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!

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