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1991 500SL Blauschwarz (57k km (35k miles))
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Discussion Starter #21
Okay guys, freak of nature problem is gone… for now or forever, who knows. I waited a few days before answering because I had to try to replicate the problem to see if it was coming back, until now, all good.

However, I am not moving away from this thread without informing of my case, it might help someone else.

Just as much, I do not know how the problem started, as much I do not know what I did to make it run well, in part because I think there was a few issues at the same time.

However, the other day after reading roadhouse's comment, and searched online for the R129 coil specification (not found), I decided to remove a coil and test. Armed with the ohms data when cold, I was going to warm up the car to the point it starts sputtering and then test the coil warmed up and see the difference. When the car got hot, it sputtered as usual, so I shut the engine and immediately tested the coils, and it was 0.35 ohms higher on average than the ~11K ohms. I said to myself, bullcrap, it’s not enough difference to be the cause of the sputtering.

I disconnected a coil but right at that moment my wife called me, I had to do something else, so when I came back in the garage, I forgot I had disconnected one coil, I started the car and realized it was running on 4 cylinders. When I realized one coil was off, I noticed the engine did not sputter on four cylinders. Therefore, I plugged back the right bank coil and detached the left bank coil, same thing, no sputtering even while revving a bit. Nevertheless, I forgot, the car was not hot anymore so it was not a good test for sputtering. So I said to myself, let me reconnect all coils and warm up the car and then do the left and right coil test again, maybe I will find which coil is problematic, if any.

Well, after each coil ran alone with a few revving, the car never sputtered again when I tried to warm up the car, and I have been letting the engine cool and get super-hot on purpose ever since, and still no sputtering. I even tried to move things around to redo the problem and to no avail.

Then on that Tuesday at 5:15PM when my son got home (he works in auto parts), he had the flexible tube he ordered for me from Mercedes to replace the broken one, as explained in the previous post I was waiting for it for Tuesday (Monday everything was closed for Labor Day here). The moment I added that new flexible tube, the engine got even smoother, better than ever, to the point, I went for a long ride around town, no sputtering.

Therefore, I have no clue why the car stopped sputtering after the engine ran and revved on one bank for a few seconds each.

However, remember what I said in an earlier comment, I also had a high idle issue, which I fixed by opening and overhauling the EZL (ignition control module). Therefore, a panoply of little things made that I fixed the sputtering; it is not just one issue.
  • I tiny part may have been related to an EZL having a dirty/rusty membrane (thus non responsive)
  • And maybe, running the engine on one coil, each alone for a few seconds fixed it.
  • And finally, the part that made it run even smoother, a vacuum hose, thus vacuum related.
That was my recipe. Sound stupid, but that is all I did and that fixed it.

Is there a possibility for an electric or mechanical component on this R129 to fix itself or reset by running on 4 cylinders? Why the sputtering went away after running on one coil?

Check for sticking control plunger in fuel distributor.
I cleaned that plate and spring a few weeks ago when I noticed it was very sluggish when pushed down. I ran some brake cleaner as far I could, near the spring area, it finally got to move it more freely but that did not stop the sputtering. Moreover, since the plate did not move fast enough, IMO, what I did is remove a piece from the plate (see photo below), that piece has weigh, without that piece it worked better by shutting against the top faster. Since the sputtering is gone, my revving is more responsive without that extra weight on the plate than when I had it and the car went well, so it is a plus without it in any case. The engine revs sportier like, it reaches down to idle much faster than before, a feature I like. I will try to put it back in a few days when I am 100% sure the sputtering is gone forever. However, until now, it is actually beneficial, as I said, after revving it goes down to idle faster, without affecting the drivability.
2650439
 

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R129 500 SL 1991
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If removing that washer made the engine more responsive, logically the KE lambda adjustment might to too rich. Have you checked the duty cycle?
 

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1991 500SL Blauschwarz (57k km (35k miles))
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Discussion Starter #23
Have you checked the duty cycle?
You were right to have asked me about the duty cycle, my low number is about 5.06V … and my highest number was about 6.12V. It cycled very close to those numbers for the time I measured.

So if I got it right, my engine is running to rich. Right or wrong?

If so, I need to adjust what on this engine to make it right?

Because if we are talking about that screw on top of the K tronic behind the plunger intake, I cannot, there is an anti-tamper ball. Never figured out how to remove it, I do not want to risk breaking anything without knowledge on how to remove that ball. Thanks for the info.
 

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R129 500 SL 1991
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... my low number is about 5.06V … and my highest number was about 6.12V. It cycled very close to those numbers for the time I measured.

So if I got it right, my engine is running to rich. Right or wrong?

If so, I need to adjust what on this engine to make it right?

Because if we are talking about that screw on top of the K tronic behind the plunger intake, I cannot, there is an anti-tamper ball. Never figured out how to remove it, I do not want to risk breaking anything without knowledge on how to remove that ball. Thanks for the info.
To convert voltages to duty cycle reading we also need your battery voltage between terminals #2 (ground) and #6 of the round diagnostic socket x11, taken during the test.

Assuming your battery voltage was roughly 13v then the duty cycle could be converted as follows: -
duty cycle [%] = [1 - (Vp3 / Vp6)] * 100
Vp3 = voltage at X11 across #2 (ground) & #3
Vp6 = battery voltage at X11 across #2 (ground) & #6

Vp3 (at idle): 5.06 and 6.12 V
Vp6 (at idle): 13.0 V (assumed)
duty cycle at 5.06 V = [1 - (5.06 / 13.0)] * 100 = 61.1%
duty cycle at 6.12 V = [1 - (6.12 / 13.0)] * 100 = 52.9%
average duty cycle : (61.1% + 52.9%) / 2 = 57% (fluctuating +/- 4.1%)

Indicating slightly lean KE baseline setting.

