Re: Help 2005 grand cherokee random misfire
Could be yes. I presume that it takes a bit for the PCM to see that an O2 sensor is bad, and in the meantime is using the output from the O2 sensor. A high O2 sensor voltage means a low O2 level, which means a rich mixture (as seen by the PCM, eventhough faulty). The PCM will downtrim the fuel injector pulse and that can go that low, that the mixture will simply not burn anymore, causing the misfire. After some time, the PCM is seeing that the O2 sensor voltage is just too high, and will not use the sensor output anymore, and goes into open loop mode. From then on, the idle will be normal again, eventhough the fuel consumption will be higher then normal, due to the fact that the PCM is not trimming the fuel anymore for the best air/fuel ratio. That would also explain why on a cold engine the problem is not there, because then the PCM works in open loop mode and doesn´t use the O2 sensor output. Before the sensor is hot enough for the PCM to use its output signal (and go into closed loop mode), the PCM already saw that the O2 sensor voltage is too high and will simply not go into closed loop.