haven't done any further work on the rough idle/miss issue. BUT i've noticed something. it seems to run smoothest on cold startup and gets rougher as it warms up... any clue there?

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R Martin
Dallas, TX (Addison)

86 560SEL, 128K

I might be able to find an extra fuel line to experiment with. I have a buddy in Chatt that has a few bits and pieces laying around. I'm in Cleveland and planning a trip down there this week.

I might be able to locate a fuel distributor for that engine if you need one. No promises but I can ask.

I believe the cold start valve injects gas into the intake manifold during the warm up period. That might be enough to make you think the idle is a little better.

Coil can cause this condition.

I assume you have changed out the leads, distributor cap and rotor all with OEM Bosch or Beru. Plugs should be non resistive types. (Be careful of erzatz Made in R.O.C. Bosch)

Use the best fuel for this car - non of that ethanol C%$p

Misses are 99.9% electrical

I have changed out a Fuel distributor on a W126 - only after it had gone 500,000 kilometres.

However - using too many fuel conditioners or ethanol fuel can cause the FD to play up.

"clemson88" i'm heading out of town Monday for the week, otherwise would be glad to meet you.

Ivanerrol, actually, No, I haven't replaced electrical items. i tested the resistance of all the components and they are within specs according to the manual.

but you're not the first one to say that. Jonathan been's saying the same thing too... so it's time to stop being so pig headed i guess and just do it anyway.

it's not really a miss as much as just a regular slight shaking of the engine. very regular shake, not a random miss

i always use premium gas, but unfortunately very hard to find ethanol free gas. don't get me started on the usual government idiots that push that boondoggle down our throats!!

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R Martin
Dallas, TX (Addison)

86 560SEL, 128K

If it is a bad wire or plug you can narrow it down by pulling a plug wire. A bad one of either should make the least difference in the way the engine idles.

I thought your tech guy checked all that and had it narrowed down to low fuel pressure on #8 hole?

Just because you have checked the resistance levels on the leads doesn't mean that the insulation on the leads hasn't broken down. Cracks develop in the distributor cap and let in moisture.

A V8 wouldn't make significant rough idle when 1 of the cylinders going not too south. You should definitely look at the ignition system first. It may still do well when the engine is cold but gets worse when warm. When then engine runs bad at night look at the engine bay to see if there's any ignition leaked outside. When that happens it means that the spark doesn't go to the spark plug but out of the combustion chamber. That's when you have to replace the cable/spark plug boot that's got the spark leaked.

Another thing is to have a very good look at the distributor cap and rotor to see if there is sign of leaking. I once had that problem with my 300E. It run like crap and stalled intermittently even when I did everything possible to the fuel system. Then it turned out that the ignition rotor had crack making the ignition just unable to reach the spark plug cable. Replaced the rotor then everything was fine.

Try tracking down the plug wire directly next to 8, swap the wires on both ends to confirm plug wire is good.

Have to disagree on the lack of poor idle w/ one cylinder down on fuel. We're going one Right now that has a dead hole on the FD, idles like Crap.

Swapping wires does work well, just like swapping FD lines around. A lot of diags can be done w/ what is on the car.

Jono

__________________
Blue Ridge Mercedes
Jonathan Hodgman
http://www.blueridgemb.com/
Enthusiast Service, Restoration & Tuning.
Follow Us on Facebook!
Located in the Atlanta area
Specializing in all pre and post merger AMG's including Hammers and DOHC M117 engines.
Mercedes Repair Atlanta

thanks for the replies. i've been offline for the past week while on a road trip. on the last leg of a 2000 mile trip. pretty unusual for this car, which generally only gets driven a few hundred miles a month.

interestingly, i've been adding a few ounces of Lucas Fuel Injection Conditioner to each tank of gas... of which there have been quite a few!! the idle issue has improved dramatically. it was never really terrible, but did make the car shake at idle. a very regular shake.

so now while idling in gear, i can barely feel the engine running.. (idles around 600-700).

i'm going to renew the distributor and rotor first when i get back home. car just turned 100K during this trip and very possibly still has the original components. i bought the car 5 years ago when it had 52K and most parts were still original.

i'll let you know if i notice a difference after new distributor cap and rotor. maybe i'll get new wires as well.

__________________
R Martin
Dallas, TX (Addison)

86 560SEL, 128K

Don't forget the correct plugs.. W9DC0 or BP6ES's @ .042.

If the cap and rotor are that old, they are TOAST>

__________________
Blue Ridge Mercedes
Jonathan Hodgman
http://www.blueridgemb.com/
Enthusiast Service, Restoration & Tuning.
Follow Us on Facebook!
Located in the Atlanta area
Specializing in all pre and post merger AMG's including Hammers and DOHC M117 engines.
Mercedes Repair Atlanta

would you replace wires AND coil too?

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R Martin
Dallas, TX (Addison)

86 560SEL, 128K

Was this problem ever solved? I have the same problem with an 86 560 sl.

Number 8 is dry at idle. Have changed seals , fuel distributor ,injectors and switched rails!!!!!!!!

the main thing that seems to have helped is running Lucas fuel conditioner through it. drove the car on a 3000 mile jaunt over a two week period and put lucas fuel conditioner in each tank. by the end of the trip, it was idling pretty good. a little bit of a shake but not a lot.

just finished replacing distributor cap, rotor and wires with bosch replacements. no change in idle

also replaced plugs (yes i always put in non-resistor plugs) no change in idle

but i keep using premium fuel and running the Lucas fuel conditioner in each tank.

runs good enough for me. once car is warmed up good and has been run it actually idles pretty good. sometimes even perfectly with no shake, but usually has a slight shake to it

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R Martin
Dallas, TX (Addison)

86 560SEL, 128K

rough idle is back again

could it be injectors? do i need to replace all 8? car has 105K.. 86 560SEL

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R Martin
Dallas, TX (Addison)

86 560SEL, 128K