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.
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OTOH misfires allow excess fuel into the exhaust, in turn the O2 sensors see less oxygen
Actually, the misfire causes the oxygen to not be consumed, and therefore the O2 sensor sees too much oxygen and tries to compensate by adding more fuel.
You want to check the freeze frame data to see if the engine was under load when the code set. If it was , check the fuel filter and the fuel pump pressure. Inadequate fuel supply can cause a lean misfire and/or the lean code. Additionally the freeze frame will show the fuel trims so you can see if the PCM was trying to compensate, and if it was compensating on one or both banks.
If the engine was unloaded look for a vacuum leak.
Jim in Central NY
'79 2WD F-150 Lariet w/302,4spd OD, 228,965 mi. (Original Owner!)
'02 4X4 Navigator w/5.4L DOHC 32v Intech V-8 (VIN R).
Bob and better half
As my brother reminds me regularly... "Life is not a spectator sport."
2007 F-250, PSD, SW, CC, LB, 2 WD, 85k, coolant filter, DashBoss, 1999 Expedition 5.4 319k plus, 32' CrossRoads CrossTerrain 5th wheel Toy Hauler, 24' Chaparral Villain lll 598 ci
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