Unable to Start When Hot After Shutdown but Starts Fine When Cooled Down?

[VonNoah]

Unable to start when hot after shutdown but starts fine when cooled down may be caused by fuel injector leakage, a faulty carbon canister solenoid valve, or fuel quality. Here are the specific explanations for the issue of being unable to start when hot after shutdown but starting fine when cooled down: Fuel Injector Leakage: Fuel injectors normally spray fuel, but leakage results in additional fuel supply. The more severe the injector leakage, the richer the air-fuel mixture becomes, leading to difficulty in starting when the engine is hot. Faulty Carbon Canister Solenoid Valve: During hot starts, fuel atomization is normally sufficient, and no additional fuel vapor is needed. If the carbon canister continues to supply extra fuel vapor to the intake manifold, it can cause an overly rich mixture, making it difficult to start. Fuel Quality: After the engine is shut down when hot, the engine compartment remains at a high temperature. If the fuel is highly volatile, it can form excessive vapor pressure in the fuel rail. When this pressure exceeds the fuel system's supply pressure, it creates vapor lock, causing the fuel injector to spray overly lean fuel vapor, resulting in difficulty starting.

[SabrinaRose]

I've encountered this situation quite a few times, and it's most likely caused by the crankshaft position sensor or camshaft sensor acting up. When the engine heats up, the electronic components inside these sensors expand due to overheating and malfunction. The ECU can't read the engine position signal, so of course it won't start. The fuel pump getting too hot may also lead to insufficient pressure, and there might be air bubbles trapped in the fuel system causing vapor lock. Additionally, carbon buildup in older engines can absorb fuel during warm operation, causing air-fuel ratio disturbances. For troubleshooting, the most direct method is to use a diagnostic tool to check for any interrupted RPM signal trouble codes. Alternatively, when the engine is hot, you can splash some cold water on the sensors (be careful not to wet the connectors) - if it starts immediately, that's basically confirmation of the problem.

[SpencerFitz]

From my experience, focus on checking the fuel pump! The fuel pump is cooled by gasoline. If the pump motor is aging or the filter screen is clogged, the fuel pressure drops severely when the engine is hot, making it hard to start. When the engine cools down, the oil thickens, increasing resistance, and the pump works harder to build pressure. Another possibility is a faulty engine temperature sensor—if it tells the computer it's sub-zero, the computer will inject excessive fuel, causing flooding. Another scenario—poor heat dissipation can cause the ignition coil to short-circuit internally when overheated, preventing the spark plug from firing. The simplest test is to press the accelerator slightly when the hot engine won't start; if it starts, it's likely a mixture issue.

[DelDaniella]

This is commonly seen in older vehicles with over 100,000 kilometers. Under high temperatures, the aging wiring connectors in the engine compartment can cause contact points to disconnect due to thermal expansion when the engine is turned off. Additionally, oxide buildup on battery terminals can lead to poor conductivity when hot, causing further issues. If you hear a clicking sound when starting but the engine doesn't turn over, it could be a classic case of overheated starter motor carbon brushes or a stuck solenoid. It's recommended to first clean the battery cables and main grounding point connections, using sandpaper to remove any greenish oxide layers—often, simply clearing away the copper corrosion resolves the problem. Moreover, internal resistance increases when the engine is hot, and an old battery's capacity can significantly degrade under these conditions.