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I have been plagued with this problem since I bought this car at 122k. I have 180k on it. I love the car but just can't seem to get this fixed. At 122k I had the top end done, valvs, injectors,fuel distributor done at 170k. The smog pum is frozen and has been removed. I am not in a state where emissions are an issue. <br> <br> Can anyone offer any suggestions to help me make this thing run the way the benz folks intended. I am embarrased to tell how many times I have brought it to dealers and spent 1000$ and up to have this problem looked at and no-one has fixed it.<br> <br> I ahve a fair amount of mechanical ability but am weak with computer systems and fuel injection. I will try just about anything at this point.<br> <br> Thanks in advance; Bill
 

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Hey, I am having the same problem with my 86 420 with 165000 on it. Mine is more intermittent though. Sometimes it will shake like a cammed out Chevy and other times it will be a sewing machine. When I took it to the dealer, they said that my cylinders were getting starved because the fuel system is dirty from age. But it sounds like to me you've about had the whole system replaced!!! I was thinking about getting the injectors done, but from what you said, I'm not so sure anymore if it doesn't do anything. However...one mechanic told me that W126 V8s love to foul plugs if they are kept under 3000 rpm for a long time(ie city driving). So what I am going to do for starters is put Bosch Platinum 4 plugs in it, Jacobs 'performance' plug wires, and a Jacobs ignition amplifier which supposedly gives it a spark that is 1600% more powerful than stock. So I am hoping that that will smooth it out a little. Good luck and I will be following this post for any other info.
 

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Tobias;<br> <br> I have replaced all plugs twice. They are all clean. This car does not burn a drop of oil either. I have not tried to hop uo the spark. I am toying with the idea of seeing if I can find a ICU from another car to trial. I hate to go out and buy one as they are pricey. Let me know how your effort proceeds.<br> <br> Thanks; Bill
 

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William...think about this. Today I was surfin around on some Benz sites and one guy made mention of motor mounts. He put new ones in his 94 S500 and he said it made a difference in the smoothness. He also said that he recommends new motor mounts if your car has over 90000 miles on it. So do you think our mounts may be bad causing it to vibrate??
 

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Yes, I replaced them, or aat least paid a dealer to replce them at about 135k. I did not notice any difference. I wonder if there is a way to check to see a motor mounts need to be replaced with replacing them?<br> <br> Bill
 

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Re: I don't know...anyone???

hey u guys I'm with ya, bought this 73 450sl to restore and hope to solve the rough idle, low speed shakes myself. I ma getting some black smoke from the exhaust and suspect mixture, but followed ur posts hoping to see some light at the end..maybe someone has an answer out there... S O S
 

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Rough idle???

Tobias was on the right track with the Bosch platinum +4 plugs, high performance wires, and ignition amplifier. I added the above with an MSD 6 (multiple spark discharge)amplifier and it made a tremendous difference on my '76 450SLC with 310,000 miles (still going strong.) Count on cleaning the distributor cap contacts every oil change and replacing it every 30k or so if you do this. Having a welding arc instead of a spark is really hard on the distributor cap and rotor.
 

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Re: Rough idle???

Thanks Don for the info....do you think the MSD or Jacobs module is better? Or would it not really make any differnce which one? Thanks again
 

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MSD or Jacobs???

Don't have any experience with the Jacobs. I have owned 4 MSD units. The advantage of these is that it provides several rapid sparks (not just one) for each 'spark.' The MSD6A I have on the Mercedes is actually a bit too much. It will jump a 1' gap with an brilliant blue arc and a loud braaappp! You could probably weld with it. My '63 Chevy pickup has an MSD5 that has a nice 1/2' blue spark and if you can find one of those it would probably be easier on the distributor cap.
 

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Re: MSD or Jacobs???

Thanks for all the ideas. I have figured out 90% of the problem. It amazes me that the many dealers and shops that I have taken it to could not identify the problem and it ws so inexpensive to fix. There was a crack in the hose that fed to the fuel recirculating pump. These are tubelike with ports that drain from the valve cover gaskets. Retrospectively they were old and soft and when the engine was running would collapse to some degree from the vacuum inside. At any rate my theory is that the vacuum leak would expand as the engine got warm and the rubber more maleable. The improvement was amazing. I can still have a very subtle vibration in the idle when the car is in neutral now but otherwise it runs very smooth. And to think how much money I have spent chasing this. Again thanks all for your input. I may try hopping up the spark. Might be fun.<br> <br> Take care; Bill Aurich
 

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Discussion Starter #13
Great, Bill, but do you think you can explain more...

I am not quite sure what exactly it is what you did. Do you think you might be able to explain more in some detail? Like where the pump is and what hose and so on. Sounds like a great idea and I would like to try it on my car. Don't think it could hurt! Thanks in advance.
 

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Discussion Starter #14
I had this problem on my '82 280SE which went away when I replaced the plugs with Bosch copper core W8DC. (The manual recommends W9D with Aussie spec pollution control gear fitted, other-wise W7D factory spec)I use LRP 98 fuel here in Australia. Car now runs as smooth as a baby's bum with no smoke. <br> <br> By the way, the old plugs looked clean and did not appear worn, so maybe one one or possiblt more were faulty. This was a cheap 'fix' !
 

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Re: Great, Bill, but do you think you can explain more...

Tobias;<br> <br> If you look parallel to the valve cover on the medial side of the cover is a rubber tube with four connections to the manifole head. They just pull up. The parallel tube is connected to the opposite side under the throttle body. On the drivers side the rubber tube connects to the (I think) recirculating pump. The other thing I noticed about rep[lacing these was that the old ones collapsed under vacuum. (ie when the engine was running). The point being they may not have been funtioning correctly even without the leak that I idntified. They were cheap to but. $17.5 a piece. Anyway worked for me. I might try running a hotter plug as well. Will give it some more thought. The car runs like new when under load. (ie in gear at idle) When in neutral I can still detect a subtle miss. Having said that it runs 95% better. Take care. Bill
 

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Discussion Starter #16
Rough Idle

I have a 1986 560SEL with this issue. I changed the plugs to Bosch Platinum 4's changed the cap rotor, air filters and breathers. I also changed the idle control valve for about $150. These changes made a big difference in idle, in and out of gear.
 
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