What is wrong if your engine cranks over but wont start?
When the engine cranks but won't start, the likely culprits are fuel delivery, ignition, or compression.
In most cases diagnosing a no-start involves checking whether the engine is receiving fuel and spark in the right amounts and timing, and whether the engine has adequate compression. Modern vehicles add potential issues from sensors and the immobilizer that can prevent starting even if everything else seems okay.
Quick checks: a first look at a cranking but not starting engine
The following quick-reference checks help you frame the problem before you start testing individual components. They can point you toward fuel, ignition, or timing issues without specialized tools.
- Fuel delivery: verify there is enough fuel in the tank, listen for the fuel pump priming when you turn the key to ON, and check for a clogged fuel filter or a faulty fuel pump.
- Ignition and spark: ensure the ignition system is delivering a spark to the cylinders (coil packs, distributor, plugs, or direct-inject spark systems). Look for worn or fouled spark plugs and damaged wiring.
- Air intake and vacuum: inspect the air filter and hoses for cracks or leaks that could cause a lean or unstable air-fuel mixture.
- Security/immobilizer: check for a security/immobilizer light on the dash; a faulty key or anti-theft system can prevent the engine from starting even if it cranks.
- Mechanical timing and compression: if the timing belt/chain has slipped or there are low compression issues, the engine may crank without starting.
These quick checks help identify whether the issue is likely fuel-related, ignition-related, or something more complex like timing or security systems.
Fuel system focus
If your quick checks suggest fuel delivery is the problem, the next steps typically involve confirming fuel pressure and injector operation, since fuel is essential for ignition and sustained running.
Deeper diagnostic steps: a systematic approach to a no-start
When quick checks don’t reveal a clear culprit, follow these structured diagnostic steps in order to pinpoint the fault. This path helps distinguish between electrical, mechanical, and control-system problems.
- Check for diagnostic trouble codes (DTCs) with an OBD-II scanner. Note any stored codes and live data from sensors that relate to fuel, ignition, or timing.
- Assess fuel delivery: measure rail pressure with a fuel pressure gauge and compare to the vehicle’s specification. If pressure is low or non-existent, inspect the fuel pump, relays, fuses, and wiring; consider a clogged filter or faulty pressure regulator.
- Verify ignition spark: perform a spark test using a proper tester or by grounding a spark plug from a known-good cylinder while cranking. Absence or weakness of spark points to ignition components or signaling sensors (e.g., crank or cam sensors).
- Evaluate air intake and sensors: inspect the air filter, mass airflow sensor (or equivalent), and throttle body for cleanliness. Look for vacuum leaks around intake manifolds and hoses, which can cause incorrect air-fuel readings.
- Test compression: use a compression gauge to check each cylinder for adequate compression. Low or uneven compression indicates mechanical issues such as worn rings, valves, or head gasket problems, or timing problems.
- Assess security and control systems: if the check engine light or immobilizer indicator is active, or if the engine refuses to crank or start after a battery disconnect, the vehicle may require dealer-level diagnostics or reset procedures.
Following these steps systematically often narrows the fault to a specific system or component. Some cases require professional testing equipment and expertise for accurate diagnosis.
Common fixes and when to call a professional
Once the fault area is identified, practical fixes range from replacing a faulty fuel pump or ignition coil to repairing timing components or replacing a bad sensor. Some issues, such as a mis-timed engine or a failed immobilizer, can require dealership-level software access or specialized tools. If you’re uncomfortable with testing the fuel system, electrical wiring, or engine timing, or if you don’t have a reliable code reader, it’s wise to seek professional help.
When to seek professional help
If the no-start condition persists after basic checks, or if you repeatedly see diagnostic codes pointing to critical sensors, the immobilizer, or timing components, consult a technician. Providing the codes you retrieved and describing the symptoms (whether the fuel pump primes, whether you hear a consistent spark, or if the dashboard lights behave unusually) will help the shop diagnose the issue faster.
Summary
A cranking engine that won’t start is usually rooted in one of three big areas: fuel delivery, ignition, or engine compression/timing. Start with quick checks for fuel presence, spark, air leaks, and security indicators, then move to a structured diagnostic sequence (codes, fuel pressure, spark, air, compression) to identify the faulty component. Modern vehicles can introduce complexity through sensors and immobilizers, so don’t overlook electronics and security systems. If in doubt, especially with uncertain safety or timing concerns, consult a professional.
