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1986 560SL
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59 Posts
Discussion Starter #1
I brought this up some months ago, but I probably wasn't clear what I was asking, because I got zero responses last time I checked.

Anyway, when warmed up, and throttle closed or barely open but in gear and moving down the road, the '86 bucked like it was on the verge of cutting out. However, adding throttle gave normal, smooth power. I've been DIY for many years, principally Audis, but the last CIS injection car I owned was an '86 Audi Turbo. Any hints of where to focus are appreciated!
 

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1987 300SL
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77 Posts
A long shot, Turbo, but check your drive line. Any slop anywhere can give you that rocking feeling in that throttle position just between on and off
 

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1986 560SL
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59 Posts
Discussion Starter #3
My friend and I took the '86 out today, first drive in months since I got spooked by the surging/bucking. I let him drive; my friends are less reluctant to floor it and wind the thing to redine if it isn't their car, which he did. I have no doubt that this may have been the first wide-open max rev experience that this engine has had in who knows how many years, since I bought it last summer and have never really jumped on it. Anyway, we drove around for a half hour or so, on and off the gas. It ran pretty fine actually. His opinion was that it only needed the "cobwebs blown out".
 

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1974 450SL, Dark_Blue/Parchment
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216 Posts
My friend and I took the '86 out today, first drive in months since I got spooked by the surging/bucking. I let him drive; my friends are less reluctant to floor it and wind the thing to redine if it isn't their car, which he did. I have no doubt that this may have been the first wide-open max rev experience that this engine has had in who knows how many years, since I bought it last summer and have never really jumped on it. Anyway, we drove around for a half hour or so, on and off the gas. It ran pretty fine actually. His opinion was that it only needed the "cobwebs blown out".
As my car didn’t get much use before I purchased it, I changed all the fluids then gave it an “Italian tune up.” Essentially same thing you guys did, drive it like you stole it! It worked wonders. (y)
 

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28 Posts
Sounds to me like the throttle switch needs adjustment. On those systems they go into fuel cut on deceleration and if your switch is still reading closed at a small throttle opening you could be feeling that. They don’t have fuel cut I believe until they go into closed loop (when the O2 sensor takes over mixture control)
 

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Your post shows you are talking about a 1986 560sl? That model has a O2 sensor. You are correct if your engine had D-Jet which is up to 1975 in the US it does not.
 

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560SL '88 Suzuki GS1000E '78
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211 Posts
Sounds to me like the throttle switch needs adjustment. On those systems they go into fuel cut on deceleration and if your switch is still reading closed at a small throttle opening you could be feeling that. They don’t have fuel cut I believe until they go into closed loop (when the O2 sensor takes over mixture control)
I agree, my car have had the same problem. I changed all the seals and rubber parts in the engine V and as well the throttle switch, because the contact in the idle position switched not well (there were high ohms sometimes). A proper adjustment is important as well. I don't know what finally the roocause was, but adjustment of mixture and switch are the both factors to solve this problem.

Now everything is perfect.
Cheers Martin
 

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1986 560SL
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59 Posts
Discussion Starter #9
Thanks for the replies. That's the kind of direction I need, from people who know these cars. A 34 year-old throttle switch, if still original, certainly might be faulty.
 

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560SL '88 Suzuki GS1000E '78
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211 Posts
Hi Turbo, the switch is easy to measure, it's plug is located near the coldstart valve. Just unplug and check if you find connection between the appropriate cables in idle and full load position. I would recommand to unplug the throttle linkage and only to move the last part. If the switch is OK you need to test the moving range and adjustment of the whole linkage, starting at the pedal. During the last 34 years there may be done a lot of "improvement" from earlier artists...

If linkage and switch operation performing well, you may need to take the long way down to injector-airsystem and the seals in the "V".

Martin
 

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1986 560SL
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59 Posts
Discussion Starter #11
Thanks Martin. Last night I finished changing all four O2 sensors, intake manifold seals, heater hoses, and fuel injector O-rings on my 3.0 A4. Now I'm all fired up to check that switch on the 560SL as you instructed.
 
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