1999 Toyota Camry 3.0L V6 Misfiring on Cylinders 1, 3, and 5 with P0300, P0301, P0303, P0305 Codes

snshevey wrote: My 1999 Toyota Camry 3.0L V6 is running sluggishly, with a smooth idle but poor performance under load. The vehicle is showing misfire diagnostic codes: P0300 (random misfires), P0301 (cylinder 1 misfire), P0303 (cylinder 3 misfire), and P0305 (cylinder 5 misfire). I have already replaced the spark plugs, ignition wires, and coil packs. The timing belt has been inspected and appears intact with no signs of wear or damage. No vacuum leaks were detected during inspection, and there is no visible intake manifold leak. The EGR system seems to be functioning correctly. Despite these checks, the misfires persist. I'm seeking advice on potential root causes that may have been overlooked. Double-check your engine timing, especially if the camshaft sprocket has rotated one or two teeth off its shaft. This issue is common and can cause misfires in cylinders 1, 3, and 5 even when timing appears correct. Perform a compression test first to verify cylinder integrity. If cylinders 1, 3, and 5 show low compression, that would point directly to mechanical issues such as worn piston rings or valve problems. This is a critical step—worth verifying before moving forward with further repairs. snshevey.

The V6 timing belt on the 1999 Toyota Camry is prone to installation errors, especially if one tooth is misaligned. Always ensure proper alignment using the factory-matched alignment marks on the new timing belt, and confirm you're using a genuine Toyota timing belt. Additionally, verify that the spark plugs installed are the correct OEM type—NGK or Denso—for this model. Using non-OEM or incompatible spark plugs can lead to poor combustion and misfires, even after replacing all ignition components. This is especially important for older 3.0L V6 engines where component compatibility directly affects performance.