A cracked engine block or cylinder head usually results from severe overheating, freezing coolant that was mixed too weak or missing, or occasionally a casting flaw. Symptoms overlap with a blown head gasket: coolant loss, overheating, milky oil, white exhaust smoke and low compression. A warped cylinder head, also caused by overheating, can often be machined flat; a crack is far more serious and usually means replacing the head or the engine.
Cracked engine block symptoms
- Coolant loss with no obvious leak, or coolant seeping from the block itself
- Overheating that returns after a head gasket repair
- Milky oil or oil in the coolant; see milky engine oil and oil in coolant
- White exhaust smoke and low compression in a cylinder
- Visible cracks or freeze-plug damage after a hard freeze
Cracked block or cylinder head?
| Clue | More likely |
|---|---|
| Coolant in one cylinder, problem near the combustion chamber | Head crack or head gasket |
| External coolant seep from the side of the block | Block crack |
| Damage after freezing | Either; block cracks often run along water jackets |
Pressure tests, dye penetrant or magnetic particle inspection at a machine shop confirm the location.
Warped cylinder head
Overheating can distort the head’s sealing surface, causing head gasket leaks. A machine shop checks flatness with a straightedge and feeler gauges and can resurface the head if the warp is within the limit for that engine. Aluminum heads on iron blocks are especially sensitive to overheating.
Can a cracked block be repaired?
Some external cracks can be stitched or welded by specialists, and sealers sometimes slow small internal leaks temporarily, but most cracks in the cylinders or water jackets mean a replacement block or engine. A sealer is a stopgap, not a cure.
Preventing cracks
- Keep the antifreeze mix correct for your climate.
- Fix cooling problems at the first sign; see engine overheating.
- Never pour cold water into an overheated engine.
How cracks are confirmed
Start with a cooling system pressure test and a combustion leak test, then pull the head if the evidence points inside the engine. Once the head is off, a machine shop can pressure-test it, check it for flatness, and inspect the block deck and cylinder walls. Overheating from a slow leak, a failed fan or a broken belt is the usual root cause, so fix that too, or a replacement engine can fail the same way.
Next steps
Confirm the crack before deciding. If the block is cracked, a replacement short block, long block or complete engine is usually the most practical fix; see short block vs long block, engine replacement cost and our engines.
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