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Old 08-23-2012, 07:49 AM   #1 (permalink)
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Date registered: Aug 2012
Vehicle: 1984 300D, 1990 420SEL
Posts: 19
Post 420SEL - Fuel Distributor Rebuild

I rebuilt my 1990 420SEL fuel distributor last night in an effort to solve a hard start condition. If the car sits for more than fifteen minutes, it would start on the first turn for a moment and then stall. The next attempt to start woudl be successful and a slight "seeking" idle would be experienced until the engine was up to temperature. If the vehicle sat for more than twelve hours.. this symptom would not appear.

Here's what's been replaced or repaired so far:

Distributor cap, spark plugs, wires, IAC (cleaned), all vacuum hoses checked, air filter, fuel filter, EHA valve, checked OVP, and repaired fuel pressure regulator vacuum line. All of these repairs significantly improved this vehicles performance but didn't solve the original symptom.

Next up was the fuel distributor as I could clearly see fuel draining into the throttle body after the engine was off and it was sitting for at least two minutes. Upon further inspection, significant varnish was found in the throttle body with an origin of just under the air meter arm below the fuel distributor.

So off it came and I spent about three hours carefully rebuilding the fuel distributor with a kit I found on ebay by American Machinist.

I'm driving it today and if it still runs well by the time I get home I'll start posting instructions on how to rebuild. Hopefully with my findings and others I've found on this forum, I can add to the knowledge base already available on BenzWorld.

Updates should be around 10:00PM EST or just a little later.
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Old 08-23-2012, 07:56 AM   #2 (permalink)
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Trek, hope this works for you. It will be great to see some instructions explaining what you did to rebuild the fuel distributor.
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Old 08-23-2012, 01:09 PM   #3 (permalink)
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I hope so too. I am having the exact same problem with Mosby. and running way to rich as well. I think the FD is the culprit, because no matter how I adjust the A/F it is still way too rich, and doing the other things you describe.
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Old 08-23-2012, 05:13 PM   #4 (permalink)
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There was a DIY rebuild thread 4-6 years ago where someone had sourced the correct o=rings etc. (i think cis flowtech still has the otherwise no longer availabe membranes), but as far as i know few of the DIY rebuild have lasted more than a few month.
A forum search should get previous threads (pls read the new member announcement on top).
CIS flowtech are top notch rebuilds (python is said to be crap).
Otherwise, you could try a wrecking yard / a FD from a well maintained accident damaged car, check the bottom piston for easy movement, and the spark plugs to see whether the FG dumped fuel into one or more cyl.
Best of luck.

Do you mean http://www.benzworld.org/forums/w126...ml#post3501473

This might help. The 928 has a similar bosch FD http://www.porsche928forums.com/down...CISRebuild.pdf
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Old 08-23-2012, 07:30 PM   #5 (permalink)
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Date registered: Aug 2012
Vehicle: 1984 300D, 1990 420SEL
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Fuel Distributor Rebuild (Removal)

Today's driving proved to be with a smoother idle and still an extended crank time.. nothing I didn't already have but from what I read this could be the fuel accumulator? Pressure is fine..

Okay, so it may not last as I've read but those of you who want to chance it: Here's what I did.

Start by loosening the fuel cap to reduce tank pressure. Then disconnect each fuel line. 12mm for the smaller ones and approximately 17mm for the larger lines.

Attached you can plainly see the varnish from the FD and the air meter arm. That roller was stuck. A bit of brake clean got it unstuck though.
Attached Thumbnails
420SEL - Fuel Distributor Rebuild-1.jpg   420SEL - Fuel Distributor Rebuild-2.jpg   420SEL - Fuel Distributor Rebuild-3.jpg  
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Old 08-23-2012, 07:36 PM   #6 (permalink)
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Fuel Distributor Rebuild (Disassembly)

You'll want to clean the exterior of the distributor to avoid getting dirt and particles into it while disassembled.

Remove the EHA and place the o-rings asside.

Make careful note of the alignment of the valve retaining nut. This will be critical as it determines the starting height for the air meter. You will have an unbelievably hard time starting the car afterwards if you don't take note of this.

Remove the outer retaining nut and take note of the single notch and it's orientation. You can't see this now but this notch indicates the orientation of the distributor injection ports internal to that center tower some refer to as the "tower of doom".

