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1998 ML320, 2000 ML320
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213 Posts
Discussion Starter #1
Theres got to be someone who has had this problem or has some kind of answer. I have a 1986 560SL with the following:
*115k miles
*above idle runs great
*at idle engine shakes
*idle speed is correct RPM

Car has new plugs, wires, rotor, distributor cap, air filter.

What I have done: pulled off one plug wire at a time during idle to see if I could isolate cylinder with missfire. On seven cylinders there is a noticable change when the plug wire is removed. On the cylinder on the passenger side rearward, when I pull its plugwire there is no change in the idle. After finding this I moved a different plugwire to this position but nothing changed. I also checked for spark at this position and had spark. I am concluding the plug and wire is good. Still if I richen the mixture the car idles and drives perfect but is finicky to start - Any new ideas would be appreciated.
 

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84 380 sl
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20 Posts
Clogged/defect injector??

For good measure, replace them all, including the injector seals. These dry out and can cause vacuum leaks resulting in rough running engines.

Good luck,
Duke
 

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1998 ML320, 2000 ML320
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213 Posts
Discussion Starter #4
To check your theories on a bad injector I moved the suspect injector from cyl#4 to cyl#1 Cyl# 4 still had miss at idle. I then while at idle pulled plug wire off cyl# 1 at distributor and idle dropped I put this wire back on and idle improved - I then pulled the wire off cyl# 4 while at idle and noticed no change in idle whatsoever (still has miss)

Just for fun under the new injector arrangement I richened the mixture and idle smoothed out perfect.

What the heck could be happening here?
 

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84 380 sl
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20 Posts
With richer setting (and smooth idle) does the #4 wire disconnect cause a rough running engine?

Does the rough engine only occur at normal operating temperatures, or also when cold?

Duke
 

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1998 ML320, 2000 ML320
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213 Posts
Discussion Starter #7
no question seems dumb to me at this point. With rich mixture and smooth idle if I pull off the wire for cyl# 4 the idle roughens. On initial cold startups car is smooth then after maybe 30 seconds or so rough idle appears and maybe rough is too harsh a word it just seems to have that annoying miss
 

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84 380 sl
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20 Posts
Since you have isolated it to #4 no matter what, plug is ok, wire is ok, fuel injector is ok: have you checked it has required fuel supply to the injector? Possibly there is a restriction somehow.

Just trying here. Thoughts anybody.....?

Duke
 

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2017 Police Interceptor Utility, 2017 Police Interceptor Sedan
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7,742 Posts
slfan - 10/3/2005 3:09 PM

Since you have isolated it to #4 no matter what, plug is ok, wire is ok, fuel injector is ok: have you checked it has required fuel supply to the injector? Possibly there is a restriction somehow.
A clogged or damaged fuel line is uncommon, but not unheard of.
 

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1998 ML320, 2000 ML320
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213 Posts
Discussion Starter #10
Had fuel line in hand yesterday and was able to blow through it - did not appear to be damaged or restricted. One curious thing though is that when I got the car someone had swapped fuel line 3 with fuel line 4 I noticed this right away and swapped it back - this was very early on when I wasn't focusing on the idle situation. Idle still wasn't pure from the start. Does anyone know exactly how the fuel dist works - whats inside.
 

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1998 ML320, 2000 ML320
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213 Posts
Discussion Starter #11
Problem solved - opened port 4 at distributor and gave it a good flushing while engine running keeping fuel mopped up with paper towels so it wouldnt go anywhere. Put all back together and car runs perfect - Thanks for all the input!

Tom
 

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One of the BW Old Guard/R129, W204 Moderator
1997 SL500- 40th Anniversary
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7,149 Posts
Thanks for posting your experiences, The investigations you performed will undoubtedly aid others with the same problems...

Keep us posted.
 

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1981 500SL
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2 Posts
Tom,

I too are experiencing rough idle issues on my 500.
Can you explain in a little more detail how you flushed the fuel distributor? I think my one has varnished up inside.

Damon
 

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1998 ML320, 2000 ML320
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213 Posts
Discussion Starter #14
Turns out the flush only worked briefly back to rough idle. If fuel cleaners don't work to solve rough idle there seems to be only one answer - (replacement fuel distributor), mine should arrive tomorrow. I will let you all know how it goes. I have done as much research as you can on these distributors and I think what fails in them is the rubber diaphragm inside - a pin hole develops and all of a sudden bad idle. I guess we really can't complain about a 20 year old piece of fuel saturated rubber membrane failing.
 

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Rough Idle

I had a Tire & Service Center for 26 years in a small town. Gas stations were famous for getting water in gas. Cheap test, use a couple of bottles of "HEET", breaks down water so small it's of little consiquense. Frank:angel
 

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2007 S550 4matic. 166k-1987 560SL,111k
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461 Posts
Did the new distributor end up fixing your issue? Where did you get it? How much?

Bill
 

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Just purchased a 1986 560SL, 92,500 miles, EXACT same idje issues. Rough idle, runs decent past idle and higher RPM’s while driving. Any new updates or detailed steps to success?!
 

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1986 560SL 1986 420 SEL
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82 Posts
This thread is 4 years old. Last post from MLSL was 2016. I don't think we're going to hear from him about how or if he solved his problem.

I have this same symptom. Had injectors tested and they were no longer giving a good spray. New injectors helped a lot but not totally. New plugs wires, dirt cap, rotor and ignition coil did nothing - I changed the just because they were old = a new tune up.

Next thing I am going to do is a smoke test looking for vacuum leaks. Probably won't get to this for a couple months and will probably start a new thread at that time.

Very interested in your efforts. You may want to start your own thread. If you do, please put a link in this thread.

Good luck..
 

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1983 380SL, ivory/dk brown, 46k miles, dual roller timing chain. 1986 560SL, red/white, 190K mile.
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6,654 Posts
I have a 1986 560SL with 190K miles.
Idles rough, basically I can feel vibration in cockpit. Smooth powerful acceleration.
Smoke tested, new injectors, new distributor cap and button, new spark plug wires, new spark plugs, premium non-Ethanol fuel ... my next move is to check Engine Mounts and Engine Shocks. Really also needs a new muffler.
 

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Good feedback and appreciate the continued interest here as this seems clearly obvious a very common PROBLEM with this car. New Dist cap / rotor coming tomorrow but 99% sure it will have minimum affect. My next move will be the fuel distributor. I bought a ‘semi’ rebuild kit that comes with new seals and a membrane element (not sure of correct terminology) that according to what I’ve been reading can get small pin holes in it causing loss of fuel pressure and erratic idle. I guess we’ll see. For kicks, I’m going to replace the fuel filter which only the Lord knows when this was replaced last. While I understand it’s a cumulation of errors I’m sure, it certainly seems there exist the main culprit to this irritating problem. I pretty new to ‘threads’ here so please be patient until I get more experience here. Thanks!
 
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