When your engine cranks but won't start it can be an extremely bothersome problem for you to have to deal with as a driver. No matter how many times you turn the key in the ignition the engine doesn't seem to want to get going. There are a handful of reasons that can lead to this condition and it might take a little bit of work to narrow down precisely what the problem is in your vehicle.
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The first thing you need to know is exactly what's happening here. When we refer to the car cranking, we're talking about the starter motor attempting to get the engine to turn on. The starter causes a flywheel to turn which in turn rotates the crankshaft and that's how your engine begins moving. The pistons will rise and the fuel injector will squirt fuel into the combustion chamber for the spark to ignite, forcing the piston back down and starting the whole process of your car moving. However, if the car doesn't actually start even though you get that initial sound like it's trying to, you're obviously not going anywhere fast. Let's take a look at what can cause your car to turn over but not start.
Fuel System Problems
There are a number of problems in your fuel system that could stop your car from turning on even though it does crank. This could be something as simple as your gas tank being empty, so it's always good to check that before you worry about something else. What moves your fuel can also have an affect on your car's ability to start properly. If a fuel pump is burned out for instance, then it's not going to be able to manage the flow of fuel from the tank through to the engine.
The fuel pump could still be working but a fuse that controls the fuel pump could also burn out. Both of these circumstances end up with your fuel unable to continue moving through the system which is obviously going to be a problem.
Your fuel injectors could also be failing you in some way either because they're broken, misaligned, or just clogged with some kind of build up or debris. Additionally, you can have an issue with the kind of fuel that you're using in your fuel tank. As rare as this is it's not unheard of for someone to accidentally fill up with diesel in their car and it's meant to take regular gasoline. One will definitely not work with the other.
Battery Problems
It's possible an issue with your battery could result in your car cranking but not starting. If you find that the engine is cranking slowly you might have a problem with a discharged battery. It could also be related to the battery terminals being either loose or corroded and unable to properly send the right amount of power.
Whether it's the result of a discharge battery or even something as simple but easy to overlook as a starter wire coming loose or breaking, you can test the battery by simply checking the voltage using a multimeter as you try to crank your engine. Your battery should normally give you 12.6 volts and when you're trying to crank it should be reading about 10 volts. If you're getting lower than that, then you definitely have a problem with the battery.
Spark Plug Problems
There are a few ways that your spark plugs can malfunction that causes your car to crank but not start. Obviously, the spark plugs are necessary to get your engine going as without a spark, you're not getting a combustion reaction and your engine won't do anything. The problem in this case could be as simple as your spark plugs being corroded or contaminated with oil or dirt build-up. A quick visual inspection will let you know if you have any black carbon buildup around the gap or other residue that could definitely be causing a problem here.
The problem could also be somewhere else in this general area such as the ignition module or even a faulty crankshaft position sensor. Whatever the case if your spark isn't happening when it's supposed to then the air and fuel mixture will be injected into the cylinder and wasted because it's not being properly ignited.
Fuse Problems
If the problem is an electrical one but it's not directly related to the battery, it's possible that you have an issue with your fuses. Blown fuses can prevent any number of circuits from working correctly relating to your fuel injection or the ECU of your vehicle. There are several possibilities for a fuse that could go bad on you resulting in your car turning over but not starting.
Compression Problems
The combustion reaction in your vehicle works when the cylinders in your engine have a precisely managed compression ratio. If you're suffering from low pressure in the cylinders, air will be able to leak out past the piston rings and could potentially render that entire cylinder useless. If that's the case, the piston will not be able to turn the crankshaft, and your engine will not be able to have the power to function.
You can suffer compression issues because of problems with your timing belt or even a damaged camshaft. When an engine overheats it can also throw off the compression ratio as well. If you have a compression gauge tester you can test for yourself, but odds are you don't have one of these in your garage and you may have to head to a mechanic to troubleshoot this particular issue.
Cold Injector Problems
There are some vehicles that are equipped with something called a cold start injector. This is used specifically when your engine is cold and the normal fuel injectors will not be able to do the job. You have a temperature sensor in your engine that will determine whether or not the cold start injector needs to turn on to get your engine working. However, if that switch or a control module fails then the cold start injector may not get the signal that it needs to turn on and the result can be that your car is going to turn over but not start properly.
