Mercedes MFI pump question

[RDM]

I suspect there's more MFI experience here than on Pelican's Mercedes board, so here we go... Yes, I'm looking at you, Jonny, and all your friends in the middle of your long, dark, cold winter! Fun fact: At the winter solstice last week, 8 hours and 9 minutes separated sunrise from sunset here.

1971 280SE with a 250SE motor in it. Bosch 6-plunger MFI pump much like on 911s of the time except the pump cylinders are inline rather than in two rows like the 911 pump. Other than that the tech is extremely similar.

I have no history of the pump other than it has been installed in this non-running car for about ten years. With much patience, the car will start, and will run OK (but prouce little to no power) until warmed up. Then it dies. I've been through all the adjustment and tuning. The MFI pump is what's left to investigate.

I have the pump out and have opened it up. I don't know enough to know what I am seeing, but what I'm seeing seems strange to me. I'm ignorant, so that strangeness doesn't mean much.

On to the pictures:

The space cam and stylus, with throttle closed. Note the meaty part of the space cam off to the right of the stylus.


When the throttle is opened, the space cam turns clockwise, and the meaty part of the space cam moves away from the stylus. The stylus moves across an essentially flat part of the space cam, and the rack moves only a faint twitch. Is that right?


As I understand it, the rack should turn the black collars at the top of each cylinder to change the amount of fuel that goes to the injector. From full throttle to no throttle, the collars don't turn more than about two hairs breadths. Note also that cylinder number 2 gets no fuel- it seems like the spring does not push the cylinder back down. I can move it with my fingers, but it doesn't move on its own.


The regulator end, with adjustment screws. Moving the centrifugal weights does exert spring pressure on the front of the space cam. but does not move the rack or the space cam.


Little known fact:Bosch makes black boxes in silver. See immediately below. Atmospheric pressure can on the right, (removed) temperature sensor on the left. These bars and linkages and springs and ladders do something. I think.


I think I read that you are supposed to be able to move the rack by threading a 4mm bolt into the end of the rack and moving it. This one is locked up tight.


This is clearly beyond me, and will likely be going off to the MFI pump day care as soon as I can locate a trustworthy one that doesn't require arms and legs in payment. I'm open to advice and suggestions and help.

Teach me. Please.

[MAH77]

It's been a while since I worked through how the MFI pump works, but could the space cam be installed backwards?

[Jonny042]

Indeed I have been hibernating.... well, skiing, actually!!! Haven't strapped boards to my feet in 27 years but it's all coming back to me..... I remember now why I went skiing 2 or 3 times a week when I was young!!

I'm sure you'll find the help you need here! And wouldn't be surprised if you can get this thing running just fine - somewhat dependent on how much you're willing to attack the pump by yourself.

OK, on to the diagnostics...... alarm bells go off for me when I see that the rack won't move with that bolt threaded into it - that's a problem, at least it would be on a 911 pump. Does the Merc pump have any sort of zero throttle deceleration fuel cut like a 911? On the 911 the rack is free to move towards the lean direction when the solenoid presses on one of those linkage arms. So with that bolt you'd be able to move a 911 rack for sure.

I see that the arrangement of the linkage arms and cold start enrichment is a bit different, too - I guess that the cold start arm can be seen at the top of one of the pics - does it have some effect on rack movement?

That #2 piston is probably gummed up a bit but not too seriously - if you flood the fuel chamber with some sort of injector cleaner or penetrant and work that plunger up and down you should be able to at least get it working like the others.

I don't see anything wrong inside the pump or with the space cam. The zero throttle "meaty" portion of the cam is the idle area and the stylus is lifted to reduce fuel at idle. It drops to increase fuelling as the space cam moves clockwise with the throttle.

As you increase throttle I would expect some rack movement but it is relatively small. But still should be measured in mm, not hair widths. If you can manually move the flyweights outwards against spring pressure, as though RPM is increasing, you should also see some rack movement.

If not, I'm going to say there's likely something awry/stuck in the linkage, or perhaps that gummy #2 plunger won't rotate in its bore (which is the movement which increases or decreases fuel delivery) is keeping the rack in place when it should be allowing the rack to move and change fuelling as the car warms up.

