If your engine cranks but won’t start, the problem is most likely a faulty fuel pump or a crankshaft position sensor. It is important to diagnose the problem before you start replacing parts. In this article, we will explain, step by step, the procedures to determine the cause of the problem using strategy-based diagnostics.
Why does an engine crank but won’t start
The most common problems that cause engine crank but no start are:
- Faulty fuel pump
- Faulty ignition module
- Faulty crankshaft sensor
- Faulty camshaft sensor
- No compression
- No spark
- Vehicle security has disabled engine start.
- The key does not belong to the vehicle.
Less common causes can cause the engine to crank but not start.
The following problems can also prevent engine start, but they are very rare.
- Faulty engine control module / ECM
- Failed timing belt or chain
- Blown head gasket
- Contaminated\bad fuel
Strategy Based Diagnostics
What is the strategy based on diagnostics? It is simply using one system to verify the function of one or more other systems. I will use a hypothetical scenario to illustrate how it works. A 2000 Ford Focus with a 2.0L 4cyl engine fails to start, but the starter engages, and the engineer sounds normal when cranking.
Opening the air cleaner housing and supplying alternate fuel in the form of non-chlorinated brake parts cleaner from an aerosol can. ( Starting fluid could also be used, but I prefer brake cleaner or carburetor cleaner because they are much less explosive, therefore, safer.)
The engine starts and will continue to run as long as the alternate fuel is supplied. Now, that we know that the fuel system is causing the no start, we have used the fuel system to verify that the ignition system is functional and the engine compression is sufficient to run.
So, we performed an ignition spark test and a mechanical compression check by spraying brake cleaner in the air filter. In a nutshell, that is strategy-based diagnostics.
Now we know it is a lack of fuel, but why?
Two subsystems make up the fuel delivery system; supply and control.
- The supply consists of the fuel, the fuel tank, the fuel pump, and all hoses.
- Fuel control determines how much fuel enters the engine.
In this scenario, the engine is port fuel injected. This means that there is an individual injector for each cylinder. The injectors are controlled electronically by the engine control module(ECM).
The ECM works with the fuel rail pressure sensor to mathematically determine how much fuel is entering the engine by controlling each injector’s “on-time.” Theoretically, if the fuel pressure is below specifications, the ECM memory should have a diagnostic trouble code.
Strategy Meets Reality The Technical Tap!
What is a technical tap test? Please take note of the word “tap,” not “hit” or “beat.” It must be replaced if you “tap” on something and it starts or stops working. You will need another person to assist you in this step. Locate the bottom of the fuel tank and have your assistant turn the key to the on position while you listen carefully near the bottom of the fuel tank.
There should be a hum as the fuel pump inside the tank runs for 2 seconds as soon as the key is turned on. If you hear the pump, you have verified there is the power to the fuel pump. If you do not, have your assistant attempt to start the engine as you “tap” the fuel tank’s bottom with a mallet. If the engine starts, chances are, it needs a fuel pump.
What if the fuel pump is running and the engine doesn’t start? What next?
With the key in the ignition, the anti-theft light on the dash should illuminate for approximately 8 seconds, then go out. If it stays on, the ECM disables the injectors, and you will have to seek a professional’s help. Mainly because of the tools required to diagnose this system.
A logic probe, a test light, or a digital volt ohmmeter capable of measuring frequency is required to check the signal to the injectors. The injection signal will be the next step if you have access to any of these checks. Otherwise, you may need to seek the help of a professional.
Ignition system/no spark
If alternate fuel is supplied and the engine still fails to start suspecting a no spark condition
- Crankshaft sensor
- Ignition control module
The vehicle in question has a coil pack to which all 4 of the spark plug wires go. Remove one wire at the spark plug end and plug a spark plug into the end of it. Lay the spark plug on top of the engine and have an assistant attempt to start the engine. Watch for the spark to jump the gap of the spark plug. (USE EXTREME CAUTION) The ignition system can produce 40k volts, so do not touch anything on the car when cranking the engine.) If a spark is seen, then the problem is elsewhere. If no spark is seen, it needs a crankshaft position sensor.
- Restricted exhaust
- Plugged fuel filter
These two things would have been preceded by a poor driveability condition that would have steadily worsened until the engine would no longer start. If no exhaust can get out, then no fuel can get in. By the same token, if no fuel goes in, there is no exhaust to go out. If it can’t poop, it can’t eat, and vice versa!
We hope you find the Engine Cranks But Won’t Start helpful. Check these troubleshooting and repair guides for more help on your vehicle.
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