Engine Cranks but Won't Start — Fuel, Spark, or Compression?

The engine is cranking — the starter works, the battery is good, the engine is spinning. But it will not fire. This means one of the three requirements for combustion is missing: fuel, spark, or compression. Your job is to figure out which one and then find out why.

Do not guess. Do not start replacing parts. A systematic check of all three takes less time than replacing a fuel pump that was never the problem.

The Big Three: Fuel, Spark, Compression

Every internal combustion engine needs these three things to run:

1. Fuel — the correct amount of fuel must be delivered to the combustion chamber and atomized properly
2. Spark — the ignition system must produce a spark at the correct time to ignite the air-fuel mixture
3. Compression — the cylinder must be sealed well enough to compress the mixture so it ignites efficiently

If all three are present but the engine still will not start, timing is the fourth check — fuel and spark may be present but delivered at the wrong time (jumped timing chain, slipped timing belt, incorrect CKP/CMP correlation).

Step 1: Check for Codes First

Before you pull a single plug or test a single wire, connect a scan tool and check for DTCs. A crank-no-start often sets codes that point you directly to the cause:

• P0335 / P0340: CKP or CMP sensor — no signal means the PCM cannot fire the coils or injectors
• P0230: Fuel pump primary circuit — no fuel pump operation
• P0016: CKP/CMP correlation — timing is off (jumped chain or belt)
• Security / immobilizer codes: The PCM is disabling fuel injector pulse as an anti-theft measure
• No codes at all: That is diagnostic information too — it narrows the search

Step 2: Check for Spark

Pull one spark plug. Reconnect it to its coil or plug wire. Ground the plug threads against the engine block (use a plug boot holder or clamp — do not hold it with your bare hand). Have someone crank the engine while you watch for a spark.

• Strong blue spark: Ignition is working on this cylinder. Spark is probably not the problem (unless only some cylinders have spark — check all if you suspect partial ignition failure).
• No spark or weak orange spark: Ignition system failure. Check:
  • CKP sensor signal — if the PCM does not see the engine spinning, it will not fire the coils
  • Coil power supply — verify 12V at the coil power feed with the key on
  • Coil driver signal — the PCM must ground the coil driver circuit to fire the coil. Check with a noid light or scope
  • Coil itself — swap with a known-good coil from another cylinder

An inline spark tester is safer and faster than grounding a plug — it provides a consistent gap and visual confirmation.

Step 3: Check for Fuel

Fuel Pump Prime

Turn the key to ON (not start). Listen for a 1-2 second hum from the rear of the vehicle — that is the fuel pump priming the system. No hum = no pump operation. Check the fuel pump fuse, relay, inertia switch (Ford), and pump power/ground circuit.

You can bypass the fuel pump relay to send direct power to the pump. If it runs with the bypass, the relay or its control circuit is the problem.

Fuel Pressure

Connect a fuel pressure gauge to the test port on the fuel rail (if equipped) or use a scan tool to read fuel rail pressure on vehicles with a fuel pressure sensor. Specifications vary by vehicle — typically 35-65 PSI for port injection, 2,000-3,000+ PSI for GDI high-pressure rail.

• No pressure: Pump is not running or pump has failed internally. Check electrical first.
• Low pressure: Weak pump, clogged filter, leaking fuel pressure regulator, or leaking injector. On GDI vehicles, distinguish between low-pressure pump (in-tank, supplies ~60 PSI) and high-pressure pump (mechanical, cam-driven, supplies rail pressure).
• Good pressure: Fuel delivery is not the problem. Move to injector operation — are the injectors being pulsed by the PCM? Use a noid light on the injector connector. Flashing noid light = PCM is commanding injection. No flash = PCM is not pulsing injectors (check CKP signal, security system, PCM power/ground).

Flooded Engine

If you smell raw fuel, the engine may be flooded — too much fuel, not enough spark. Hold the throttle wide open and crank for 10 seconds (clear flood mode — PCM cuts fuel or reduces it significantly with the throttle open). If it starts, something was delivering excessive fuel — leaking injector, stuck-open fuel pressure regulator, or the engine was cranked repeatedly without starting and fuel accumulated.

Step 4: Check Compression

If you have spark and fuel but no start, check compression. A relative compression test with a PicoScope takes 30 seconds and tells you if all cylinders are relatively equal. If one or more are significantly low, perform a traditional compression test on those cylinders.

Low compression across all cylinders on a crank-no-start usually means:

• Jumped timing belt or chain: Valve timing is off — the valves are open when they should be closed, and the cylinder cannot build compression. Check cam timing marks.
• Broken timing belt: On interference engines, this also causes valve damage. On non-interference engines, the engine simply will not start but is not damaged.

Low compression on one or two cylinders usually will not prevent starting — it causes a rough idle or misfire, not a complete no-start.

Step 5: Check Timing

If fuel, spark, and compression are all present but the engine will not start, timing may be off. The fuel and spark are being delivered at the wrong point in the engine cycle.

• Visual timing check: Remove the timing cover or use an inspection window to verify the timing marks on the camshaft and crankshaft sprockets are aligned. If they are off by one or more teeth, the chain or belt has jumped.
• Scope check: A PicoScope CKP/CMP correlation waveform shows the relationship between crankshaft and camshaft signals. If the cam signal is shifted relative to the crank signal, timing has jumped.
• P0016 / P0017 codes: These directly indicate CKP/CMP correlation is out of range — timing has shifted.

Step 6: Security System

Some immobilizer systems allow the engine to crank but disable fuel injection. The engine spins but never fires because no fuel is being delivered — even though the pump runs and pressurizes the rail.

Check for a security light on the dash. Check for immobilizer DTCs. If the PCM is not pulsing injectors (noid light does not flash) and there are no ignition or CKP faults, the anti-theft system is a strong suspect.

Common Crank-No-Start Patterns by Symptom

• Cranks normally, no attempt to fire, no codes: Check CKP sensor — if the PCM does not see the engine spinning, it will not fire coils or injectors, and it may not set a code if the signal is completely absent.
• Cranks and almost catches, then dies: Fuel delivery problem — low pressure, failing pump, or clogged filter. The engine gets enough fuel for a partial fire but cannot sustain it.
• Starts momentarily then stalls: Anti-theft system allowing a brief start then cutting fuel. Or a failed MAF/MAP that causes an immediate stall after start.
• Cranks normally, strong fuel smell: Flooded. Ignition system failure — coils not firing. Or leaking injector flooding the engine.
• Cranks faster than normal: Loss of compression — jumped timing, broken belt. The engine spins faster because there is no compression resistance. This is an audible clue if you know what normal cranking speed sounds like.