Come to think of it the gummy #2 plunger (and perhaps several others not moving freely) are the most likely culprits from sitting for so long.

Hope that helps!!! Don't be too scared to poke around a bit more the MFI pump isn't actually that delicate, and there aren't too many things that you can mess up at this stage provided you don't take it apart too much further. Even then, if you are careful to put things back where you found them you can't do too much harm. However don't loosen the clamp screws of the 6 collars that index the plungers to the rack as you'll absolutely have to have the pump re-calibrated.

Working ahead a little bit, but once you get the rack moving freely you can work on priming the pump head with gasoline and injector cleaner, and checking that the plungers are all delivering fuel and that the non-return valves (check valves) are working, then hook up the lines and injectors on the bench and spin the pump with a drill so you can test the fuel delivery off the car.

Fun stuff! It's such a neat system, and even though it's pretty sophisticated, it's actually made up of simple parts. Levers, gears, cams, springs, pistons, etc. all stuff you can see and figure out.

[RDM]

Skiing is a great way to spend this part of the year. I'm jealous. I was a daily cross-country skier in my much younger years. I suspect that the ski areas are locked down here, though I haven't looked.

It's late and I'm not going back downstairs to play, but I can tell you that

1. I gently tapped on the bolt head with a hardwood handle and the rack will now move freely. It spins the measuring valves for all six cylinders.

2. There doesn't seem to be any sort of natural rest position for the rack- it's not spring loaded in either direction.

3. Indeed, none of the regulatory or control mechanisms seem to affect the rack- including the throttle and stylus. I need to look at this again.

4. I understood the space cam/stylus function backwards, just by assumption. However, the meaty portion of the space cam never lies under the stylus. The first picture last time was throttle closed- as CCW as it goes. Should it rotate further? Not properly installed?

5. Cold enrichment- Belt, suspenders, and staple gun.
a)there is a solenoid (currently inoperative) mounted on the back cover that pushes on a plastic lever (sticking up out of the top of the opening in the below picture) that pushes directly on the main rack adjustment screw. Activated by the thermo-time switch. Pump expert says this one is not necessary if the cold start injector is working.


b) there is a seventh fuel injector on the intake manifold, also connected to the thermo-time switch.

c)There is also an auxiliary air valve that sits on top of the MFI pump that works in concert with a cold enrichment function activated through rods and levers. Controlled by the temperature of a special coolant circuit for just this purpose. This is Mercedes-Benz, after all.

6. No fuel cut-off solenoid. In fact, with the auto transmission there is a mechanical device to dampen the fuel cutoff at zero throttle.

Maybe lots more cleaning required. I've heard ATF is the bomb for such things. Unfortunately, I just recycled all of mine.

This has become an adventure. Thanks, Jonny!

[Jonny042]

That's good news - if you've freed up the rack you will no doubt find out more about how it is, or isn't, working, as you play around. A great first step. Get that #2 un-gummed (which should be easy now that it's also rotating) and make sure everything is working freely. I'm sure it will start to make sense.

The meaty portion of the space cam is really just the transition from one diameter to another - the fuel delivery map ends just at the part that starts to ramp up. No reason to think anything is wrong there the stylus looks to be hitting the right spot for 0% throttle. Be aware too that the fuel delivery for a given RPM actually goes DOWN as RPM increases, especially on something like a torquey sedan. The map shape is a bit more complicated than that but it would be easy to assume the opposite and think something was wrong internally because the rack moves towards lean at some point in the RPM range. You will not likely move the flyweights by hand out past a position equivalent to a few thousand RPM.

Any adjustment of any internal portions of the pump beyond what's necessary to get things cleaned up should not be necessary - if the car isn't running right, and the pump is un-gummed and working correctly, the poor running is most likely because of some other external system (like that whacky warm up system!!! I'm assuming the air valve provides some sort of automatic fast idle which is neat.

Hope my hasty answers are making sense!!!

[RDM]

Much progress made today, but guests are due in less than 60 seconds. More later!

