What is the situation when the engine experiences intake backfire under load?

[DiJulia]

There are many reasons that can cause intake manifold backfire, which may involve issues with the fuel supply system, ignition system, and other aspects. Engine intake manifold backfire: Refers to a fault phenomenon where the air-fuel mixture combusts within the intake manifold, causing flames to shoot out from the intake port. Fuel system faults: Intake manifold backfire occurs due to insufficient fuel supply, resulting in an excessively lean air-fuel mixture. A lean mixture slows down the combustion speed, causing combustion to continue until the end of the exhaust stroke when the intake valve opens. The fresh air-fuel mixture then encounters unexpelled combustion gases in the cylinder, igniting and flowing backward through the intake manifold, resulting in backfire from the intake port. Ignition system faults: Intake manifold backfire can occur due to delayed ignition or poor spark quality. On one hand, this delays the start of combustion; on the other hand, it also slows down the combustion speed, causing combustion to continue until the intake valve opens, leading to intake manifold backfire. Accompanying symptoms may include: slow engine speed increase during acceleration, noticeable backfire during rapid acceleration, sometimes backfire during gradual acceleration, muffled exhaust sound from the tailpipe, and the engine temperature rising easily. If severe engine vibration, exhaust backfiring, and significant power loss accompany the intake manifold backfire, it is often caused by ignition misfiring. Due to ignition misfiring, when a cylinder is in the intake stroke with the intake valve open and the spark plug fires, intake manifold backfire can occur. If inspection confirms that the high-voltage wires are correctly connected to each cylinder, the issue is likely a distributor cap breakdown. It is important to note that with minor distributor cap breakdown, the engine may operate normally under no-load conditions, but backfire may occur during heavy load climbing when the engine is hot, accompanied by noticeable power loss. If the engine runs normally at idle but occasionally backfires during rapid acceleration or deceleration, exhibits irregular backfire at high speeds, or frequently backfires when driving on uneven roads, for traditional contact-point ignition systems, it is often due to poor distributor grounding. For electronic ignition systems, it may be caused by poor grounding of the ignition module or loose sensors in the ignition system. A poorly grounded distributor or ignition module may generate high voltage when engine vibration causes grounding issues, equivalent to disconnecting the low-voltage circuit. If the distributor rotor happens to point to a cylinder in the intake stroke, it can cause intake manifold backfire.