Whenever I disconnect my battery

[jaywood]

My car runs horribly for the next few days (very low idle that fluctuates). After a few days it gradually starts to idle better. Stock chip, m20, no engine mods

Why?

[Cabriolet]

it resets the brain. happens to all of us.
it should reset after 30min of driving.

[jlevie]

If everything is right with the engine and management system there should little to no affect from removing all power from the DME. But things wrong with the engine or it's sensors can upset the A/F ratio. Based on feedback from the O2 sensor the DME can learn to run richer or leaner mixture and compensate. When all power is removed from the DME that learned adaptation is lost.

I'd smoke test the intake, check rail fuel pressure, and have the injectors professionally cleaned, rebuilt, and flow tested.