I had an issue with being powerless when engine was cold. I've recently tuned my truck with plugs/cap/rotor/f filter/ and cleaned out the TB, but it still struggled when cold. I changed the o2 sensor yesterday and it made a world of difference, truck ran great (HUGE IMPROVEMENT, if you've never changed it and have high miles, do it now!), even improved it when cold.

My question is, why would I see an improvement from a new o2 when it is cold if it is supposed to be in open loop when cold? I would think that a new or old sensor would equally have no effect until closed loop.

could this mean it is starting in closed loop when cold? could I have a bad coolant temp sensor?

 

As I understand it (and I'm just learning this open and closed loop relationship stuff myself), the closed loop fuel trims do effect the A/F ratio in open loop. I would have thought it would take the ECU at least a few cycles to start effecting open loop though.

 

On my '85, the O2 sensor starts cycling within about 2 blocks of starting (as seen on the A/F gauge). So, even though the engine is not warmed up yet (still in fast idle mode), the ECU seems to be adjusting the A/F ratio based upon the O2 sensor output. Probably not full closed loop mode, but seems that the sensor is in the loop.