Engine Cranks But Won’t Start
If your engine cranks but refuses to start, the problem is likely a faulty fuel pump or crankshaft position sensor. It is important to determine the proper cause 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 the engine to crank but not start are:
- Faulty fuel pump
- Defective ignition module
- Malfunctioning crankshaft sensor
- Dirty or damaged camshaft sensor
- No compression
- No spark
- Vehicle security has disabled engine start.
- The key does not belong to the vehicle.
A faulty engine control module (ECM), blown head gasket, or bad fuel can also cause the problem, but these problems are very rare.
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Strategy Based Diagnostics
Strategy-based diagnostics is the process of using one system to verify the functionality of other systems. For example, A 2000 Ford Focus with a 2.0L 4cyl engine fails to start, but the starter engages, and the engine sounds normal when cranking.
The first step would be opening the air cleaner housing and supplying alternate fuel, such as non-chlorinated brake parts cleaner from an aerosol can. (Starting fluid could also be used, but brake or carburetor cleaner are not as flammable and are, therefore, safer.)
If the engine starts and continues to run with the alternate fuel supplied, we now know that the fuel system is preventing the engine from starting. The fuel system was used to verify that the ignition system was functional and that the engine compression was sufficient to run.
The next step would be performing 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 enters the engine by controlling the timing of each injector. 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,” instead of “hit” or “beat.” If you “tap” on a component and it starts or stops working, it must be replaced. 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 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? The fuel pump was, in fact, not the problem, and you should try some other troubleshooting strategies.
Anti-Theft/Immobilizer System
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, it means the ECM has disabled the injectors, and you will have to seek help from your dealership or trusted mechanic, mainly because of the tools required to diagnose this system.
Injection Signal
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 can access any of these checks. Otherwise, you may need to seek help from a professional.
Ignition system/no spark
If alternate fuel is supplied and the engine fails to start, it could indicate no spark when the ignition is turned to the on position.
Common causes
- 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 new 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. Look for a 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. A new crankshaft position sensor might be needed if no spark is seen.
Other Causes
- Restricted exhaust
- Plugged fuel filter
These two things would have been preceded by a poor driveability condition that steadily worsened until the engine no longer started. If no exhaust can get out, then no fuel can get in.
We hope you find the Engine Cranks But Won’t Start helpful. Check these troubleshooting and repair guides for more help on your vehicle.
