Originally Posted by 03GRS
Hi Alloro...I was told that if there was a bad vacuum, PCV valve, etc that the PCM would lean out the engine, causing the misfires.
Vacuum leak misfires are old school predating engine computers as we know them today. In today's world with a vacuum leak the computer can compensate to a point by enriching the mixture. OTOH misfires allow excess fuel into the exhaust, in turn the O2 sensors see less oxygen and figure the system is running rich, so the computer leans out the mixture, hence the P0174 code.