P0172 & P0300 Misfire Issues in 2002 Lexus LS430 Dual-Fuel System

Hi, I own a 2002 Lexus LS43-0 that has been converted to run on both gasoline and propane. The vehicle normally starts on gasoline and automatically switches to propane once the engine coolant reaches a warm temperature. Recently, I've noticed the car running roughly on gasoline only — producing thick white or bluish smoke from the exhaust, with a strong gasoline odor. Diagnostic trouble codes P0300, P0301, P0305, and P0307 have appeared (all related to misfires on Bank 1). The most frequent issue is P0301. Additionally, code P0172 — indicating a rich fuel mixture in Bank 1 — has surfaced. When the engine reaches operating temperature, switching to propane results in smooth operation within 1–2 seconds and the engine warning light (including 'Cat Danger') stops flashing. The codes clear after this transition but reappear when I switch back to gasoline. I took the vehicle to the original propane conversion installer. After they cleared the codes, the problem disappeared entirely — the car ran smoothly on both fuels. However, two days later, the same symptoms returned. Before a full inspection, the technician recommended replacing the spark plugs (last serviced at 120,000 miles). I asked him to clear the codes again as this had previously resolved the issue. While he connected the scanner, the car suddenly began running normally on gasoline — and the problem seemed to vanish. I then inspected the spark plugs myself and found that all were in good condition except for Cylinder No. 1 (the primary misfire source), which was visibly soaked with gasoline. This suggests a fuel delivery issue rather than ignition failure, as the engine runs fine on propane. I also cleaned the Air Flow Meter to rule out sensor-related problems. Could anyone provide insight into this behavior? Key observations: 1. The problem occurs only in Bank 1; 2. It does not happen when running on propane; 3. Clearing codes or allowing the engine to warm up resolves it temporarily — and switching back to gasoline immediately triggers a recurrence. Any ideas would be greatly appreciated! Best regards, Mike