1999 Buick LeSabre P0172: Misfire, Hard Starting, and Stalling Issues - How to Diagnose and Fix
I own a 1999 Buick Lesabre that is experiencing severe starting difficulties. After cranking for an extended period, the engine starts but immediately misfires and often stalls before I can reach my destination. Pushing the accelerator helps me rev it up and temporarily keep it running. Once the car is moving, if I stop, it won't restart until I let it sit for at least 30 minutes. At idle—such as at a red light—the engine loses power and misfires repeatedly, making it feel like it's about to die, so I have to keep pressing the gas pedal to maintain operation. I recently had diagnostic codes read, and I believe the primary code was P0172 (System Too Rich). The technician also mentioned a secondary misfire code, possibly P0300. I replaced the spark plugs and ignition wires with new AC Delco parts, but the problem persists. Before investing in expensive repairs, I'm seeking advice on what to check next. The mechanic suggested replacing the oxygen sensor, though he noted that this is a common recommendation without always being the actual root cause. He advised checking other potential issues before jumping to replacement parts. Any insights or troubleshooting steps would be greatly appreciated.
Check for vacuum leaks, as they are a common cause of misfires and rich fuel trim conditions. Verify fuel pressure and perform a leak-down test to rule out fuel system issues. If you have an OBD2 scanner, check the fuel trims—especially long-term and short-term—during idle and under load. This can indicate whether the engine is running too rich (P0172). Are your spark plugs AC Delco? Confirming the brand helps assess compatibility. Also inspect for a restricted catalytic converter or exhaust system, which can cause backpressure and affect engine performance.