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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).

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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:
  • Water in various components e.g. driving through a large puddle (may count as "obvious" though).
  • Aliens.

Also some vehicle-specific issues, for example:

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.

  • I feel like there's some valve train issues that should be on this list; but I don't know if things like stuck valves, rockers, etc. actually happen. I just know what they do in the engine not how they fail. – Jason C Jul 17 '16 at 20:22
  • That's a great list, lots to think about. I've been mostly driving / working on diesels for the last 15 years or so, so I'm loosing touch with what goes wrong in gasoline engines. Would something like fouled plugs cause a sudden shutdown? I'd really like to have something of a definitive (or at least well qualified like you did with the small/marginal engine and sudden load) list of things that somebody could look for when troubleshooting. It seems that we get a lot of "mysterious stall" questions. – dlu Jul 17 '16 at 20:55
  • @dlu Well I imagine that if the spark plugs are fouled in such a way that prevents them from sparking properly (or at all), then poor combustion could lead to a stall, and it could be a transient situation because perhaps it is dependent on temperature / pressure / RPMs / other factors, etc. It's just that, there's a zillion reasons for an engine to stall. Unfortunately I just don't have the experience to say what the most likely causes are, so this list is in no particular order. – Jason C Jul 17 '16 at 20:58
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    @dlu In addition to a list you might also want to put together a list of helpful information that the poster should include, e.g.: Warning lights on dashboard, ECU error codes if available, noises, smells, recent other problems, driving conditions (hills, etc.), driving behavior (did it happen when braking, shifting, at speed, while turning) etc. Those kinds of things can really narrow it down. – Jason C Jul 17 '16 at 21:11
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    Great list, Jason. I'd not put most mechanical things on here as mechanical doesn't usually solve themselves. There are a few I can think of, but for the most part, not happening. Also, another thing could be an overheating fuel pump, which quits pumping when hot (thermal overload), then starts working again when it cools down. Just a thought. – Pᴀᴜʟsᴛᴇʀ2 Jul 17 '16 at 21:11
1
  1. Electrical trouble tripping the autoshutdown relay.
  2. Overheating.
  3. Loose electrical connection at either fuel pump, ignition switch or spark plugs.
  • How would over heating cause a shutdown? Via the ECU and the auto shutdown relay? – dlu Jul 17 '16 at 19:40
  • @dlu Yes. Most cars have this feature nowadays. :) – tlhIngan Jul 17 '16 at 19:48
  • Cool! (heh, heh) Amazing what you can do with software :-) I don't think our VWs have that feature (Mk4 TDIs). I guess what this really comes down to, is any transient fault that could be bad and that can clear itself. – dlu Jul 17 '16 at 19:51
  • Maybe it's different nowadays but I would think most cars would throw an ect sensor failure code on overheat rather than shutting down, letting other effects of overheating shutdown the engine rather than the temperature reading itself? Mine behaves that way at least. It seems weird and unsafe for the engine to shutdown in response to say a loose ect sensor wire, when it's easy enough to indicate overheat conditions to the driver and let them deal with it. P0115, P0116, P0118, P0119, P0125, P0128 are all ect error codes, there's a lot of ways for it to fail that won't make the car undrivable. – Jason C Jul 17 '16 at 20:01
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    Does use of the auto-shutdown relay usually (always) result in a code being set? – dlu Jul 17 '16 at 21:00
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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.

1

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).

-1

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.

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    The former would be obvious, I'd think :-) and I doubt the later happens, or at least it seems unlikely to happen. In fact SAAB used to demo the emissions improvements that they made over the years by hooking the exhaust of a two-stroke model 93 or 96 to the intake of a modern car (a 9000 if memory serves me). The result was that the air coming out of the tailpipe of the 9000 was cleaner than the air going into the engine from the two-stroke. – dlu Jul 17 '16 at 19:58
  • Two things: - if the OP was only locking for internal causes, he may specify this further. The "obvious" is not so obvious as there are not more external causes I could think of (which allow a restart of the engine) - Your point about the engines is just wrong, except one of the cares produced oxygen instead of running on it. Cleaner (whatever that means) is possible, but for sure, the oxygen level will be lower. Or you will get a Nobel-prize instantly ;D (assuming normal fuel) – Mayou36 Jul 17 '16 at 20:28
  • My point was mostly that it seemed unlikely that exhaust from another vehicle would be able to shut the engine down. SAABs "stunt" wasn't an attempt to win a Nobel prize, it was to show that the emissions controls on their newer cars did a good job, presumably burning or otherwise (catalyst) mitigating the pollutants in the exhaust of the older car – it's very unlikely that anything in that setup was liberating oxygen… – dlu Jul 17 '16 at 20:36
  • Ah sure, my fault :D Still, the experiment says nothing about the oxygen level. You are not very used to the chemical reactions happening inside a car engine? Simple: the fuel (consisting of C and H) burns by making new connections to the O in the air, resulting in CO, CO2, H2O (and more). Two effects appear: less oxygen in the air plus pollutants. Modern cars convert (by catalysators) most of the CO to CO2 and more (which is much less toxic). This is a way of "cleaning" the exhaust gases. Still, there is les oxygen -> mixture in the engine of the second car may gets too rich. – Mayou36 Jul 17 '16 at 20:43
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    @JasonC :-) I had one of those cars (the two-stroke), I hate to think of how much junk it was putting out. Loved it but would feel guilty driving it today. It amazes me whenever I'm out on the road walking or biking and I get passed by a "classic" car how much they stink. Emission control systems have come a long way! – dlu Jul 17 '16 at 20:50

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