GM Vortex P0300 Fault: Humidity-Related Engine Shutdown & Fuel Flow Issues
I'm experiencing a recurring issue with my GM Vortex van, specifically code P0300 (Random/Multiple Cylinder Misfire Detected). The problem has worsened over the past two weeks. After rain or high humidity conditions—especially in the morning—the engine will not start and may only run for 1–4 seconds before shutting down completely. I’ve ruled out ignition-related causes: spraying starting fluid into the intake sustains combustion, confirming that fuel delivery is not the primary issue. The van sits overnight in a dry parking lot, so the engine compartment remains dry. However, high humidity after rain consistently triggers the failure. A key update: when I disconnected both the MAF (Mass Air Flow) and Air Temperature sensors simultaneously and sprayed ether into the intake, the engine started immediately. This suggests that one or more of these sensors may be sending incorrect signals to the ECU under humid conditions, causing it to cut off fuel flow. I’ve already replaced critical components including ignition coil (1 year old), spark plugs, distributor cap, rotor, TPS, fuel filter, fuel pump, and catalytic converter (less than a month old). All tests show normal readings. Fuel pressure remains stable at 55–62 psi when the key is on (not running) and drops to about 53 psi at idle—within specifications—and does not fall below 55 psi during operation. Fuel injectors have been tested individually and all show resistance within 12.5–13 ohms, indicating no electrical faults. The engine runs briefly without ether but stalls immediately after, with very rough performance. I’ve read that GM Vortex engines lack a dedicated air density sensor and rely on sensor data and historical values to calculate fuel mixture. This may explain why high humidity disrupts the ECU’s fuel calculation—causing it to lean out the mixture excessively. My question: Are there any sensors or combinations of sensors (like MAF, temperature, or barometric) that could cause the ECU to misread air density under humid conditions and reduce fuel delivery? What is a reliable fuel pressure gauge for this application? I’m considering purchasing one from Advance Auto but would appreciate recommendations. Also, are there known issues with aftermarket distributor caps or ignition wires on Vortex models? I’ve tried disconnecting the battery to reset ECU memory—no effect. The behavior appears consistent and not intermittent; it’s specifically triggered by high humidity, especially in the morning after rain. Any insights into sensor-based fuel mixture control under humid conditions would be greatly appreciated.
Check the wiring harness along the frame rail that connects to the fuel pump—intermittent connections can cause fuel flow interruptions, especially in damp or humid conditions.