An engine is usually called blown when it has a major internal failure, such as a spun bearing, a broken connecting rod, a holed piston or a cracked block. Common signs are a loud, deep knock from the bottom of the engine, heavy smoke, metal particles in the oil, oil and coolant mixing, very low compression, or an engine that will not turn over at all. Several less serious problems can sound or look similar, so have a qualified shop confirm the diagnosis before you buy parts.
Common signs of a blown engine
- Loud knocking from the lower half of the engine that gets louder with rpm or load.
- Heavy smoke. Blue-gray smoke suggests oil burning; thick white smoke that does not clear after warm-up suggests coolant burning.
- Metal in the oil. Glittery particles on the dipstick, in drained oil or in a cut-open oil filter point to internal wear.
- Oil and coolant mixing, seen as milky oil or oil in the coolant reservoir.
- Low oil pressure warnings, especially together with new noises.
- Low or no compression on one or more cylinders.
- A seized engine that will not crank and cannot be turned by hand at the crankshaft bolt. A dead battery or failed starter can also cause a no-crank, so rule those out first.
- Visible damage, such as a hole in the block or oil pan after a connecting rod breaks.
Rod knock symptoms
Rod knock comes from excess clearance at a connecting-rod bearing, which lets the rod hammer against the crankshaft journal on every revolution. Common rod knock symptoms include:
- A deep, heavy, rhythmic knock from low in the engine
- A knock that speeds up with rpm and usually gets louder under load or when the throttle is blipped
- A knock that is still there, or louder, once the engine is warm and the oil has thinned
- Low oil pressure readings or a flickering oil light
- Metal particles in the oil or filter
Rod knock does not fix itself. A knocking rod can eventually break and punch through the block, which is why shops usually advise you to stop driving once a knock is confirmed.
Lifter tick vs rod knock: how can you tell the difference?
As general guidance, these are the usual differences. A shop should confirm which one you have.
| Trait | Lifter or valvetrain tick | Rod knock |
|---|---|---|
| Sound | Light, sharp ticking or tapping | Deep, heavy knock or thud |
| Where it seems to come from | Top of the engine, near the valve covers | Lower engine, toward the oil pan |
| Rhythm | Half crankshaft speed, because the valvetrain runs off the camshaft | Once per crankshaft revolution |
| Cold vs warm | Often loudest at a cold start and may quiet as oil pressure builds | Often present or louder once warm |
| Under load | Usually changes little | Usually louder under load or on throttle changes |
Other noises can confuse the picture. An exhaust manifold leak often ticks loudest when cold and quiets as the metal expands. Piston slap is typically a hollow knock on a cold engine that fades as it warms up. Fuel injectors on many engines tick as part of normal operation. On the 5.7 Hemi, ticking has its own list of common causes, covered in our Hemi tick guide. Shops often use a mechanic’s stethoscope to locate a noise, and may disable cylinders one at a time to see whether a knock changes.
Spun bearing symptoms
A spun bearing happens when a rod or main bearing shell loses its grip in its bore and turns with the crankshaft, usually after oil starvation, low oil pressure or overheating. Common spun bearing symptoms are:
- A sudden knock that was not there before, often after a low-oil event
- Low oil pressure
- Metallic flakes in the oil or filter, often copper-colored material from the bearing’s inner layers
- An engine that becomes very hard to turn or seizes
A spun bearing usually damages the crankshaft journal and the bearing bore, so the repair is at least a machine-shop job and often leads to a short block or complete engine.
How does a shop confirm a blown engine?
- Oil analysis, a lab report on wear metals and coolant in the oil
- Oil filter inspection for metal debris
- Oil pressure test with a mechanical gauge
- Compression and leak-down tests to find damaged cylinders, valves or gaskets
- Teardown, such as removing the oil pan or cylinder heads, the only way to see bearing and cylinder damage directly
Repair or replace a blown engine?
Most truly blown engines need major machine work or replacement. If the damage is in the bottom end, a short block replaces the block, crankshaft, rods and pistons, while a long block also includes the heads; see our short block vs long block guide. Our engine replacement cost guide covers what affects the total, and we carry remanufactured engines and long blocks. Find and fix the cause of the failure, such as an oil-supply or cooling problem, before a replacement goes in.
Related guides: spun rod bearings and seized engines · compression test · engine knocking · low oil pressure · how long engines last.
More guides: is it worth replacing an engine? · crate engine vs rebuild · seized engine · hydrolocked engine · cracked engine block · low compression in one cylinder · engine or transmission problem? · what comes with a long block.
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