How to Stop a Car Engine Turning Over Without Firing
An engine that cranks but will not fire is one of the most frustrating faults, because the fault that is actually present is not usually the one people assume. The starter is turning, so the battery and the starter are working, and that eliminates a group of causes before the diagnosis even begins.
This guide covers what to check and in what order.
What Cranking Without Firing Rules Out
The starter is rotating the engine, which means the battery has delivered enough current for the starter to operate, and the starter itself is functional. That is a significant amount of information.
It makes a weak battery much less likely, though not impossible: a battery that can turn the starter may still not be able to sustain the voltage needed for the ignition and the fuel pump, and on a vehicle where the fuel pump is electric that matters. It makes a failed starter unlikely. It makes a mechanic's starting fault unlikely, because a starter fault produces a crank that does not rotate the engine.
What remains is fuel, ignition, compression, or a mixture problem, and those four are where the diagnosis starts.
The Order to Check
1. Fuel Level
The simplest check and the one that resolves more cases than people expect. A vehicle that has run dry or is close to dry will crank without firing, and it will continue to do so until fuel is added. On many vehicles the fuel pump is quieter or inaudible when the level is low, which is a quick indicator. If there is fuel, move on.
2. Is There a Spark?
This single test splits the problem in half and it takes five minutes. Remove a spark plug, connect it to the plug lead or hold it firmly against a grounded part of the engine, and crank the engine.
A spark means the ignition system is working and the problem is fuel or compression. No spark means the ignition system is at fault, and the possible causes are the plug itself, the plug leads, the coil, the coil pack, the crank or cam sensor, the ignition switch, the immobiliser, or the wiring between them.
Test across more than one plug, because a single plug can be faulty or a single lead can be damaged, and a vehicle that has three working cylinders and one not will still crank without firing.
3. Does the Fuel Pump Run?
On a vehicle with an electric pump, listen for it with the ignition on. On a vehicle with a mechanical pump, check whether the engine free-revs when the ignition is off, which tests it directly.
4. Is the Immobiliser Engaged?
An immobiliser that has not recognised the key, or one that has lost its power supply, will crank without firing and with no warning. Some vehicles show an immobiliser light. On some vehicles the anti-theft system can be triggered by a low battery, and on some a battery disconnection resets it and it needs the key programmed again. That is a genuine consideration on a vehicle that has recently had its battery disconnected or replaced.
5. Is There Fuel at the Injectors?
Where the pump runs and the spark is present, the fuel delivery to the injectors has to be confirmed. A fuel pressure test gives a direct reading, and it also confirms the regulator and the filter. Where the pressure is correct, the mixture problem is upstream or the injectors are not delivering.
6. Compression
An engine with no compression will crank but never fire. This is rare but it happens, usually with a seized or burnt valve, a broken chain or belt, or water damage. A compression test establishes it immediately.
The Fuel Side in Detail
A Blocked Fuel Filter
The most common cause. Where the filter is restricted, the pump cannot deliver the volume needed, particularly during the high demand of cranking and starting. A filter that is blocked on the supply line is a straightforward replacement.
A Failed Fuel Pump Relay
Where the relay has failed, the pump does not run at all, which is directly audible or measurable. Swapping the relay with an identical one is a two-minute test.
A Failed Fuel Pump
A pump that has failed, or one that is failing and cannot sustain pressure, produces exactly this symptom. A vehicle that starts when the pump is connected directly to a battery has a pump or a relay fault.
A Failed Pressure Regulator or Sensor
A regulator that cannot hold the correct pressure means the injectors do not receive the right amount of fuel. On a vehicle with a returnless system, the pressure sensor reporting incorrectly causes the control unit to reduce the pump output, which produces a cranking fault rather than a running one.
Air in the Fuel System
Where a hose has been disconnected, or where the vehicle has been standing for a long period, or where a recent fuel service was not properly primed, air in the system prevents the pump from drawing. The vehicle will crank without firing until the air is purged, which can take a number of attempts with the ignition on.
The Ignition Side in Detail
Fouled or Failed Spark Plugs
A plug that has fouled, is wet, or has failed will not spark. A plug that is very wet usually means fuel is present but is not being ignited, which points back to the fuel side and to a plug that is being contaminated repeatedly. A plug that is dry and clean points to the ignition supply rather than the plug itself.
A Failed Coil or Coil Pack
A coil that fails produces no spark at that cylinder. Where the vehicle has multiple coils, one failing is enough to stop the engine firing. On a vehicle with a coil pack, the whole pack fails as a unit more often than individual coils do.
A Failed Crank or Cam Sensor
Where the engine has no spark and the coil and the plug are proven good, the sensor that tells the control unit where the engine is has failed. That is covered in our guide to the crankshaft position sensor, and it is one of the more common causes on older vehicles.
Failed Plug Leads
Cheap, extremely common on older vehicles, and frequently overlooked because people assume the plugs are the problem. A lead that has a crack inside the insulation, or that has been connected to the wrong plug, produces no spark at that cylinder. Testing the leads with the plugs removed and the leads shaken identifies the intermittent case.
A Failed Ignition Switch
Where the switch is failing, the signal to the coils is interrupted. It may produce a crank without a spark, and it may do so intermittently. The wiring in the steering column is a common failure point on high-mileage vehicles.
Other Possibilities
A Flooded Engine After Repeated Cranking
Where the vehicle has been cranked many times, the intake and the cylinders fill with fuel. It will fire eventually, or after the battery is disconnected for a period to let the fuel evaporate, or after the plugs are removed and the engine is cranked to clear it. Where the fuel smells strongly of petrol from the airbox, that is what has happened.
A Failed Fuel Pump Fuse or Relay
Trivial to check and worth checking early. A blown fuse on the fuel system prevents the pump from running, and it is a two-minute check.
A Fuel Line That Has Collapsed
A flexible fuel line that has lost its shape, or a fuel line that is kinked, can collapse and prevent flow under suction. It is a known failure where a vehicle has been stationary under load.
Gearbox Oil in the Right Place
A vehicle that has had the wrong fluid put in, or a plug that has been fitted in the wrong filler, will crank without firing. It is not common but it is worth knowing.
How to Diagnose Efficiently
Start with the fuel level and the spark test, because those two between them halve the problem. Then branch: with a spark and no fuel pressure it is the fuel supply; with a spark and correct pressure it is the injectors or the mixture; with no spark it is the ignition and the sensor chain.
Read the codes as well, because a control unit that is not receiving a crank signal or an immobiliser signal will have recorded that, and the code identifies the circuit directly. The procedure is in how to read and clear a check engine light.
The Battery Point
One thing to be aware of on a modern vehicle: a battery that can turn the starter may not be able to hold enough voltage for the fuel pump and the ignition electronics to operate. Where the vehicle cranks slowly rather than normally, the battery may be the whole problem, and charging it and retesting will establish that. The tests are in why a car dies when you turn the key.
If this guide was helpful, you may also want to read why a car turns off in the middle of driving for the related running faults, five signs a fuel pump is failing for the fuel side, and why a car's engine light comes on with no symptom for what the codes mean without a driving symptom.
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