What Would Make a Car Engine Turn But Not Start?by Derek Odom
If the engine is turning over but not starting, we can immediately eliminate the starter, the ignition switch, and all the wiring in between. We can also eliminate the battery as the cause, because if the battery has no juice left in it, the starter will not engage, and the engine will not turn over. Now that we know it isn't the battery, the ignition switch or the starter, let's start looking elsewhere on the vehicle for potential problems.
The fuel pump on older vehicles is in the engine compartment and is known as a "mechanical" fuel pump because there are no electric parts to it. A good way to test whether this is the problem is to disconnect the line from the fuel pump to the carburetor or throttle body and have someone crank the engine for a second. If no fuel spurts out of the line, the fuel pump could indeed be the culprit (assuming there is gas in the vehicle's tank). On newer-model vehicles, the fuel pump is located inside the fuel tank and is most likely an electric model. Turn the key to the "On" or "ACC" position and listen to determine if you can hear the pump engage for a few seconds. If it is working properly, you may notice a humming sound. If the vehicle is completely silent, the fuel pump may be broken or not getting power.
In order for an internal-combustion engine to start, there has to be a spark to ignite the fuel in the combustion chamber. The spark is sent by the coil and the distributor to the spark plugs. If the engine is getting fuel, check that the coil is good and that no wires to either it or the distributor have broken or come loose. This may require tracing the wiring all the way into the passenger compartment to check for continuity. Make sure the spark-plug wires are all in one piece and going to the correct cylinder as per the user's manual for the automobile. On some vehicles, you can check the coil by unhooking the plug wire going from it to the center of the distributor and holding it against either the body or the valve cover and turning the engine over for a few seconds. If there is a bright spark traveling from the end of the wire to the metal, the coil is good, and we can eliminate it as the issue. The distributor cap can be removed to inspect the rotor and contact terminals. If there are noticeable black spots or no metal at all, it's time to replace your cap and rotor.