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1984 300D, 1990 420SEL
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Discussion Starter #1
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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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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4,468 Posts
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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'82 Euro 500SEL, '85 Euro 500SEC AMG WB Cabriolet,'86 Euro 500SEC RUF
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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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CH4S Admin , Outstanding Contributor
1985 500SEC, 1991 190E 2.6.
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40,674 Posts
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-s-se-sec-sel-sd/1447046-fuel-distributor-rebuild-kit-advice.html#post3501473

This might help. The 928 has a similar bosch FD http://www.porsche928forums.com/download/manuals/CISRebuild.pdf
 

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1984 300D, 1990 420SEL
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19 Posts
Discussion Starter #5
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.
 

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1984 300D, 1990 420SEL
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Discussion Starter #6
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.
 

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1984 300D, 1990 420SEL
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Discussion Starter #7
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)
 

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1984 300D, 1990 420SEL
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Discussion Starter #8
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.
 

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1984 300D, 1990 420SEL
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Discussion Starter #9
Fuel Distributor Rebuild (Reassembly Part 2)

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

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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.
 

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CH4S Admin , Outstanding Contributor
1985 500SEC, 1991 190E 2.6.
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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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1984 300D, 1990 420SEL
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Discussion Starter #11
Source of Kit

Thank you. I lack the expertise and equipment to properly recalibrate these but was careful to get "close enough" results. I'm sure it could run better if I had the time and money. This was more of a stop-gap measure. It did seem to work though.. Would have loved to see that shop. These injector units are clearly an art.

Here's a link to the current kit:

Mercedes W126, W463, Coupe C126, SL R107 M116, M117 Fuel Distributor | eBay

Pretty good kit for the money. It came with o-rings for the distributor, a gasket/diaphragm, and two neoprene o-rings for the EHA.
 

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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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Trek,

Great job! Love the pictures and explanations. Takes a lot of the mystery out of our fuel distributors.

Thank you!
 

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'95 993 / 2011 F150 Raptor SC / Land Cruiser D4D V8 / 500 SEC
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Thanks for that link Dvorak, I just ordered a kit. It's one of these things you don't need, and can't find anywhere when you do.
 

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78 280S, 81 300SD, 82 300D, 92 300E 2.6, 90 260SE, 94 S280, 10 E300,
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Thanks for the great write up, I ordered 2 FD kits from the same eBay seller a while back, the pics posted will be very useful once I tackle my FD.
 

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1984 300D, 1990 420SEL
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Discussion Starter #15 (Edited)
You're very welcome! I'd appreciate seeing if you all find the same varnish. That o-ring in the base of the fuel distributor takes a beating as the air meter valve actuates it constantly. It's definitely a wear part.
 

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1984 300D, 1990 420SEL
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Discussion Starter #16 (Edited)
Final Adjustments

I drove the 420SEL for about a week before garaging it due to an overheated catalytic converter and a lack of time to fiddle with it. Turns out after a few hours of testing, I have determined the following:

After a distributor rebuild and probably before you go down this route should test and check:

- Air flow potentiometer voltage at idle reads .7VDC. Mine read 0VDC as it was defective.. Dead spots wear into these just like any other sweeping arm potentiometer.
- Duty cycle across pins 2 (ground) and 3 (lamda controller) reads approximately 50% at idle and fluctuates about 10% under load.
- O2 sensor response is quick. Mine turned out to be defective as it was stuck on .1VDC
- Air Fuel adjustment tower has been readjusted to match rebuilt fuel distributor spec. This can be CAREFULLY achieved by monitoring the O2 output with the EHA unplugged as to not foul your readings.
- Check fuel pressure regulator only allows blead off of fuel pressure with applied vacuum.
- Check temperature switch as it is responsible for operation of the cold start injector below 35 degrees C. Verify injector sprays for 1-5 seconds during startup and only when switch is below 35 degrees C.
- Check idle air controller for free movement and listen to it with a scope to see if it is "struggling". Mine was jammed. A couple good sprays of brake cleaner fixed that right up.

Hard start after engine has reached operating temperature no longer exists.

More to come as I get his car up and running. Had to give in my camera (phone) after I moved down to Palm Bay for business but will post some decent pictures once I'm back up and running.
 

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'82 Euro 500SEL, '85 Euro 500SEC AMG WB Cabriolet,'86 Euro 500SEC RUF
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I took your lead and pictures and rebuilt my FD in my Euro 1984 500SEC. I thought it might turn out to be rocket science, but turned out to fairly straight forward. My FD is the Gen I type with no EHA, And it has the outside port adjusters.
Found the tower of doom not to be that off putting, but straight forward as well. I believe you are referring to the O-Ring inside the tower of doom. that would see a lot of wear. the O-Ring on the bottom of the FD does not move with the exception of mounting the FD onto the Air Plenum and when it is removed.
All in all I want to thank you for this thread, I would never have attempted the rebuild without it and the CIS rebuild Porsche article.

I also did not try to adjust any settings, the springs and discs(plastic or nylon, looked new as well, the O-rings were all very tired and the diaphragm was more tired, no pin holes, but very worn. I too, used the rebuilt kit from Ebay.
Car started and ran fine with a good idle and correct rpm with a couple small A/F adjustments.
Thanks again for th9is thread and I gave you a shout out in my "Blue Bird II...Maybe Not" thread.
 

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1984 300D, 1990 420SEL
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Discussion Starter #18
This is great to hear! I was wondering if anyone would have similar results. The o-ring I mentioned was the inner one like you saw and it does see a lot of wear. That bottom one seems to be there to create a seal to the throttle body to avoid un-metered air from passing the plate.

I definitely wish mine were as simple to adjust as yours. Since you do not have an EHA to worry about, your vehicle doesn't run O2 either I assume and is purely a mechanical adjustment?
 

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'82 Euro 500SEL, '85 Euro 500SEC AMG WB Cabriolet,'86 Euro 500SEC RUF
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It is a pure Euro car with no ECU to run the O2 sensor, EHA (not used on Gen I's), or Frequency Valve.
 

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2008 ML350 ,1986 560SL
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Trex, first let me thank you for the great info and the time it took to collect and disseminate. Now as for this oring mentioned in the latter part of this thread, are you referring to the oring on the slotted cap in the bottom through which the spring loaded plunger protrudes? Or should there be an oring on the top of the distributor tower that goes into the top half? Picture 1 of your reassembly part 2 does not show one and for the life of me I don't remember seeing one on my when I took it apart. Please explain I just reassembled mine and I want to be sure all the parts I removed are in place, but I'm not sure if it was dissambled prior to me.
Thank you.
 
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