Suggest you re-attach washer to AFS plate and report back with duty cycle. Extra weight of washer should make KE baseline slightly richer, and therefore bring duty cycle value closer to 50%.

I suggest a target average duty value for your engine would normally be around 48% for optimal running.

Can you post a photo looking down on the KE adjustment port? Normally there is a yellow plastic tamper plug, which you can hook out with a small hook tool. Sewing pin or very small flat bladed watchmakers screw driver would also work.

Others have reported a ball bearing, but I have never seen this type. Maybe in Canada or US. If you search Benzworld you should find how others have removed the bearing.
 

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1992 500SL, 1988 560SL, 1977 450SL, 1999 E320 4matic
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robm.UK, bobterry99 or anyone
I also have a sputter problem when the engine is warm. However, it has been mention several times in this and other posts about changing the AFP. So I thought it would be a good idea to change mine with 275,000 miles on the odemeter, so I purchase one, but I'm not sure just how to remove the old AFP and you need to remove the outer cover to access the 4 screws even on the new AFP. I have attached 2 pics the explain further. Thanks...
AIR FLOW POTENTIOMETER~1.jpg


Air Flow~2.jpg
 

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R129 500 SL 1991
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I believe the posts suggest checking the AFS potentiometer before changing.

Pry up banana shaped covers (white arrows) with small watchmakers flat bladed screwdriver, you can then access the screws.
Before loosening screws, mark the position of the old potentiometer on the alloy AFS housing. Dot of white paint is good.

So, you will see the holes for the screws are elongated. This is so that, once loosened, it it possible to rotate the potentiometer. This is necessary to calibrate new potentiometer.

So first, make marks on alloy base and pot. Then remove screws and old pot. Line up new and old pots, and put mark on new pot in same position as old. Then fit new pot, lining up marks. Refit plug. Tighten screws lightly so that pot can still be rotated with mild force.

Next, while idling, measure voltage across pins #1 and #3. To do this I pull the plug out slightly p, but enough to get croc clips on pins to multimeter. You’re looking for a voltage of 4.6-5.1 V at idle. Rotate pot to set required voltage.
Same with pins #1 and #2, 0.55-0.95 V at idle.

2650960
 

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robm.UK,
Thank you for the info. You say "pry Up" , you mean to pry up the top cover as labeled in photo. As of tomorrow I will try to remove old AFP.
AIR FLOW P showing pry here mark.jpg
 

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R129 500 SL 1991
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The reverse side, does that square black cover come off to reveal the pcb and carbon tracks?
 

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Here is the AFS potentiometer (red arrow)
View attachment 2650033

Checking the AFS falls into simple mechanical and electrical tests with a multimeter.

Basically you’re checking for a sticking AFS plate and worn potentiometer on side of AFS. Check plate and clearance by feel. Depress plate by hand and check there is 1-2mm of free movement before resistance is felt as it engages the plunger in the furl distributor. Before doing this test ensure fuel system is 'primed' by cranking for 10 secs.

Check for sticking control plunger in fuel distributor. Deflect AFS plate gently by hand all the way and release. The plate should return to rest position smoothly. Depress plate again. Gentle resistance to movement should be felt.
AFS potentiometer. Black box on side of AFS.
Pull off the electrical connector.

There should be 3200 to 4800 ohms of resistance between terminals 14 and 18, and 560 to 1060 ohms between 14 and 17. The reading should increase to 3760 to 5640 ohms when the AFS plate is depressed to its lowest point. The readings should be smooth, like gravy 😋. If they are erratic or out of spec, renew potentiometer.
Looks like a good test to check the pot. I will be trying that on mine. Dam robm.uk that's a clean motor.
 

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1991 500SL Blauschwarz (57k km (35k miles))
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Discussion Starter #32
Suggest you re-attach washer to AFS plate and report back with duty cycle.
The voltage at the pin #6 = 13.7
During the cycle test the pin #2 minimum = ~5.56 V and the max was ~6.48.
This is ~56% average duty cycle, which is wrong because it's far from the 48% preferred average.

As requested, below is the photo showing the anti-tamper ball. BTW, as you can see with the photo the "washer" was reattached on the AFS plate for the new duty cycle result.
2650989


@carltwo
Thanks for asking that question about the meter, I was also wondering how to get that thing open. I just found a pic online how it looks inside, but never how to open it.
 

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R129 500 SL 1991
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So, what happens is, after 50k+ miles or so, the carbon tracks wear out around the idle position, which makes the idle uneven. (I have an old VAF kicking around somewhere - will post photo). Then they wear out across the mid range.
With sluggish acceleration and hesitation below 3000 rpm it is recommended first to check the AFS potentiometer (VAF).

2651161


But also fuel pressures and coolant temperature sensor (ECT). ECT is cheap and quick.
❗When you check the voltages at the VAF be sure not to short out the contact pins. I use small croc clips with insulator covers.
 

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Today I removed my AFP from my 1992 500SL. Can anyone comment on the condition of the old AFP. Even the condition on the mechanism in the Air Flow Intake.
Thank you...
AIR FLOW INTAKE.jpg


OLD 92SL AFP~1.jpg


OLD 92SL AFP~2.jpg
 
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