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Old 01-16-2011, 12:30 AM
Registered User
Join Date: Mar 2004
Posts: 26
560SEL rough idle and missing (oh yes, again)

Hey everyone,
I've spent the last several months trying to diagnose an issue with my girlfriends 91 560SEL. It's the seemingly common miss and high idle problem.
Here are the symptoms:
Starts fine even when it's only 10 degrees out.
The warmer it get's, the higher the idle gets. Usually settles at about 1100.
It will then shake pretty badly and miss pretty much randomly.
HOWEVER ... if I put it in drive or reverse or anything else it'll calm down to 600 or so RPM, and idle perfectly. Quiet and smooth. Smoothes out with a little throttle even at idle.

Here is all the stuff I have done before:
Cleaned the injectors
Replaced injectors
Replaced fuel distributor
Replaced potentiometer on fuel distributor
Replaced EHA
Replaced all temperature sensors
Replaced Cap, Rotor, Spark Plugs
Replaced Ignition Control Unit
Replaced Idle Air Valve
Replaced Idle Air Control Unit
Replaced OVP
Replaced Fuel Pump Relay
Replaced Fuel Pressure Regulator
Replaced Engine Computer under dash
Double, Triple and Quadruple checked for vacuum leaks
Checked to make sure timing chain was ok twice
Checked all vacuum routing
New Thermostat
Checked O2 sensor
Replaced Cold Start Valve
Replaced Fuel Filter
Tweaked the mixture a bit but that doesn't help

I'm pretty much lost. And I thought my 74 Porsche with D-jet injection was problematic. I was wondering if there are any other things I can try. The parts came off an identical 560SEC that ran great but got taken out by a pickup.

I actually messed with it a little tonight. The only thing I hadn't checked was the vacuum connection on the modulator on the transmission, so I unplugged it, then started the car to see if there was a change. The car actually ran pretty good when it first started, but once it got warmer it idled higher and started missing. Even when I plugged it back in it changed nothing. I'm thinking it might just be a fluke.

Anybody have any pointer?

Thanks for reading this long winded post!

1984 300SD, 384K miles. Gold/brown.
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Old 01-16-2011, 01:05 AM
Registered User
Join Date: Mar 2007
Posts: 159
The list of parts does not have plug wires in it. Just a guess.
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Old 01-16-2011, 01:15 AM
Grumpy560W126's Avatar
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Join Date: Dec 2010
Location: Estill Springs Tn, 37330
Posts: 17
By what method did you happen to check for Vac leaks? I have found in the past that the air distribution hoses along side the valve covers and connect to each injector can sometimes be hard to diagnose vac leaks on. They get brittle and crack, thus letting air bypass the ICV and give you the above symptoms when the engine warms up. Did you check the crossover vac tube under the upper intake below the FD that connects the air distribution tubes together? I know you mentioned above that you have checked for vac leaks but I would try one more time. Did you check the voltage at the ICV? Use a unlit plumbers propane torch to check for vac leaks around the Intake and injectors.
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Old 01-16-2011, 01:40 AM
Registered User
Join Date: Mar 2004
Posts: 26
Oh you are right I didn't do plug wires! Duh.... I need to look into that.

The voltage at the ICV is good. I have the shop manual and that checked out according to speck. I checked the vacuum lines allover by means of WD40. Maybe that's not thin enough to get through the cracks.

I also tried isolating a single cylinder by removing ignition leads, but that didn't change anything. I then tried cracking the fuel lines while it was running, that didn't really change anything either.

I suppose I'll re-check the crossover tubes and the injector seals.

1984 300SD, 384K miles. Gold/brown.
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