I've been trying to solve my cold start idle up and down and my rough idle when after it warms up for about a year now. I had just about given up and was just going to live with it, when today I was checking my fog lights and found something that might be the cause for the poor idle.
I turned the ignition on, but did not start the car and turned the lights on. I noticed that the front lights went bright and then would go dim and then cycle to bright and dim again. The car was very cold and seemed to do this until I turned the ignition off.
Does anyone know what might have made the lights do this and if so, is there possibly a relay that might cause this. Whatever caused my lights to to from bright to dim might be the some thing that could be causing my cold start idle to go high and low and my idle to run rough. This whole idle issue might be electrical. Any suggestions.

 

The headlights do have a relay and a headlamp control unit, they are both located in the left side engine panel by the fuses.

The issue could be connected to your problem in that the ECM uses battery voltage to make the final adjustment to the fuel injection pulse width. Try measuring the voltage at the battery under the same conditions. If your battery voltage is going up and down, it will affect engine performance. Then of course it becomes a hunt for why

 

(3 green convertible, Definitely worth looking in to. Good luck.
RagJag