A hydrolocked engine has liquid, usually water, in one or more cylinders. Because liquid cannot be compressed, the piston cannot complete its stroke, so the engine stops abruptly or refuses to turn over. Hydrolock most often happens when driving through deep water with an intake that sits low, but coolant from a failed head gasket or fuel from a leaking injector can cause it too. Never keep cranking a suspected hydrolocked engine: the force can bend connecting rods.
Hydrolocked engine symptoms
- The engine stalls suddenly while driving through water
- The starter clunks or the engine will not turn over at all
- The engine turns slowly and stops at the same point
- After a restart: knocking, rough running, low compression or smoke from a damaged cylinder
What causes hydrolock?
| Cause | Notes |
|---|---|
| Driving through deep water | The most common cause; low-mounted cold air intakes raise the risk; see what does a cold air intake do? |
| Coolant leak into a cylinder | Head gasket or cracked head; see signs of a blown head gasket |
| Leaking fuel injector | Fuel fills a cylinder while the engine sits |
| Boats and marine engines | Water entering through the exhaust or intake |
What to do if you suspect hydrolock
- Do not try to restart the engine.
- Have the vehicle towed, not driven.
- Remove the spark plugs (or injectors on a diesel) and turn the engine slowly by hand with a wrench on the crankshaft bolt to push the liquid out.
- Check the air filter and intake for water, and the oil for contamination; change the oil and filter.
- Run a compression test before trusting the engine again.
Can a hydrolocked engine be fixed?
If the engine stopped gently and nothing bent, draining the water, changing the oil and drying the intake can be enough. If a connecting rod bent, that cylinder will show low compression and the engine may knock; a bent rod means a teardown or a replacement engine, and a rod that later breaks can destroy the block; see how to tell if an engine is blown.
Hydrolocked engine sound
When cranked, a hydrolocked engine often makes a single clunk and stops, or the starter strains without turning the engine. That differs from the rapid clicking of a weak battery; see car won’t start.
Repair or replace?
When internal damage is confirmed, a replacement engine is often the faster and more predictable fix; see engine replacement cost, is it worth replacing an engine? and our engines, listed by engine code.
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