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1976 Euro 280SL; 1987 560SL
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53 Posts
Discussion Starter #1
Good afternoon everyone,

I am having an issue with my 87 560sl. The idle in neutral is hovering around 1200 rpm in neutral and park.

So far, I have replaced the following parts:

Fuel hoses, fuel injector seals, basically all vacuum lines
Idle Control Valve
Plugs, wires, distributor cap

The car is in the shop currently and they tested a new idle speed control unit and the car still idled high. Have been pinning out for bad voltage readings and nothing.

What am I/we missing?

side note I have also noticed since the problem occurred that it feels like my brake are pulsing as if my anti-lock brakes are wanting to kick in just moving a low speed out of the parking garage.

Any help would be GREATLY appreciated.
 

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1986 560SL
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26 Posts
Disclaimer: I'm a novice, new owner of '86 560SL

If the idle stays the same whether the "new" ICV is plugged in or unplugged, I'd be suspect. If the ICV is okay, I believe there is also an associated Idle Speed Control Unit behind the glovebox. While I don't know, it would make sense that if the relay is bad, you won't get proper function of the idle control system.

Also, has the o2 sensor been checked?

Has the vacuum system been smoked, or just replaced a bunch of hoses?

Lastly, with the brakes.. does the pulsing slow as the car slows (Warped rotor) or is the ABS activating at low speed (wheel speed sensor / ABS Sensor)?

Good luck. There's quite a few smart folks here!
 

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Registered
1986 560SL 200,000 km
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80 Posts
Sounds like a good smoke test is needed to find a well hidden vacuum leak or leaks.
Double check that all throttle linkages are moving free and clear.
 

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65 Posts
One thing not mentioned was the idle switch on the throttle body. If the switch is open circuit a lot of the early idle control systems go to a default slightly open to reduce unburnt hydrocarbons on decel. when I work on any car with a idle control issue i pinch off the line to the idle control valve. If the engine stalls its not a vacuum leak issue but a control issue.
also with ABS activation at low speeds is always a wheel speed sensor issue. Either too large of a sensor to reluctor ring gap or broken wires in the sensor cable. Most always a front wheel due to the amount of Cable flex with turning and suspension movement.
 

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Registered
1976 Euro 280SL; 1987 560SL
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53 Posts
Discussion Starter #6
One thing not mentioned was the idle switch on the throttle body. If the switch is open circuit a lot of the early idle control systems go to a default slightly open to reduce unburnt hydrocarbons on decel. when I work on any car with a idle control issue i pinch off the line to the idle control valve. If the engine stalls its not a vacuum leak issue but a control issue.
also with ABS activation at low speeds is always a wheel speed sensor issue. Either too large of a sensor to reluctor ring gap or broken wires in the sensor cable. Most always a front wheel due to the amount of Cable flex with turning and suspension movement.
It is going back into the shop this Thursday. Here's what has been checked:

  • Pin tested idle speed controller, sensors in engine bay, nothing out of sorts
  • Pin tested the OVP, even popped another in, still same high idle issue
  • Vehicle has been smoke tested, ended up replacing boot on rear of motor that holds MAF, replaced fuel injector seals, little bits here and there that had a leak.
The mechanic that I had it at threw in the towel and I am off to another that has a great reputation in the area.
 

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1986 560SL
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26 Posts
I recall another member who posted about high idle issue on their 560sl. After 4 months and $5k replacing a bunch of good parts, what finally sorted it was when replacing the 'Klima' (Climate Control) relay, they swapped their old ISCU back in the place of a rebuild they replaced along the way, and it sorted it out.

-> 560sl high idle hell - possible causes, i fixed it!

*edit - also as Ken S states, I would make certain there's no binding in the throttle linkages, even disconnecting throttle and cruise linkages to see if it improves idle. Even a little binding can introduce 'false air' to the system increasing idle.
 

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65 Posts
Good choice to find a good Indy that specializes in the older cars. Most of this knowledge has been lost and the new generation of tech relies heavily on the scan tool. So you wind up with a tech firing the parts cannon at your car.
 
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