Idle problems on '86 300E

[lewise]

Hi guys.

Here's what's going on in a nutshell. The other day I replaced my aging and non-working Becker radio with a Pioneer unit. As instructed, I disconected the negative battery cable during the install. Radio works great. Car ran fine for about 4 days, then it started to do this weird thing at idle. It will idle at about 650 rpm for a bit then jump up to 1500-1800 rpm. It will do this cold, or hot, in park or in gear, A/C on or off. Then I also noticed that it surges a little while driving, but the rpm change is no where near as noticable. I did a search for idle problems, and did some testing on my own. I checked the voltage to the idle speed/aux air valve, and with the engine running I get about 12.7 volts. I also checked duty cycle of the valve and it fluctuates wildly between 15% and 100%, and you can hear the valve opening and shutting frequently and you can see the air sensor plate in the fuel distributor bobbing up and down. With the engine off, and applying battery voltage straight to the idle valve, you can audibly hear it open and shut. I replace the Over Voltage Protection Relay hopeing this was it, but the problem continues. Could the idle valve just be very "sticky" and bad? It looks relatively easy to replace. Is there something else I should be looking for? The temp sensor was replaced last year when I completely rebuilt the A/C system, the aux fan comes on when it should. I hope you guys can help me, I hope to keep this car for at least 4 more years.

Thanks
Evan