How do I diagnose a hard start when the engine is hot?
Short answer: Hot-start problems often come from fuel pressure leaking down, leaking injectors, vapor issues, a failing crank sensor, or a starter weakened by heat; check fuel pressure hold and sensor signals when hot.
Engines that start fine cold but crank a long time when hot often lose fuel pressure after shutdown. A leaking injector, a pump check valve or a regulator that bleeds off lets the rail drain, so the engine must rebuild pressure before it starts. A leaking injector can also flood a cylinder, causing a rough start with black smoke.
Check fuel pressure hold with a gauge after a hot shutdown and compare it with specifications. Look at the starting sequence with a scan tool: rail pressure on direct injection engines, crank and cam signals, and coolant temperature reading. A coolant sensor reading cold on a hot engine adds too much fuel.
Crank sensors sometimes drop out when hot, so check the crank signal when the problem happens. A starter that slows when hot, from worn brushes or heat-soaked cables, can also cause hard starts. Relieve fuel pressure before disconnecting lines, and keep ignition sources away from fuel.
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