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Starter spins but the engine won't crank

You press the button, the starter spins (sometimes with a flaring rev or a shrill metallic noise), but the engine isn't turned over and won't start. It's different from a starter that struggles or a weak battery: here, the starter spins freely. The most common mechanical cause is the starter clutch (sprag clutch / one-way clutch) slipping: this part no longer transmits rotation to the crankshaft. The starter pinion or its teeth may also be damaged (a grinding gear noise). More serious: if the timing has jumped (belt/chain) or the engine has lost all compression, the starter turns the crank but the engine can't run normally — these cases are for the professional. You tell them apart by the noise and the context. Don't keep hammering the starter: you risk worsening the wear on the starter clutch.

Possible causes: starter spins but the engine won't crank

These are the most common causes, from most to least likely. The guided diagnosis above helps you find yours, test by test.

  1. Slipping starter clutch

    Intermediate

    The starter clutch (sprag clutch, one-way clutch) transmits the starter's rotation to the crankshaft in one direction only, then disengages as soon as the engine runs. When its rollers or sprags wear and slip, the starter spins — often flaring up with a rattling or shrill 'whirr' — without turning the engine, especially when cold. It's a classic fault on many high-mileage engines. The starter clutch is generally located behind the alternator cover: replacing it is a substantial mechanical job but feasible for a well-equipped DIYer; otherwise, a workshop. Don't keep pressing the button, it wears it further.

  2. Starter pinion / damaged teeth

    Intermediate

    The starter turns the crankshaft via one or more gears (starter pinion, starter ring gear). If a tooth is broken, chipped or badly meshed, the starter spins producing a grinding or clattering gear noise, without cranking the engine firmly, sometimes intermittently depending on the crankshaft position. Checking it means removing the cover concerned to inspect the teeth and replace the damaged pinion. A medium mechanical job, within reach of a methodical DIYer on many models.

  3. Jumped timing (belt/chain)

    Expert

    If the timing chain or belt has jumped teeth or snapped, the starter does turn the crankshaft, but the valves no longer open at the right moment: the engine can't start, spinning 'freely' with sometimes abnormal compression. It's a serious case: on many engines (interference type), jumped timing bends the valves. Don't keep hammering the starter. The diagnosis (timing check, compression) and the repair are for the professional.

    Repair to be left to a professional.

  4. Loss of engine compression

    Expert

    If the engine has lost its compression (burnt or seized-open valves, dead rings, head gasket), the starter turns it without any combustion being possible: it spins 'too easily' and the engine won't catch. Suspect it when the cranking seems abnormally free and fast, with no compression pulse. A compression test with a gauge settles it. Collapsed compression means a professional engine diagnosis.

    Repair to be left to a professional.