What Causes a Car to Stall While Driving?

Modern cars will usually stall because of a sensor failure. All of the sensors are important, but anything with "position" in the name is critical. The throttle-, crankshaft- and camshaft-position sensors are three that your computer just can't compensate for if they malfunction. Mass airflow sensors are important, especially at idle, because they tell the engine how much air is going in. If the MAF is dirty or malfunctioning, the computer will think there's less air going in than there is. That could easily throw it into a "lean stall" before the oxygen sensors have a chance to tell the computer that things have gone awry. Manifold air pressure sensors are almost as important, and just as important in cars that don't have MAF sensors. But the TPS, CPS and MAF sensors are always primary suspects in stalling.

Believe it or not, idling is one of the hardest things an engine has to do. At idle, engines are more sensitive to even the slightest variation in air, fuel and spark delivery. Often, engines that stall only at idle do so because they're getting either too much air, or not enough fuel. Leaking vacuum lines and intake induction parts are the classic suspects, but these days you have to account for exhaust gas recirculation valves stuck in the open position, and idle air control valves doing the same. An IAC valve is like a metered air leak that the engine uses at idle. If the IAC is stuck open, the engine gets more air than it should, and can stall at idle. As mentioned, sensor failures are always suspect, but these kinds of failures will generally manifest as problems at higher rpm as well.

Pressing the gas pedal means opening the main throttle plate, which introduces more air into the engine. The engine has to respond by delivering more fuel, and more aggressive spark timing to set it off. Massive vacuum leaks can cause stalling under acceleration, but more often the cause goes back to a failure in fuel delivery. That means a bad fuel pump or regulator, or a clogged fuel filter or injectors. Bad gas or gas contaminated with chemicals or water is another possible cause here. A weak ignition system or bad ignition timing will also cause stalling under acceleration. Check your ignition module, plug wires and spark plugs on a newer car. Owners of older cars with distributors should check for proper timing advance. Stalling under deceleration almost always goes back to either excess fuel or not enough air -- especially when accompanied by a backfire. Check that you IAC is opening, and isn't clogged with carbon and debris.

Sudden stalling under cruise conditions is usually electrical in nature. Air and fuel failures will generally cause the vehicle to surge and drop off in power before stalling; electrical or sensor failures will kill an engine outright, even if it's already at cruise speed. Two often-overlooked causes are bad engine ground connections and overheating ignition coils. A failing coil will get very hot under consistent use. The hotter it gets, the more it resists electricity, and then the hotter it gets. If your vehicle runs fine for a few minutes, and then starts acting up while you're going down the highway, check the ignition coil or coils. They shouldn't sizzle when you put a drop of water on them. Fuel pressure failures will also cause stalling under cruise, but it's usually more gradual -- like running out of gas. If you get sudden, random stalls under cruise, check the electrical system first.