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  #1  
Old 08-16-2019, 03:32 AM
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87 300SDL intermittant high idle?

The car will normally idle around 700 rpm or so indicated on the tach, but sometimes when I come to up a stop it will stay at 900-1000, and at that speed its pushing against the torque converter pretty hard so it takes holding the brake pretty firmly to get it to stay in one place. If I keep my foot on the brake and rev it up slightly it will drop to normal, and the accelerator pedal is definitely all the way up when my foot isn't on it. It does it every time I drive it but usually only a couple of times, and the rest of the time it behaves normally. It doesn't seem to have anything to do with engine temperature or any other particular situation. Any idea what might cause this?
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  #2  
Old 08-16-2019, 10:11 AM
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Is the throttle linkage lubed?
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  #3  
Old 08-16-2019, 10:34 AM
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This is a quirk of the EDS computer on the 86/87 cars. The redesigned (incompatible with 86/87) version in the 90/91 cars doesn't do this (it has it's own quirks). I had good success replacing the capacitors in my EDS computer on my '86 SDL and greatly reducing the irritating high idle issue. I have a thread on this forum somewhere where I documented it.

The other option is to unplug the EDS solenoid on the back of the injection pump and use the mechanical idle adjustment to set the idle speed to ~650 RPM.
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Old 08-16-2019, 04:37 PM
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Quote:
Originally Posted by Diseasel300 View Post
This is a quirk of the EDS computer on the 86/87 cars. The redesigned (incompatible with 86/87) version in the 90/91 cars doesn't do this (it has it's own quirks). I had good success replacing the capacitors in my EDS computer on my '86 SDL and greatly reducing the irritating high idle issue. I have a thread on this forum somewhere where I documented it.

The other option is to unplug the EDS solenoid on the back of the injection pump and use the mechanical idle adjustment to set the idle speed to ~650 RPM.

So other than grabbing an extra computer next time I have the chance and trying that it's probably not a big deal? On my old 300SD the governor failed and once it got over a certain RPM it would climb until you shut it off, so I wanted to make sure this wasn't something dangerous like that.
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Old 08-16-2019, 05:01 PM
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No, the intermittent 900-1000 RPM idle is a well documented issue with the 86/87 cars. The cause is not clear, but it's definitely electronic. Even after a recap of my EDS computer, it still happened from time to time but more like once or twice a month instead of 3-4 times every time I drove the car. The base mechanical idle is set very low, 450-500 RPM and the EDS solenoid brings it up to ~650-700 RPM. At full extension, the max RPM of the EDS controller is ~1000 RPM.
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Old 08-16-2019, 07:53 PM
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I seem to remember some members found that the connector to the EDS from the pump was not making a good connection resulting in your issue. Maybe check that?
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