What are the main reasons for difficulty in starting a hot engine?

[LaCollin]

Here are the reasons that may cause difficulty in starting a hot engine: 1. The fuel injector may be leaking. While fuel injection is normal, leakage constitutes additional fuel supply. The more severe the fuel injector leakage, the richer the air-fuel mixture becomes, leading to difficulty in starting a hot engine. 2. Engine carbon buildup. Engine carbon buildup is a long-standing issue. Carbon has a strong adsorption effect on gasoline. If there is significant carbon buildup on the intake valves, throttle, pistons, etc., it can adsorb some fuel, causing the air-fuel mixture to become too lean and affecting startup. This situation can also occur during cold starts. 3. Poor fuel quality. After the hot engine is turned off for a while, the engine compartment remains at a high temperature. If the fuel itself is highly volatile, it can easily form high vapor pressure in the fuel rail. When this pressure exceeds the fuel supply system's pressure, vapor lock occurs, causing the fuel injector to spray overly lean fuel vapor, making it difficult to start. 4. Intake air temperature sensor issue. The intake air temperature sensor, similar to the coolant temperature sensor, adjusts the fuel injection amount. If the coolant temperature sensor fails, it can cause difficulty in cold starts; if the intake air temperature sensor fails, it can cause difficulty in hot starts. 5. Damaged carbon canister purge valve. During a hot start, fuel atomization is normal, and no additional fuel vapor is needed. If the carbon canister continues to supply fuel vapor to the intake tract, it can cause the air-fuel mixture to become too rich, leading to difficulty in starting.

[WilliamAnn]

I've found that difficulty in hot starts is mainly due to several common issues. One is that spark plugs cannot function properly after aging. In hot conditions, the spark plug gap increases, leading to poorer ignition performance and making the engine harder to start. Another issue is a faulty fuel system, such as insufficient fuel pump pressure or vapor lock in the fuel lines during hot weather, where excessive fuel evaporation prevents fuel from reaching the cylinders, resulting in a weak start. Additionally, sensor failures, like a misreading coolant temperature sensor, can cause the engine control unit to deliver insufficient or overly rich fuel mixtures, affecting startup. I often encounter these situations while repairing cars and recommend regularly replacing spark plugs and checking the fuel pump, ideally performing a thorough maintenance every two years to avoid trouble. Also, ensure the battery connections are secure, as battery efficiency drops in hot conditions, adding to the problem. Paying attention to these small details can significantly reduce starting issues.