Problem - my idle oscillates mildly after it is through warming up. RPMs will swing between 600 and 800 every couple of seconds while idling. The vehicle performs fine during the warm up period when the choke is on, and also when the throttle is open. I just had the idle control valve replaced (dealer said it was broke) and it helped alittle. I took a trip and got 18-22 mpg which is right. It starts rough, as if no gas, coughs alittle as it is starting, will quit if I don't goose the pedal, then it will smooth right out. There are 235,000 on her. I had the upper end, including valve guides, rebuilt at 225,000 and compression is fine and the plugs are carbon free. The O2 sensor is fine.
What is making the idle oscillate? Injectors? Vacumn leak? Air in the antifreeze? Fuel distributor?

If someone has suggestions, I could use some right now.
Thank you, Dave

Have you ever replaced the OVP module? Both the modules on my 88's went out within the last year and were still running on the original - they are dated so you should be able to tell. That alone was the cause of the rough startup. I would imagine it could also play some weird games on the idle speed at full operating temp.

One of them also needed a new water temp sensor (located at the back of the head near the firewall on the driver's side, has 2 connectors attached to it) before it would start properly.

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(2) 1988 300 SEL W126