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1991 500SL Blauschwarz (57k km (35k miles))
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28 Posts
Discussion Starter #1
When the engine start cold it does everything it supposed to do by revving higher and then drops to the desired RPM of about ~650 and ~500 if set to D.
All that without a single sputter, smooth engine.

But, as the engine temperature reaches the 80 degree mark, while the idle seem to be continuing to be good, if one revs or try driving (under load), a distinctive sputter occurs until about 3000RPM or so.

This problem started gradually about a month ago and getting worst.

  • Spark plugs changed (proper F8DC4)
  • Fuel injector all good because I removed them and made it test by a specialist with proper equipment plus a clean with ultra-sonic.
  • All vacuum lines and air tubes sealed and I know it worked because the RPM crept to 900RPM and now never does. Thus, air leak solved.
So my question is:
If the distributor caps were bad, it would be sputtering cold and at rest when hot as well, not sputtering just when accelerating up to 300RPM. Right?

What you people think I should check next or change? The ignition coil? Or bad distributors can create these problems as well?
 

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1999 E320, 2000 E320, 2003 E320 Wagon, 2005 C230K SS, 2010 Accord LX w Eibach & Koni FSD's
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3,254 Posts
You can try cleaning the contact points on the distributor cap and rotor with fine sandpaper and see if there's any change.
 

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1991 500SL Blauschwarz (57k km (35k miles))
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28 Posts
Discussion Starter #3
You can try cleaning the contact points on the distributor cap and rotor
Done as a first step, but I will not lie, they are not in good shape visually. However, why it revs perfectly when cold? If the cap and rotors were that bad, the sputtering would be noticeable from the get-go, I presume.
Trying to find some info on the specs for the ignition coil for this model year so i can do some testing, cannot find anything yet. :(
 

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Outstanding Contributor
R129 500 SL 1991
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1,180 Posts
First, check the AFS potentiometer resistances are in spec as you deflect the AFS plate. Then perform a duty cycle test.
 

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Greek God of the R129
SL500-500SEL-190E
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8,432 Posts
Common problem.
There is water in the distributor caps. When you start cold the water is at the bottom of the caps.
As temperature increases, usually in about 20 minutes it turns in to steam.
Check for arching marks.
Clean/replace caps, rotors.

Regards,
aam.
 

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300 CE 24V Sportline
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997 Posts
And a golden rule with these is to ensure that your coil, cap, rotor, shield, leads and plugs are 100% OK. MB original is best if most expensive.

RayH
 

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1991 500SL Blauschwarz (57k km (35k miles))
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28 Posts
Discussion Starter #7
Hi guys, sorry for not responding since I asked my question, I was waiting for a few parts such as rotor, caps and camshaft position sensor, which was literally cracked at the base of the sensor and hardly noticeable. For some reason the higher than normal idle returned, thus will do a smoke test in the upcoming days, the time to setup my own smoke machine. I have to make sure everything is 100% tight before going forward with more diagnostics.
 

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1991 500SL Blauschwarz (57k km (35k miles))
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28 Posts
Discussion Starter #8
Another update
Well smoke test yielded no leak, electric tests were okay, except that the output of the ignition module was weird. Disengaging the air hose leading to the ignition module made the engine run smoother at idle, and worst when reattached. Therefore, I decided to remove the ignition module and start opening it (very hard), and I found it was very dirty and full of rust. Dirt removed, I am now removing the rust stuck on the metal, which touches a membrane. You can see on the photo even the membrane is dirty with the rust. I am not sure if it was airtight, but for sure, it will be airtight once I finish with it. Will post the result when back on the car in a few days, time for the sealer to set.
2649391
 

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Outstanding Contributor
R129 500 SL 1991
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Do you think it will work again?
 

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1991 500SL Blauschwarz (57k km (35k miles))
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28 Posts
Discussion Starter #10
I hope it will work, I won't be sure at 100% until I get my part (119-090-01-82) I broke yesterday. Damn hose cracked in half while pulling on it… should get it Tuesday or Wednesday, then I will try the ignition module, with that hose broken it might skew the result if I try now. At least it gives time for the sealant inside the module to cure. I know the Permatex sealant I use does not emit odors when dry, and it still smell a bit when I smell close to the vacuum hole of the ignition module. Inside the ignition module it is quasi airtight, only that small hole for the vacuum present, so the sealant might take some time to cure.

Either way, at least I know the module was causing the high RPM and rough acceleration when the engine got to its peak temperature. I will post the result for sure because if there is a way to fix this module, or at least try as I did, many will be happy because it is an expensive peace of hardware, even used. :(
 

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Outstanding Contributor
R129 500 SL 1991
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... Disengaging the air hose leading to the ignition module made the engine run smoother at idle, and worst when reattached...
When you temporarily detached the hose did you seal it off?
 

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1991 500SL Blauschwarz (57k km (35k miles))
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Discussion Starter #12
When you temporarily detached the hose did you seal it off?
When I unplugged it from the module, I tried both blocked and unblocked, it just ran worst when plugged into the ignition module. The module really messed up the idle and revving output once the hose was back on and sucked air from the module.