Begin carefully removing all torx bolts except those oposite of the fuel injector ports. These are plugs and exist only to block ports that resulted from the milling process of the distributor.
Attached Thumbnails
420SEL - Fuel Distributor Rebuild-4.jpg   420SEL - Fuel Distributor Rebuild-5.jpg   420SEL - Fuel Distributor Rebuild-6.jpg  
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Old 08-23-2012, 07:43 PM   #7 (permalink)
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Fuel Distributor Rebuild (Disassembly Part 2)

Be sure to keep this unit upside down during disassembly or you chance having parts fall out without first taking note of their positioning.

Once the halves are apart. You can start by laying out the springs in the order they are removed. The only parts that will not be reused in this kit are the o-rings and gasket/diaphragm.

Use a pick to carefully remove the o-rings, clips, and fuel filter (CAREFUL NOT TO SCRATCH ANYTHING) and associated snap rings. Once the filter is cleaned, installation is the reverse of the removal more or less.

See next reply for filter removal photo. (didn't fit)
Attached Thumbnails
420SEL - Fuel Distributor Rebuild-7.5.jpg   420SEL - Fuel Distributor Rebuild-7.jpg   420SEL - Fuel Distributor Rebuild-8.jpg   420SEL - Fuel Distributor Rebuild-10.jpg   420SEL - Fuel Distributor Rebuild-11.jpg  

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Old 08-23-2012, 07:50 PM   #8 (permalink)
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Date registered: Aug 2012
Vehicle: 1984 300D, 1990 420SEL
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Fuel Distributor Rebuild (Reassembly)

Inspect and clean any surface that looks varnished or has particles / dirt. It may help to run brake clean or carburetor cleaner through your distributor to clean out passages. Mine was fairly clean as it seemed it may have been rebuilt by the last owner (terrible job lining up the diaphragm..)

To reinstall the o-rings, pinch the clips with your fingers and slide them back into place (opening faces down). Use a pick to CAREFULLY place the o-rings back onto these clips. It may be necessary to guide the clips out of their slots using a pick to get these o-rings to stay put.

Place the small metal discs back onto the distributor injector ports and place the diaphragm back on the distributor half.

Carefully clip the springs back to the seats (white discs) and avoid bending the springs while doing so. With your best judgment, place them onto the diaphragm.
Attached Thumbnails
420SEL - Fuel Distributor Rebuild-12.jpg   420SEL - Fuel Distributor Rebuild-13.jpg   420SEL - Fuel Distributor Rebuild-14.jpg   420SEL - Fuel Distributor Rebuild-15.jpg  
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Old 08-23-2012, 08:00 PM   #9 (permalink)
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Date registered: Aug 2012
Vehicle: 1984 300D, 1990 420SEL
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Fuel Distributor Rebuild (Reassembly Part 2)

-------

Take a look around for any left over parts or mistakes before this step as there's now no turning back.

-------

Pick up the other half (top of the distributor) and CAREFULLY without bumping or moving the bottom half, slide the top back onto the tower of doom.

Carefully rock the head back and forth in ever-so-slight degrees of movement until it starts to slide on. Be patient. The longer it takes the more sure you'll be that you did this part right. The last thing you want to happen is an o-ring falling off.

Inspect the springs and seats as you compress the halves. Adjust as necessary with a pick. Positioning is critical to operation.

Once the halves are compressed. Inspect the diapgragm seal. No wrinkles or rolls should be visible.

Reinsert and carefully tighten with a crossing pattern from the center of the distributor outwards. I was not able to find a torque spec for these but your best judgment should do. Tighten to feel.. If anyone has this spec, please post it in a reply. We'd be glad to hear it.

Reinstall it on your Benz and you be back to riding in style with a smoother idle and potentially a little bit of regained performance. I can't take responsibility for those who take this on but I wish you the best if you're crazy enough to try... I was pretty nervous about this one.
Attached Thumbnails
420SEL - Fuel Distributor Rebuild-15.jpg   420SEL - Fuel Distributor Rebuild-16.jpg   420SEL - Fuel Distributor Rebuild-17.jpg   420SEL - Fuel Distributor Rebuild-18.jpg  
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Old 08-23-2012, 08:21 PM   #10 (permalink)
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You are a brave man.
How did you source the big seal between the halves (3rd pic post #7)?
I would be interested in how the unit holds up.
Cheers

P.S. My pal and old time Mercedes tech Jerry (RIP) had a retired re-builder pal in Encino, who showed me the elaborate test bench he still had in his old work shop. They rebuild my FD there.
Used to restore gullwings and other MB classics on Ventura Blvd in Encino.
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