Car hard to start when hot? Check for lost fuel pressure
A hard hot restart usually means fuel pressure is lost after shutdown, through a leaking injector tip, a leaking pressure regulator or the check valve in the pump. Vapor lock on older cars and a crankshaft sensor that drops out when hot cause the same symptom.
Can you keep driving?
Usually, yes, once it starts. It is a fault to have diagnosed, not an emergency. If you also smell fuel or see a leak, stop: that is a fire risk.
What it means
When the engine is switched off, the fuel system should hold its pressure for a while, known as residual pressure. If that pressure falls quickly after shutdown, a hot restart becomes hard.
A rapid fall points at a leaking injector, a leaking regulator, or a check valve in the pump. Two other faults give the same symptom: vapor lock on older cars, and a crankshaft sensor that drops out when it gets hot.
Common causes
- An injector tip that leaks after shutdown
- A leaking fuel pressure regulator
- A leaking check valve in the fuel pump
- Vapor lock on older cars
- A crankshaft sensor that drops out when hot
What to do now
- Note that it happens when the engine is hot, not on a cold start.
- Ask for a residual pressure test: the system is pressurized, switched off, and the pressure is watched.
- Have any fault codes recorded before they are cleared.
- If you smell fuel or see a leak, stop driving and have it checked first.
- Don't rely on fuel additives. An additive cannot repair a leak.
Hot start or something else?
- Hard to restart hot, fine cold: lost residual pressure, vapor lock on older cars, or a crankshaft sensor dropping out hot.
- A fuel smell or a visible leak as well: stop, because that is a leaking or stuck injector or seal, and a fire risk.
- A rough cold idle on a direct injection engine: possibly intake valve deposits, among several other causes.
What people often get wrong
That another bottle of cleaner will fix it. An approved additive may help some deposit complaints, but it cannot repair a leak or a faulty valve. A second bottle is a decision not to find out.
When the car must go to a workshop
Pressure testing and anything that opens the fuel system. The workshop follows the manufacturer's procedure for releasing pressure before any line is opened.
Read next
- Car won't start: find the fault by the sound
- Weak fuel pump symptoms: misfires and power loss under load
- Car stalls while driving: what to do first
This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.
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