What can cause a gasoline engine to suddenly & mysteriously stop (non-catastrophically)

Question

I was thinking about this question and found myself wondering what could cause a gasoline engine to suddenly stop, and then be restartable? So not things like running out of fuel, or suffering a timing belt failure.

The only thing that I've been able to come up with is an intermittent ignition glitch (say a failing ignition switch) or something that would cause the ECU to shutdown fuel.

I think it would be interesting to distinguish between the problems that could/should set a code and the ones that won't or can't. For example an ECU initiated shutdown, say on overheating, could (and arguably should) set a code, but a faulty ignition switch is probably indistinguishable from turning the switch off and that would be hard to diagnose (except heuristically, for example when the ignition cuts out at 60 MPH in gear).

Answer 1

This is what I can think of, in my limited knowledge. Overlaps with some existing answers. Trying to limit to things that aren't obvious, also trying to think of direct physical causes rather than things that may or may not trigger certain ECU software to make an explicit shutdown decision (I assume there's a lot of variation here, so it's kinda hard to cover all cars, plus that's more of a symptom than a cause):

• Faulty ignition switch / loose wiring.^†
• Loose/failing power input to ignition coil.
• Electrical faults (short / open) in ignition coil.
• Loose/failing ignition wire.
• Failing distributor cap / rotor^† (maybe? do they have transient failure modes?)
• Faulty crankshaft position sensor / loose wiring.
• Enough loose/failing spark plug wires that misfires lead to stalling.
• Fouled or faulty spark plugs.
• Clogged fuel filter.
• Faulty fuel pressure regulator.
• Fuel pump power issue (ranging from wiring to perhaps a loose or semi-blown fuse).
• Fuel pump overheating (perhaps due to e.g. failing bearings, failing pump, etc.)
• Fuel injector faulty wiring (in systems with EFI).^†
• Any conditions leading to momentarily improper fuel trim, e.g.:
  • MAP/MAF sensor failure or faulty wiring leading to momentarily improper fuel trim.
  • Any other faulty sensors (TPS, O2, thermostat esp. when engine cold, etc.) / faulty sensor wiring leading to the same thing.
  • Leak in intake downstream of MAF sensor.
  • Vacuum leaks.
• Failed or failing ECU.
• Idle air control valve faulty wiring.^†
• Defective speed sensor (leading to failure to idle when vehicle stopped).
• Obstruction in intake (perhaps it shuts down at high rpms but you can still start it, or the obstruction moves around or clears itself). Perhaps as simple as a dirty air filter.
• Obstruction in exhaust, e.g. back pressure from a clogged or failing catalytic converter that clears when the engine stops.
• Faulty EGR valve wiring.^†
• Coolant leaks, air in coolant lines can throw off the ECT sensor causing the engine to behave poorly and possibly stall (source is personal experience).
• In cars with smaller engines, or cars that are having other problems and running on the border anyways, anything that puts an unexpected load on the engine esp. when it is idling (e.g. too much electrical load + a possibly inadequate/failing alternator, power steering issues if stalled while turning, things like that).
  • Also wiring faults leading to e.g. the ECU failing to notice that the A/C has been turned on.
• I'm sure fuel quality issues / impurities that get past the fuel filter must be on this list somewhere but I don't know specifics.
• In automatics: Low automatic transmission fluid (causing excess engine load from torque converter at low driving speeds) or failing torque converter.
• In carbureted engines:
  • Stuck or poorly adjusted automatic choke
  • Stuck heat riser valve
• Water in various components e.g. driving through a large puddle (may count as "obvious" though).
• Aliens.

Also some vehicle-specific issues, for example:

• Faulty ASD relay (causing random shutdowns, esp. older Chryslers)
• Faulty PCV valve (cold weather issue with 2003-2004 Ford Focus)
• ECU bugs, solvable with firmware updates, for example:
  • This page also raises the possibility that on some vehicles "the factory [ECU] programming may not provide enough idle speed when the A/C is on, when the alternator is under high load or when the temperature is extremely hot or cold."
  • 2001-2003 Ford Escape idle / emissions calibration problems
  • "The National Highway Traffic Safety Administration (NHTSA) is currently investigating reports of sudden stalling with Toyota Prius hybrid electric cars. For reasons which have yet to be explained, the engine will suddenly shut down at speeds of 35 to 65 mph. In some cases the vehicle can still be driven in the electric mode, but in others everything goes dead and the car has to be towed. No word yet from Toyota of the cause, but it will probably turn out to be a PCM programming issue."

For all of the "faulty wiring" things above, the fault could be in a number of locations:

• Connectors at the components
• Wiring near the components
• Buried in the wiring harness or connectors elsewhere
• Fuse boxes, etc.
• Also electrical contact failure in the component itself, even though this may not commonly count as "wiring". I'd probably call that a "failed component" rather than "faulty wiring", myself.

In the above lists, general internet consensus seems to identify the most common causes of intermittent stalling as:

• Faulty sensors
• Vacuum leaks
• Intake leaks / obstructions
• Fuel obstructions.

Uh... I'm sure there's more, I'm trying to work this out in my head from what I know about engines. I know nothing about diesel. I also know nothing about carbureted fuel systems, or turbo/super charged engines (although you could lump those in with intake issues, I guess), mechanical fuel injection, or hybrid cars.

I've excluded most overheat related issues since things just get weird, any number of strange things could start to happen (e.g. sensors malfunctioning out of their temperature spec, etc.) that clear when it is cool again - I feel like that list could be infinite; plus I'm counting overheating as "obvious" or at least "visible".

^† More likely wiring or electrical faults, as mechanical failure of these components unlikely to be transient.

Answer 2

1. Electrical trouble tripping the autoshutdown relay.
2. Overheating.
3. Loose electrical connection at either fuel pump, ignition switch or spark plugs.

Answer 3

I've seen this happen with a faulty after market immobilizer of the type which cuts power to the fuel pump to prevent theft.

Basically, it was going bad and would occasionally not let the car start, or cut power to the fuel pump relay while driving. Then the car would start back up fine, or start after X number of tries.

Disconnecting the immobilizer from the fuel pump relay solved the problem.

Answer 4

There is of course one factor that seems to have been missed; driver error.

Stalling could cause a motor to stop and it could then easily be restarted.

Flooding, although this is associated with older carburettor fed cars, could also cause a motor to stop but it could be restarted.

Incorrect setting of the choke (again, you're looking back to older manual choke carburettor cars).

Incorrect setting of the ignition advance / retard (harks back to even older cars when the ignition advance / retard was controlled manually, usually by a control in the centre of the steering wheel).

Answer 5

An external cause: if there is too less oxygen in the intake air.

This can intentionally be accomplished by using a CO2-fire-extinguisher (a fire-fighter technique).

Maybe this is also possible if the car takes in too much exhaust gases from the car/truck in front of him.