560sl black plugs and a miss. Help please?

Dear Group,
I have a 1986 560sl with 100K the engine has become rough running at low speed. It shakes at idel and becomes smooth as you start to drive.
It sounds like a solid miss at idle but any plug wire pulled off will decrease RPM. I have ohmed out all wires and found them to range 1.1K to 1.2 k the distributor looks great too. After pulling each plug I have learned that my dry compression is 145 to 150 each cylinder. I found four plugs to be quite dry black and the other four to be lean looking ,showing white at the porcelain and ash gray at the tip. I put new plugs in and ran the engine for say ten minutes, the cylinders that had the black plugs now have new black plugs. I think I have a rich fuel problem. Would this be problems with injectors? Can some one tell me what to check next.
Thanks in advance Bruce Buchanan. Brujan(NOSPAM)@the-i.net
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Which cylinders showed black? All on one side?
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T.G. Lambach wrote:

5,6,7,8 and 2,3,4 look real good #1 is, well, not as black as the rest.
Bruce, Buchanan
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It's probably a problem with a clogged injection ditributor. DO NOT EVER run injector cleaner through this engine as it can cause a lot of crud from the injection system to be sent down stream and in a worste case scenario foul your intake valves.
The problem is that this car runs a Constant Injection Stream semi-mechanical injection system as opposed to the Electronic Fuel Injection in almost all other mercedeses.
My guess is that you need a rebuilt injection head installed on the car, which is not cheap. Anyway, don't let anyone put injector cleaner in the tank, and remember to keep the fuel filter clean. Best of luck.
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Well yes, I have put injector cleaner into the tank but after this symptom had developed. In an attempt to try and solve the problem. So I am rich because crud has jammed open something in the injection head? Is this something that can be cleaned? I am pretty good if I know the goal and have a trusted diagnosis. Thanks Bruce Buchanan
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Bruce,
Where are you located?
I'm in Atlanta (have a '78 450SL) and I (although I do work on the car myself) get my engine work done by Mat Cama at http://oldworldauto.com / - he's in Lawrenceville on the NE side of Atlanta - very knowledgeable and meticulous - highly recommend his services.
Thomas Kase
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Mat's a good friend of mine and has worked on my cars. He's good alright.
Sounds like you need a new fuel distributor. Not a cheap part Im afraid, the only other car that uses that part is a Ferarri 308. But it's 20 yrs old and they don't last a heckuva lot longer than that.
--
Need Mercedes parts ? - http://parts.mbz.org
Richard Sexton | Mercedes stuff: http://mbz.org
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Sounds like I need a new fuel distributor. How do I determine for sure that I need this? Are having plugs black on just one side of my motor an indication of any thing? Bruce Buchanan
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Hey, Since you already used the injection cleaner, it sounds as if you are still alright because your compression test was OK. Like I said, it's a worse case scenario, but injection cleaner is a bad idea with this injection system and specifically not reccomended by Mercedes Benz.
Yes, the uneven richness does tell us something, but I'm blanking on what. I haven't worked on one of these in about three years, but if memory serves it points to an uneven pressure between the two heads and might point to a cheaper fix than a whole injector head???? but like I said, I can't remember. You'll have to wait for Tom to repost and see what he was getting at. Sorry that I can't be of any more help. All the best.
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Thanks Hazey , I will wait and see. I suspect that the fuel distributor is the culprit. I can believe that crud or what ever would allow it to give extra dosage to some cylinders. This problem has been with this car since I bought it a year ago but has worsened recently which is why I did this investigation Sunday. The car had been garaged for a year without running until I bought it. This may explain crud in the fuel. I am not so much looking for the cheapest repair as I am wanting to understand things about my car. I would enjoy changing the distributer but don't know much about it yet. I know how my diesel Benz works in all ways but this fuel injection system on this car is a little Greek, well German to me. So as well as fixing it I hope to also understand more when I am done. Will wait and see if Tom has an angle to the plugs being black on one side. Thats got my attention. Bruce,
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Just thought of something, maybe unrelated but food for thought. The cold start injector is on the 5,6,7,8 side of the engine. If it were leaking, maybe it could be causing the running rich problem. Remove it, disconnect the electrical connection, leave the fuel line hooked to it. Block the hole where it was removed from with the corner of a shop towel. Put the injector in a small cup or something. Start the engine and look for fuel coming out of the injector. If it is spraying even a small amount or just dripping, replace it. If there are no leaks, reconnect the electrical plug and repeat the test. If it now leaks or sprays, you have an electrical problem. It should only spray when the engine is cold and the starter is operating. Paul

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Excellent food for thought! I will snack on that. I suspect Mr. Lambach was onto that when he asked about "what side was black"? I will look into this. Thanks Bruce Buchanan
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No, I was NOT onto that great idea!
I know very little detail about these V-8s and was looking for further symptoms that might illuminate a possible a cause.
One or two overly rich cylinders would point to some fuel injectors but so many..... I thought which ones and whether they were all on one bank .... thinking ignition or valve stem seals. But a leaking cold start valve is a much more elegant and logical idea.
Looks like you have a strong possible cause to check out.
First consider the simple stuff, there's always time later for the complicated and expensive stuff.
Tom
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On 2006-02-07 12:03:44 -0800, "T.G. Lambach"

This theory sound right to me also...
I don't know this engine, but the one side only things makes the fuel distrib. theory seem bad...
Consider the air intake also. If this is dirty/plugged on one side it could be the issue.
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