1989 300se high idle
 

My 300se will start easily and run well for the first two or three minutes. Then (if in park) the vacuum gauge (economy) pegs almost to the right and the idle drops to almost stall and it runs rough. If driving during this period it acts as if starving for gas and has a top speed of 45mph. After a few minutes the gauge goes back to no vacuum and the car then runs smooth except it idles at 1500 rpm in park and 800 rpm in park. I have changed the plugs, fuel filter, cleaned the ICU, and replaced all leaking vacuum lines. Does anyone know what I should try next?

The m103 has a idle switch. If you look closely at the throttle linkage you will find the throttle microswitch. This switch needs to be hit by the linkage when the car is at idle otherwise it will sit at 1500 RPM.

I dont know if thats related to your other problems.

Also inspect your spark plug wires, might be time for new ones too.

I would also get a multimeter and read the duty cycle of the engine, x11 port, on the driver's fender. BOSCH KE3-JETRONIC MIXTURE ADJUSTMENT

If that value is FIXED and not moving after the engine has been running for >1 minute or so..then that is indicating a fault with a sensor or component in the CIS system.

Please proof read what you print! I assume it idles at 1500 in park and 800 in gear? Also no vacuum would have the needle to the far right, max vacuum would have it far left in the black. Assuming good plug wires, the duty cycle adjustment fixed mine.(1988 300SE) It was starting cold too lean. Too lean caused it to idle too low and stall from time to time cold. Now, since adjusted, it starts cold and runs 900 RPM in park, then settles to 700 in gear within 2 blocks. Running too lean can also contribute to the lack of power you mention. Cleaning the ICU is well worth doing but does it have voltage to it? When running, you should be able to lay your hand on it and feel it buzzing.

Thanks for the ideas. I'll post on my progress.

I had a few mins before work so I took the air cleaner off and started the car. I could feel the ICU working. After starting the idle was a little high for about half a minute then slowly decreased until the car was running rough with the econ gauge almost in the red and fluctuating. Then the idle went up to 1500 as the gauge pegged solid to the black. I shut the car off and immediately start it and it idled perfect at about 650 and the vacuum gauge not quite pegged to the left. After a few minutes the idle climbed back up to 1500 and the vacuum gauge was again pegged to the black. All the while the microswitch on the engine was touching the linkage and the linkage never moved. Next I will get a meter and check the duty cycle.