Re: Please help!! Rough idle,stall, piston mosfire
When the engine runs fine cold, but starts to idle rough when hot, then it could be a bad O2 sensor. On a cold engine, the O2 sensor readings are not used by the PCM ( open loop mode), but as soon as the sensor is hot enough, the PCM will use the O2 readings( closed loop mode). A bad sensor could cause a too lean mixture and cause misfires due to this. You will need a scanner with live data to see if the O2 sensors (1-1 and 2-1) are fluctuating frequently between 0.2 and 0.8 V. Then you can also check the fuel trim figures, which should give single digit values. (Normalley they are around +- 5% during stable operation. When going over 10%, then there is probablly something wrong).
A blocked exhaust (loose baffle in silencer or a plugged converter) could also cause rough idle and bad acceleration.
As a third item, which many times is related to heat, is a bad crank sensor.