A seized engine will not turn: the crankshaft is locked, so the starter cannot crank it and it cannot be turned by hand. The most common causes are oil starvation that welds a bearing to the crankshaft, severe overheating that makes the pistons seize in their bores, hydrolock, and rust inside an engine that has sat unused for a long time. A seized engine is different from a bad starter, which can make a healthy engine appear stuck.
Seized engine symptoms
- A loud knock while driving, then the engine stops suddenly
- The starter clunks or hums but the engine does not rotate
- The crankshaft will not turn with a wrench on the crank bolt
- Low or no oil on the dipstick, or metal in the oil
Seized engine or bad starter?
| Test | Seized engine | Bad starter |
|---|---|---|
| Turn the crank bolt with a breaker bar (plugs out) | Will not turn | Turns normally |
| Starter sound | Single clunk or strain | Click, whir or nothing |
| History | Knocking, oil loss or overheating before it stopped | Engine ran fine until a no-crank |
See car won’t start for battery and starter checks.
Common causes
- Oil starvation: low oil, a failed pump or a blocked pickup; bearings overheat and seize; see spun rod bearings and oil pressure light.
- Overheating: pistons expand and scuff the cylinders; see engine overheating.
- Hydrolock: water or coolant in a cylinder; see hydrolocked engine.
- Long storage: rust between rings and cylinder walls.
Can a seized engine be fixed?
An engine seized by rust from storage can sometimes be freed with penetrating oil in the cylinders and patient turning, followed by a careful inspection. An engine that seized while running almost always has bearing, crankshaft or piston damage, which means a rebuild with machining or a replacement.
Freeing an engine that sat for years
For a stored engine, remove the spark plugs, add a little penetrating oil to each cylinder, and let it soak for a day or more before trying to rock the crankshaft gently back and forth by hand. Never use the starter to force it. Once it turns, change the oil, inspect the cylinders with a borescope if possible and run a compression test before starting it.
Seized engine sound
Before seizing, a failing engine usually knocks louder and louder; see engine knocking. When it seizes, it stops abruptly, often with a bang or grinding noise, and the starter afterward only clunks.
Repair or replace?
For most seized engines, replacement is the practical fix. Compare costs in engine replacement cost, decide with is it worth replacing an engine?, and browse our engines by engine code, including short and long blocks.
More guides: car won’t start, just clicks.
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