Practically, from a good idle, as per the title, it started to idle bad the first time I put back the module after trying fiddling with the module. What happened is that I tried to separate the module, not being able, I put it back, and that is when it started to idle bad and accentuate the bad sputters until ~3000RPM. Therefore, fiddling with the module accentuated the problem, which makes me think there was a slight initial leak at the module.
 

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R129 500 SL 1991
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The correct way to test the function of the vacuum diaphragm on the EZL is with a vacuum gauge, not by hacking it apart. Does it hold vacuum?

I think you have probably ruined the EZL. Maybe there’s a slim chance of getting it working again. Lesson learned, get a good secondhand. Good supply of EZL’s in US, which is where I got my spare EZL (which I carry for emergency) for $80. Important to match part numbers and years of production.

The EZL vacuum advance does not function at idle. When the idle speed (closed throttle position) switch is closed the EZL uses a fixed advance. In a fully functioning EZL and vacuum system pulling the vacuum line off the EZL and plugging it should make NO difference to idle. The connector block for the CTP switch is on top of the manifold, making it easy to test the function of the CTP switch.

If, before you raped the EZL, the idle was good but hesitated during pick up below 3k revs, check the function of the air flow sensor potentiometer. Black box on side of AFS. Also check operation of idle position switch.
 

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1991 500SL Blauschwarz (57k km (35k miles))
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Discussion Starter #14
pulling the vacuum line off the EZL and plugging it should make NO difference to idle.
It did make a difference, when vacuum pulled away the idle dropped to normal, which is not normal. And the warmer the module got the worst it performed, in fact when I reinstall the module I will make sure it makes good contact on the wheel well, now, it hardly touches at 3 little area (shown with photo) and suspect it overheats.
2649743


When the module is cold, idle and acceleration is more than perfect. Hence that is why I decided to try to open it, everything else seem fine and or with new parts.

Where do you get 80$ modules? Let me know I buy right away. And thanks for the input so far, much appreciated.
 

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Outstanding Contributor
R129 500 SL 1991
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1,180 Posts
18 months ago. Pick n’ pull in Kansas. Just checked eBay. Wow! Prices of used EZLs have rocketed.
Average seems to be $300-400.
 

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1991 500SL Blauschwarz (57k km (35k miles))
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Discussion Starter #16
Wow! Prices of used EZLs have rocketed.
That's why when you said 80$... I said to myself; get a few at that price. LOL

However, there is no need for a module, it works. I could not wait for Tuesday until I got the new tube, I taped it for now so I can test the module right away since the sealant was fully cured on Sunday. Everything is back to normal idle wise.

You can check a video I recorded yesterday for that purpose. I took a capture of the title of this topic on my wife's phone so you can see it is not an old video. Proof, the module is not that hard to open and clean up, it will work once back. You made me sacred when you said I might have broken it, thinking you might know a thing or two about it. Because there are items out ther that once you open it, they do turn sour. LOL
But, as long one understand "sealing" it is very important, there should not be a problem. Of course, opening it carefully is another story altogether… freaking hard!



However, even if the idle is back to a more normal speed, what you do not see in the video is that 5 minutes after that, when the engine starts to really get hot, the idle turns to crap and sputters all kinds, as it did before. I still cannot get my finger on the problem, in other words this topic is still open. :(

I need to find the problem soon enough, I need to sell this 500SL to get myself an easier car to work on and beat the crap out of it for some summer fun (C5 corvette), as this car is boring for me because I take too much care, too clean, too nice to maltreat it. It is a true survivor as you can see, 57K km and mint condition, look at the cover valve in the video... showroom as with everything.

Anyways, I have to test more, such as what you said from the beginning, to test the AFS potentiometer resistances. The problem, I can't get info how to do that. Any old post I have missed on the topic on how to perform such task? thx
 

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'82 500sec euro, '95 Ford F150
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Check your coils. Ohm them out and compare with a known good one. Many times as the bad coil heats up, it starts dropping out, and causes misfires.
 

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1991 500SL Blauschwarz (57k km (35k miles))
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28 Posts
Discussion Starter #18
Ohm them out and compare with a known good one.
Easy to say, not easy done when I do not have good ones laying around. Moreover, I cannot find the info online, as if no one done this before… BS!
 

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Outstanding Contributor
R129 500 SL 1991
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Google search is your friend. You just have to ask the right question :)
Check out this link
 

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Outstanding Contributor
R129 500 SL 1991
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1,180 Posts
Here is the AFS potentiometer (red arrow)
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.

Ignition off. Connector unplugged. There should be 3600 to 4400 ohms of resistance between terminals 1 and 3. Slowly deflect AFS plate, the resistance between terminals 1 and 2 should increase steadily up to 2/3 of AFS plate travel and then slowly drop off again.
The readings should be smooth, like gravy 😋. If they are erratic or out of spec, renew potentiometer.

Ignition on. Engine idling. There should be 0.55 V to 0.95 V across terminals #1 and #2, and 4.6 V to 5.1 V between #1 and #3.

2651258
 
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