1999 Chevy Astro P0155 Code: Intermittent Stumbling, Hard Starts, and Misfire Solutions

Hello, I'm experiencing persistent issues with my 1999 Chevrolet Astro cargo van. I've been troubleshooting for over two years and am now at a loss as to what's causing the problem. Symptoms include: - Intermittent stumbling at idle and low RPMs under load - Occasional hard starts—cranking repeatedly with no response - OBDII diagnostic codes: P0300 (random misfires across cylinders) and P0155 (right-hand oxygen sensor heater malfunction) Troubleshooting history: This issue began about two years ago. Initially, I replaced the right-front oxygen sensor based on the P0155 code, assuming a faulty heater element was at fault. However, this did not resolve the problem. I later suspected electrical issues and replaced spark plugs, ignition wires, distributor cap, and rotor—no improvement. Using an oscilloscope, I observed that misfires occurred alongside abnormally high voltage readings, suggesting mixture instability. I then investigated the fuel system, as spark performance is dependent on air-fuel ratio. Given known history with California gasoline causing fuel injector poppet valves to stick, I used a fuel injector cleaner (connected via air compressor). This temporarily improved engine operation—enough for smog compliance—and allowed me to manage hard starts by using the cleaner during cold or rainy weather. I took the vehicle to a GM dealer under their extended emissions warranty with instructions to inspect for stuck injectors. They concluded that the fuel injectors were fine but recommended replacing the post-catalytic converter oxygen sensor, charging $240 for diagnostics. After paying and installing the new sensor, there was no improvement. Per GM Special Service Bulletin 99066G, I replaced the original fuel injector spider with a newer MFI (Multi-Point Fuel Injection) system—eliminating poppet valves—and also upgraded the fuel pressure regulator and fuel filter. I expected this to resolve all issues permanently, but it did not. The truck still intermittently stumbles off idle and occasionally starts very hard, especially on cold, rainy days. The pressure cleaner fixes the issue temporarily. OBDII codes P0300 and P0155 persist. I've also considered ignition system issues—though I only misted components with water while running, no improvement was observed. According to GM bulletin 99066G, a fuel filler pipe may degrade due to California gasoline, causing melted plastic to enter the fuel system. While this seems far-fetched, I’ve ordered a replacement pipe. I've installed a pressure gauge on the injection rail and recorded the following readings: - Ignition turned on, engine off: 61 PSI - Ignition off, wait 10 minutes: 45 PSI - Engine idling: 54 PSI - Full throttle under load (uphill): 58 PSI These readings appear stable but may indicate a subtle fuel delivery issue. Has anyone encountered similar symptoms or diagnostics with their 1999 Chevy Astro? Any insights would be greatly appreciated. Best regards, Jerry Kaidor