Help! My Car Stalls While Driving

The first time your car stalls while driving can be a scary experience. I've personally been in dozens of different cars that have stalled while driving, and it's not a fun experience. Let's first talk about why a car stalls out in general, and then we'll get into some more detail about specific reasons your car might be stalling. Think of this word picture. A car engine is like the pedals of a bike, only most engines have 4 pedals instead of 2. If you're riding a bike, you have to be moving at a certain speed in order to stay on the bike. If you're going too slow, the bike won't stay upright and you'll fall off. Think of this minimum cruising speed as idling. With an engine, in order for it to stay on, it has to idle with a certain amount of power. If something happens to cause the power to diminish slightly, the engine will begin to shake. The idle of an engine is usually set to barely the minimum power to keep the engine on and running. Why is that? The main reason is to save you money on gas when you're stopped at red lights. If the idle were set higher, and it can be set higher, you'd just waste more gas when stopped.

There are a number of reasons why a car might stall while driving. In almost all cases, it's due to a lack of power while the car is at idle.

1. Water and/or Oil in the Combustion Chamber -- Your combustion chamber is where your spark plugs ignite and your pistons fire. Ordinarily, the only compounds present are oxygen and gas. These combine to create the fuel we burn for combustion. But because an engine runs hot, oil is needed to lubricate the pistons as they move up and down, and water/coolant is needed to keep the engine cool as the pistons are firing. The only thing keeping the water and oil out of the combustion chamber are seals. These seals can wear out over time and can begin to leak other fluids into the combustion chamber. When this happens, you might start to lose power because the fuel you're burning is being watered down. This can cause mild shaking and stalling, and can lead to misfiring pistons.
2. Misfiring Pistons -- If the leak into the combustion chamber is severe enough, you might experience a misfire, which is what happens when the fuel won't ignite and the piston won't fire. If this happens, you'll experience a big power loss, and your check engine light will generally come on. If your car is shaking at idle, has a lack of power while driving, and your check engine light has come on, it generally means that one or more of your cylinders isn't firing. This can be a simple fix or an expensive one depending on what's causing it. Seal leaks and blown head gaskets are the most common causes of misfiring. Seals we've already talked about. The head gasket is what keeps the water/coolant from entering the combustion chamber. If you have a blown head gasket, you'll definitely experience these symptoms, and there should probably be a bit of white smoke coming out of the exhaust pipe. This is the water/coolant being burned.
3. Lack of Compression -- This is a more rare cause of stalling. What is compression? Compression is what happens when a piston pushes back up after firing. When fuel is sprayed into the combustion chamber, the piston compresses it to get it all tightly packed into a small area. That way, when the spark plug fires, it'll explode more powerfully and give the maximum bang for your buck. If you have a compression problem, it will lead to lack of power, shaking and stalling while driving. It's more rare, but it can happen.
4. Combination of Factors -- If your car shakes and stalls while driving, it could be a combination of factors. If your air filter is clogged, you're leaking a little bit of oil and/or water into your combustion chamber, and your spark plugs are old, you'll experience a lack of power from a number of different sources. If this is the case, you'll want to get a standard tuneup, which will consist of changing your spark plugs, cleaning or replacing your air filter, and a number of other things that will lead to higher performance and more power.
5. Exhaust System Clogging -- Your exhaust system is fairly basic, but there are a few points at which it can clog up. If your exhaust system gets clogged up, it can't pass exhaust away from the combustion chamber efficiently and can lead to shaking and stalling while driving. There are a handful of places where a clog can occur in an exhaust system. One is your catalytic converter. Another is in the initial exhaust manifold. If you're experiencing and exhaust system clog, your check engine light will generally come on to let you know there's a serious problem.