Here's the basics on troubleshooting a car that won't start. I'll reference this in the context of it being a car that recently ran and suddenly won't start or run. Cars that have sat a long time, or that were bought not running, can follow the same process, but can also have deeper issues, like seized motors.
A lot of this is geared towards modern fuel injected cars with DIS (distributorless ignition systems), but I will try and cover all bases as best I can
The basic components of an engine that make it run are: Fuel, Air and Spark. Remove one of these and you get a dead motor. So let's look at some common problems and solutions.
- When you turn the key, you get nothing or get fast clicking sounds.
First check to make sure the battery cables are secure and not overly corroded. A loose positive cable will give you a no start. Especially if you were just driving and now it won't even click.
Next, make sure the battery is charged, or jump start the car with another car. If it starts after a jump, but won't start again after running for 20 minutes or more, it's time to replace the battery.
If you know that the battery is good, you might have a bad starter. It could just be stuck. Sometimes you can tap it with a hammer and get it to crank again.
- Car turns over, but won't start.
First, I like to listen for the fuel pump, as this is easy to do. Turn the key from the off to on position (not start) and you should hear the pump priming. The sound can vary in location and pitch, but you should hear something.
If you hear the fuel pump, or don't know what to listen for, the next step is to get a little fuel into the motor and crank it again. For modern fuel injected cars, you'll need to locate the throttle body (larger or performance vehicles may have more than one) and spray some starter fluid in. It's helpful to have 2 people for this, so one can spray while the other cranks.
For older carbureted cars, you can pour a little gas (doesn't take much!) or use starter fluid right into the carb. Pro tip: Brake cleaner is essentially the same as starter fluid and can be used here too.
If it starts with some fuel added, but quickly dies, then you probably have a clogged fuel filter or a bad fuel pump. If it continues to run, but doesn't start again after you shut it off, or dies at, say, a stoplight, then it's probably the pump. Sometimes the engine starting is enough to pull fuel through once it fires, but it won't last.
If it still won't start with fuel forced upon it, you may have a spark issue.
Start by making sure the connection to your coil pack or distributor is solid. Also look for any beaks or melted spots in the wire(s). If everything looks good, check for spark. To do this, pull one of the spark plug wires and stick a paper clip in the connection that normally mates up to the plug. Hold the wire by the insulation, so you don't get a shock (gloves may be a good idea too, if you're nervous), and hold the paper clip a few millimeters away from something grounded. And engine bolt is usually a good spot. Have some one crank the car and look for the spark to arc from the clip to the bolt.
If you have no spark, you might be looking at a new distributor or coil pack (as applicable).
- Car dies while driving.
If not, did you notice any warning lights come on, specifically the battery light? If so, your alternator is probably the problem. Sometimes they will linger and may only trip the battery light if you're using a lot of accessories, like the radio, headlights, heater/defroster and windshield wipers. Or you'll notice the lights dim at stop lights before it dies.
It is also possible that you have something else draining the system. I had a car with a short on the starter wire that acted like a bad alternator, once upon a time.
If the lights are still on and bright, you may have a bigger problem, like a broken timing belt.
Does it idle then die when you hit the gas? Or does it die pulling away from a stoplight? This is often a fuel pump issue.
- Cars runs, but not well.
If so, you probably have one or more bad spark plugs, spark plug wires or your coil pack (or distributor) is going bad or a bad/clogged fuel injector.
Check all the wires to the plugs, coil pack (or distributor) and fuel injectors (if applicable). Make sure they are tight and look for shorts of breaks.
If the wiring looks solid, next pull a plug wire, while the car is running. If it gets worse, that cylinder is good. Put it back and go to the next. When you pull one and there's no change, you've identified the problem cylinder. Now you can check the plugs, swap wires, etc. to try and determine if spark is the problem. If you swap plug wires to troubleshoot, don't just pull a wire from the plug end and switch it. You need to pull it from the other (coil pack, distributor) end as well, or the firing timing will be off!
If the wires and plugs seem good, try disconnecting the fuel injector (if applicable) for the problem cylinder(s).
Does it run worse when give it gas, or make a strange noise when you do so?
This can be the sign of a clogged air filter, or a catalytic converter going bad. Or someone has stuffed a sock in your tailpipe (it happens). Or maybe even a seized turbine in a turbocharged car.
Check the tailpipe first, since that's the easiest.
Next, remove the air filter. Don't leave it out long, especially in in dusty/dirty environments.
If these check out, you'll need to disconnect the exhaust upstream from the cat to determine if it is the problem.
Does it rev up and down, especially at idle? Or otherwise seem "off"?
This can be from a bad Idle Air Control (IAC) sensor, or the Throttle Position Sensor (TPS). Check the wiring to any sensors you can see, especially on/around the throttle body. Sometime removing the IAC and just cleaning it will fix the problem.
These few things should cover most no start issues you come across. I've dealt with all the above several times and most of them can be checked out with minimal tools. Replacement of things like fuel pumps, alternators, and even spark plugs, can vary greatly form car to car. Some are easy to locate and replace, others can be an all day project requiring specialty tools. Once you have identified the problem, check around the internet for forums for your particular vehicle, or pick up a repair manual at the local automotive store for tips, tricks and how-tos for specific repairs.