Power loss after start?

[tazjig]

Hi, i own a 1990 s13 and whenever i leave my car overnight after driving it, when i start it up in the morning it has lots of balls for the first 3 or 4 minutes, but after it starts to warm up it almost cuts the power in half, its a durastic drop too, i was thinking it could be something like vacuum hoses or something but im not too sure, so if anyone could help me out it would be much appreciated. Thanks.

[drjohn]

Check the O2 sensor. The O2 sensor is inop during warm up or open loop and doesn't effect engine. once the engine is warm the fuel system goes into closed loop and starts using O2 sensor input to adjust mixture. The length of time for open loop is controlled by the ecm and it's inputs.