S50 swap rough idle

[pancakechef]

Recently got my 89 325i with s50 swap running. Has everything except the full exhaust setup (headers are on with o2 sensor in). She starts right up every time and idles beautifully for about 2 minutes.. then it starts to idle a little rough. RPMs only waver slightly if I don't touch the throttle, but if I open the throttle a little then the RPMs are pretty wild once I let off.

All sensors are plugged in.. MAF, o2, etc. There doesn't seem to be a vacuum leak bc I replaced all of the hoses with cracks and tested some connections by spraying starting fluid with no effect on the idle. Once the idle starts getting rough the brake lights and dash lights dim in and out along with the RPMs. The fuel economy gauge also goes back and forth from both extremes along with the RPM swing. What are some possible causes of these symptoms? (Besides the ICV)

[M3PO]

I was gonna say try unplugging the water temp sensor for the ECU and see what that does. But the last few sentences sounds like you have an wiring/electrical issue.

[sweet3]

Have you tried the stomp test? Turn the key to the "run" position and press the accel pedal 5 times all the way down. Then let the check light flash. The codes are 4 digit codes

[pancakechef]

Now I've noticed that at about the same time the engine starts to run bad the green light on the dash, one spot to the left of the service indicator lights, comes on. I tried a stomp test but the CEL never flashed.

Also tried swapping the ecu but the problem still happened.

EDIT: The light that comes on is the emission-related indicator, two spots left of the fog light indicator. Not the light previously stated.

[jt223539]

Sounds to me like a charging issue of some sort. The dash lights do crazy things when there is a drop in voltage.

Have you hooked up all the required grounds? Also make sure you have tightened down the alternator wires, as well as the red power wire that connects to the starter.