cover that in detail at another time.  With the extinguisher finally chosen the next task was mounting it securely.

 

 

 

The transmission tunnel was chosen as a mounting location for it’s accessibility to both driver and co-driver alike.   Using the seat belt attachment points rather than drilling new holes was a no-brainer.

First a cardboard mock up was made to accurately locate the seatbelt threads, and bracket holes, then it was transferred to aluminum sheet.

Cardboard, especially press board like that cereal boxes are made of is a great tool for templates.  The cardboard is stiff enough to keep shape but unlike the corrugated version, bends easily, and cut edges are clean and accurate.

Some templates for projects coming soon to the F-Bomb Studio

A large washer was used to form a radius on the ends both to protect the carpet, and for a cleaner appearance.  The aluminium was cut with a fine blade jig saw, and cleaned up with a drum sander on a die grinder.  

A step drill makes drilling various sizes quick and simple

While simple rivets connect the mounting strap to the extinguisher mount, we felt that the connection needed to be more secure for occupant safety so the strap was placed over the bracket.  While this is perhaps less attractive it is hidden once the extinguisher is in place, regardless function first on a ‘race’ car.

The stickers were peeled off the extinguisher as they were too painfully modern.  They may even be replaced by faux vintage stickers at a later date.  Both the extinguisher and the bracket were subjected to some forced weathering – a close look at the bracket will reveal some peeling paint and rust on it despite being brand new. Efforts to dull the bright white strap ends are planned, or they may be replaced altogether.

 

No vintage rally car is complete without some auxiliary driving lights.

For the Finnie we went with some vintage well used Cibie Super Oscars that were sourced after lengthy research.  Finding these locally, already with a convincing patina helped the decision making. As a big plus they were sold as a set of 4, so we have spares if needed (every minute the car sits in a parking lot is sheer anguish).  

As we did with the extinguisher mount, we wanted to avoid drilling any holes to mount these lights.  Using the existing bumper mounts was a no brainer here however a cardboard template had serious limitations. As such the first one made was truly a test run.

A chop saw was used to cut four lengths of steel.  Each was cut the same length for simplicity, and if a mistake was made at any point, the bracket could be flipped in hopes of salvaging the bracket.

Like we did for the extinguisher mount, a large washer was used to mark a radius – a curved end just makes for a cleaner look.   The radius was rough cut with an angle grinder and finished with a flappy wheel replacing the cutting disk.  

A couple of quick beads, and flat stock becomes a bracket.

Again with redundancy in mind each piece was rounded even though one end is hidden from view when installed.  We have been known to make mistakes…there IS after all a reason the garage is called  the        F-Bomb Studio.

 

 

Granted adding some very straight forward brackets to mount a fire extinguisher and driving lights to a Mercedes Fintail is by no means a big project , but with safety AND appearance covered, it is one with a big impact.

Here the large 7 inch driving lights can be seen in place but still awaiting support rods, and wiring.  Stay tuned to the F-Bomb Studio for more on that project.

 

© Daimler AG.

The global copyright remains the property of Daimler AG.

I want to thank Daimler AG for making the historic photos available to all enthusiasts, visit their archives for some spectacular views into their competitive past.