Many times in the past as an automotive technician, I would be given a car to diagnose that either had a problem that prevented the car’s engine from turning over or that prevented the car’s engine from starting. These are two completely different scenarios and these problems are approached differently. Sometimes when the service writer would create a work order, he wouldn’t convey the REAL problem that the customer was experiencing. If the customer or the service writer would make sure the correct explanation of the problem is written, some unnecessary trips out to the car with the wrong tools would be avoided. For instance if the car doesn’t crank over at all, I would need to take my jump box with me. And if the car cranked over but didn’t start, I would want my scan tool and some starting fluid.
A car that cranks over but doesn’t start, in many cases will be due to a bad fuel pump, a broken timing belt, an inoperative cam sensor or crank sensor. The first step in diagnosing a no start is to scan for codes with a code reader or scan tool. Approximately 40% of the cars towed in that I worked on in the last 25 years, either had a bad fuel pump or the fuel gage read incorrectly and the car was just out of fuel. Yes, some people actually have their cars towed in when all they need is some gas! If the fuel gage indicates that there is fuel in the tank, most people will assume naturally that the car does have fuel. But if the fuel sending unit or gage is off slightly, the car’s gas tank could actually be empty. One of the easiest ways to check for a fuel pump related problem is to spray a little starting fluid into the air intake while trying to start the car. If the car starts momentarily, then there’s a good chance the fuel pump is not working or the car is out of gas. You can also listen for the fuel pump’s humming sound when the key is first turned on. Also if a inoperative fuel pump is suspected, a good technician will always check to verify there is power and ground being supplied to the pump, to eliminate the possibility of a bad relay or electrical issue. Fuel pressure can be checked with a pressure gage and if there is pressure, pulse can be checked at the fuel injectors in several ways including the most common, with the use of noid lights. If the injectors don’t have pulse the engine won’t start.
Broken timing belts also caused about 40% of the tow ins in my experience. A broken timing belt can fail causing the car to seem as if it ran out of fuel while driving. When trying to start the car though, the engine typically spins over faster than normal due to the engine having low compression from the timing being off and the valves being open at the wrong time. If the timing belt breaks, there sometimes could be a code set in the vehicles ECU (Electronic Control Unit) for no signal from the cam sensor. If this code is present, you can sometimes check to see if the cam is moving by removing the oil cap while someone engages the starter. Also the upper timing cover can be removed to allow access to the timing belt for inspection. Most of the time when a timing belt breaks the teeth (or cogs) are stripped down at the crank pulley the bottom sprocket preventing the belt from turning the camshaft.
Lack of pulse from the fuel injectors as stated before can prevent a car from running. This brings me to the third most common cause of a car that cranks over but doesn’t start, approximately 20% of the tow ins that I worked on had the following problem. Many times the cause of lack of pulse is because of a bad cam or crank sensor. If the computer doesn’t see either the crank or the cam turning, the fuel injectors won’t be commanded to pulse. One quick indication that a cam or crank sensor may be bad is to check for RPM movement on the tachometer (if equipped) when trying to start the engine. If you have a scan tool not just a code reader, the cam and crank sensor data can be checked on many vehicles by accessing the appropriate screen. If no revolutions are shown when attempting to start the car, the respective sensor could be bad.
Obviously there are many vehicle makes, with different ignition and fuel delivery systems that may require a service manual to properly diagnose or to delve deeper. But with the above tips and tricks that I’ve used over the years you’ll be checking the most likely causes, that hopefully will send you in the right direction.