[RDM]

Prodigious amounts of WD40 (excellent against varnish and sludge), brake cleaner, and toothbrush has made the rack move smoothly and the number two piston return to duty.

Also, my neighbor's application of screwdriver and light hammerwork has repaired the threads for the #1 check-valve connection, so tomorrow may be set aside for reassembly, reinstallation, and maybe even a test drive.

The big takeaway: My SC needs MFI. It's just so amazingly cool.

Edit: Electronics: bah!

- The rack moves freely and quickly in both directions, and moves as quickly as the rebuilt pump I have on the shelf for the 280 motor that's coming.
- The throttle and atmosphere/temp compensation Goldberg also moves the rack.
- There is a neutral point where the rack will settle, from either direction.

[Scott Douglas]

That is the same feeling I got when I finally got my SPICA injection working correctly on my Alfa.

[RDM]

Speaking of which, the Alfa I buy will have SPICA (when the day comes!).

Berlinas are about all that remain in my hypothetical price range. I like the 1750s for the very cool dash.

What do you have?

[Scott Douglas]

Uh, '73 or '74 Spyder (I forget which it was) w/2000cc engine.
I'll hunt down a picture and post it, if you don't mind my messing up your thread.

[RDM]

This is off-topic- everything goes! Well, except politics and religion, of course.

[Scott Douglas]

This was actually my second Alfa. First one was yellow/green but needed full restoration. Friend found this while taking a vacation out in Palm Springs. I went out and bought it the following weekend.

[Scott Douglas]

This was my first one, free to me as a finders fee from my brother, who also got it free just for towing it home.

[RDM]

Here's a picture of my first Alfa... No SPICA on this baby.



(I'm looking for more)

[RDM]

Finally... here we go.

[Scott Douglas]

Very nice!

[RDM]

The good news: The pump is back in, and the car runs better than it ever has for me. It doesn't quit running when it's warm, it doesn't refuse to go up the hill because it doesn't produce enough power. It doesn't even produce as large a cloud of unburnt fuel as it did before! Somehow, having the rack unstuck and fuel injected to all the cylinders makes a difference.

The bad news- maybe the work yet to be done- is that it will require some adjustment, which is an exacting and lengthy process, if what I read on the internet is true.

If you want to get an idea of how truly fascinating this entire system is, I recommend this Bosch Lehrfilm (teaching film). Turn on the subtitles and translate to English. This same system was still used in Mercedes diesels until not long ago.



So here's how I got there.

End cover back on. Behind this cover are all the adjusting screws. On this cover is also the special little dipstick for checking the oil for the regulator half of the pump. So in order to make any adjustments other than idle speed, the oil must be drained, then replaced once adjusted. The cold idle valve (air and fuel) is installed on top. Note also the empty start enrichment solenoid.


Side cover back on. The 280 MFI pump can be seen in the background.


In order to time the injector opening to the inlet valve opening, the pump must be turned so that the missing spline on the pump (inside the scribed collar) lines up with the mark on the pump body (visible below the top right screw...


AND the motor is set to 20 degrees after top dead center.


The distributor indicates cylinder 1 (just like on a 911!)


and in goes the pump, correctly timed. Having extra joints, say between your wrist and elbow, really helps in getting the flange bolts on. Mercedes has lots more crap- I mean features- on their car than Porsche, even in 1971.


then layers and layers of pipes, hoses, wires, and connectors.

[Nostril Cheese]

Thanks for this. Great info. Great video.

[Jonny042]

Great progress!!! I'd say you're well on your way to becoming an expert!

If the injectors for Mercedes are available at a reasonable price I would replace them. You'll find on the internet that you can clean them, I wouldn't waste my time, I ruined a complete set of injectors with the so-called cleaning/testing kit.

With crappy injectors that dribble and leak the car will run but won't be everything it can be. Also if it was spewing clouds of smoke the plugs could be fouled and the car might run a lot better with a fresh set.

Have fun!!

Oh and yes to MFI on the SC - it's a long road to travel but SO worth it in the end!!!!

[matthewb0051]

I've followed this guy for a bit. Bought his injector pump thing a few years back after seeing it on Wheeler Dealers with Edd China.

https://mercedessource.com/