Security System Problems
Believe it or not but a problem with your car security system can actually cause your car to turn over but not start properly. That's because many security systems include an engine immobilizer that's part of the anti-theft system. If the security system itself is malfunctioning, then it may not recognize that you are the actual key holder attempting to start the car. If it believes the car is being stolen, then the engine immobilizer can come on or it could disable the fuel regulation system as well. Troubleshooting this issue will probably involve checking out either your vehicle manufacturer or the manufacturer of the security system if it's an after-market one.
EGR System Problems
A faulty EGR valve or exhaust gas recirculation valve can definitely have an effect on your car's ability to turn over but not start. The EGR valve in your vehicle recirculates gases from your exhaust through the intake manifold so that they can be burned again as a method of reducing pollutants from heading out of the exhaust as well as keeping the temperature of your engine down. Unfortunately, if this gets clogged or it malfunctions and ends up stuck in the on position it will not be able to redirect these gases the way it's supposed to.
Typically, you will experience problems with your engine stalling before the EGR valve fails on you completely so you can get some advance warning of any issues you may need to be on the lookout for.
Canister Vent Valve Problems
Inside your car is something called the evaporative emission control system, which most mechanics will refer to as an EVAP. The canister vent valve works as part of your evaporative emission control system that prevents harmful vapors from being released through the exhaust of your vehicle. The canister stores them so they can be redirected to the intake manifold and then back into the engine where they can be burned again. However, if the canister vent valve fails then it's not able to send those gases back to the engine which can throw off the fuel to air ratio and can potentially stop your engine from working properly.
Sensor Problems
There are a number of sensors in your vehicle that relay important data to your car's computer to allow it to work properly. If any of the sensors are malfunctioning, then you may end up having incorrect signals transmitted to your computer which in turn is preventing it from operating the way it's supposed to. Things like the crankshaft position sensor and a camshaft position sensor can relay incorrect information about where those parts are in your engine which in turn throws off your vehicle's ability to manage a proper combustion reaction. A bad throttle position sensor may prevent the spark from actually occurring in the cylinder so that you get no combustion at all.
The engine coolant temperature sensor, also known as the ECT sensor is the sensor that monitors the amount of fuel the engine needs and also when your engine is at proper operating temperature. If it fails, then ignition timing and cooling data will not be transmitted correctly which could cause a problem with your car turning over but not starting.
Vacuum Leak Problems
You could experience vacuum leaks in your engine in a number of different places if any particular valves or gaskets fail. This will cause air to leak out and will reduce the overall functionality of your engine up to the point that perhaps your car is going to turn over and not start properly as a result. Vacuum leaks can occur in the EGR valve, the intake manifold gasket, and various hoses and more.
There are some methods that you can use to figure out where you have a vacuum leak that can be done at home. The first thing you can do is actually listen when the car is running to see if you can pinpoint a hissing sound coming from somewhere in the engine. That's going to identify an air leak. If you have a general idea of what you're looking for, you can also use a cigar to help detect the precise location of the leak by removing one of the hoses and blowing smoke from the cigar inside. This video here shows you how to do it step-by-step if you are interested. It's a bit of a hack method of doing things, but it can get the job done and not cost you very much at all.
Carburetor Problems
This one is only relevant on older vehicles since newer vehicles don't actually have carburetors. Modern vehicles use a fuel injection system to add fuel to the combustion chamber, but this job was accomplished by carburetors in old vehicles. A flooded carburetor could be the cause of a car that cranks but won't turn over as it’s unable to properly supply fuel to the engine.
The Bottom Line
As you can see there are a host of issues that can cause your car to turn over but not actually start. Because there's such a wide range of problems that can occur, including some rare ones we didn't even touch on in this article, diagnosing the problem is not as easy as you might hope.
One of the best things that you can do if you're interested in figuring this out on your own is to buy an OBD2 scanner. The onboard diagnostic tool is the same kind of scanner a mechanic uses to diagnose problems in your vehicle. This will help you pinpoint where the problems are in your vehicle when something goes wrong and eliminate a lot of the guesswork in trying to figure out a problem like why your car is turning over but not starting. When you know exactly where you need to look to fix something, it can save a lot of aggravation.
Whatever the problem is, it's good to know where you need to look so you can get it fixed as soon as possible.