Hard to start when hot: the fault that hides on a cold morning

Cold starting is the hard case for most engines — thick oil, cold fuel, poor vaporisation. So a car that starts perfectly cold and struggles hot is behaving backwards, and that reversal is informative.

Related: engine cranks but won’t fire.

What heat changes

Four things, and each maps to a category of fault:

Electrical resistance rises. A marginal connection or a failing sensor works cold and fails hot.

Fuel vaporises more easily. Vapour in a fuel line is compressible; liquid is not. Vapour lock means the pump moves gas instead of fuel.

Components expand. Clearances change, and a marginal mechanical fit becomes a failure.

Residual pressure bleeds away faster in a hot system.

The causes, in order

Crankshaft or camshaft position sensor

The classic hot-start fault, and the one worth suspecting first.

An inductive sensor breaking down internally works cold and fails as it heats. The engine cranks and never fires because the ECU has no timing reference.

The signature is unmistakable: drives fine, stops or refuses to restart when hot, starts perfectly after half an hour standing.

P0320 to P0322 engine speed input, P0336 crankshaft sensor range/performance and P0341 and P0344 camshaft sensor.

Fuel pressure not holding

A leaking injector or a failed check valve in the pump lets rail pressure bleed away after switch-off. Hot fuel does it faster.

The test: switch the ignition on for two seconds without cranking, off, on again, off — priming the rail several times — then crank. If it starts noticeably better, pressure was not being held.

P0087 fuel rail pressure too low and long crank in the morning.

Leaking injectors flooding the cylinders

Injectors that do not seal drip into the cylinders after switch-off. Hot restart then has an over-rich mixture to deal with.

The tell: a fuel smell, and better starting with the throttle held open — which on many vehicles triggers a clear-flood mode.

P0201 to P0206 injector circuit diagnostics.

Vapour lock

More common on older vehicles and in very hot climates. Fuel vaporises in the lines near hot components, and the pump cannot move vapour effectively.

The starter motor

Heat raises resistance in the windings and the solenoid contacts. A starter that turns the engine briskly cold and slowly hot is a genuine candidate — and this is one of the few cases where the starter itself is the likely answer.

P0616 and P0617 starter relay circuit.

Coolant temperature sensor

The ECU uses it to set enrichment. A sensor reading cold when the engine is hot makes the ECU over-fuel a hot engine, which floods it.

Check it plausibly reports temperature across a warm-up: P0116 to P0119 coolant sensor performance.

Battery and connections

Cranking demand rises when the engine is hot and the oil is thin but the starter is drawing more. A marginal battery copes cold and fails hot.

Battery terminals corrosion.

The diagnostic that costs nothing

Note precisely what happens.

Observation	Points at
Cranks briskly, does not fire	Sensor or fuel pressure
Cranks slowly	Starter, battery or connections
Fires and dies	Starts then dies immediately
Better with throttle held	Flooding — injectors
Better after several ignition cycles	Fuel pressure not holding
Recovers after 30 minutes	Heat-related sensor

What to log

Leave logging running and try a hot start:

• RPM during cranking. Present means the crank sensor is working; absent means it is not.
• Fuel rail pressure. Whether it builds.
• Battery voltage during crank. Below about 9.6 V is a supply problem.
• Coolant temperature. Whether it is plausible.

Capturing one failed hot start in a log is worth more than a week of speculation.

Diagnosing intermittent faults with OBD.

Step by step

1. Characterise the crank — brisk or slow?
2. Try the multiple-prime trick for fuel pressure.
3. Try holding the throttle open to test for flooding.
4. Log a failed hot start, especially RPM.
5. Check battery voltage under crank.
6. Read all codes, including pending.

Pair an ELM327 adapter, open the free tier of the app and leave logging running before a hot restart attempt. Whether RPM appears during cranking splits this diagnosis cleanly in two.