What Causes Car Engine To Fail? Top 5 causes

Car engine failure is a horrible feeling, coasting to the side of the road, your heart sinks. The last time it happened to me I was almost home, just coasting to a stop sign and “CRUNCH” – Timing belt snapped.

What causes a car engine to fail? Car engines commonly fail from one of the five following reasons:

  1. Lack of oil
  2. Engine overheats
  3. Hydro-locked
  4. Wear and Tear
  5. Timing Assembly Fault

In this post you’ll learn why most engines fail, you’ll also learn what the symptoms they display and how to diagnose.

1 Lack Of Oil

oil dip stick

Oil is no 1 cause of engine failure for good reason. Lack of oil is the fastest way to kill an engine guaranteed. The oil forms a protective film between metal components, fresh oil causes them to glide across the surface. Oil doesn’t just reduce heat by reducing friction, it also moves heat away from the engine, often to an oil cooler where the oil is cooled and returned to the heart of the motor to repeat the process.

Less oil therefore means lower oil pressure and less heat removed from the motor. Less oil also means less glide and more contact. As temperature rise, components start to expand and become tight, which causes even more heat, you can see where this is going.


Eventually the metal fuses together, just like you welded it, and your engine is now dead. Repairing this type damage likely isn’t worth the labour and parts and besides the other components have been exposed to metal fillings which would likely cause other problems down the road.

Bad oil is almost as big a killer as low oil. Fresh oil contains cleaning agents which help neutralise fuel and carbon deposits. As the oil ages the oil becomes thin and the cleaning agents are overcome by the contaminates.

The engine oil turns acidic and your engine is bathing in it. Acidic oil will not only not offer protection, it will also eat at metal, gaskets and rubber seals.


This one is usually pretty simple, just dip it for oil, check the level and quantity. A faulty oil filter is rare but could starve the engine of oil even though the engine oil is full.

Oil filter

2 Engine Overheating

The over heating engine isn’t as common as once it was but it still does a ton of damage when it happens. Blown head-gasket if your lucky but warped cylinder head is common too.

Coolant system info graphic

Coolant as you know makes its way around your engine in a circuit, removing heat from the cylinders and cylinder head. The system is pressurised as a pressurised system and so must be sealed. Overheating commonly happens from something as simple as a bad rad cap seal. Hard to believe a 10 dollar item could cause so much damage.

Here’s a list of common coolant system component failures that cause the engine to overheat:

  • Low coolant level
  • Leaking coolant reservoir cap
  • Stuck closed thermostat
  • Blocked Rad
  • Faulty fan
  • Faulty water pump
  • Loose/broken water-pump drive belt

Cost of replacing the head-gasket will be counted in thousands and if the cylinder head is badly warped or cracked a new engine will likely be a cheaper solution.


A blow head-gasket usually blows from the combustion chamber into the coolant passage. A test kit sourced from a auto parts store will check for the presents of exhausts gases in the coolant system. As it’s not the only place the gasket can leak, a leak-down tester and a coolant system pressure tester is conclusive.

Head gasket failure chart

3 Hydro-locked Engine

A hydro-locked engine may not be common in the warmer climates but it your region get above average rainfall you’ll likely be familiar with stories of cars driving through puddles and suddenly stopping. Hydro-locking is an engine killer I’ve seen it a ton of times. Don’t underestimate the strength of water.

So what is hydro-locking?

Hydro-locking commonly happens during the rainy season when there’s a lot of standing water around. Driving through deep puddles causes the engine to suck up and ingest water. As water fills the cylinder and isn’t compressible, the piston is simply stopped in it tracks like it hit a brick wall. The piston energy needs and outlet so it often bends or breaks a con-rod (con-rod connects crank to piston).

I’ve worked on lots of cars that started after they were dried out only to find a slight miss in the engine. The miss is caused by the piston being a few mm lower than the others – con-rod slightly bent. This type damage can be repaired but it’s expensive.


A compression test will show lower readings on the effected cylinders. It’s usually only one cylinder or possibly two.

Compression test infographic

4 Wear & Tear

Engine wear and tear causes lots of problems with various symptoms. Common faults and symptoms include:

  • Worn rings with high oil consumption and lots of smoke.
  • Excessive crankshaft bearing clearance which causes deep hammering noise.
  • Excessive camshaft wear causing top end symphony of ticking noises.
  • Lots of oil leaks.
  • Worn valves causing misfiring.

Modern engines aren’t rebuilt anymore, instead a salvage yard is going to be a cheaper solution.


Compression test infographic
Wet test

Run a compression test followed by a wet compression test.

5 Timing Assembly Fault

Engine stripping

Engines are designed in two halves (ends), the bottom end containing the crankshaft and the top end containing the valve train. Both ends must be timed very precisely, otherwise they collide and it’s like a horror story.

As the pistons and the valves of most engines occupy the same space at different times, it’s critical they’re kept apart (Interference engines). Two methods are common, a toothed timing belt or timing chain stretched around a toothed pulley at the crank and the camshaft.

Both require maintenance but belt driven is a ton more likely to break if neglected. Typically timing belts are replaced at about 90k miles. Accessing the belt is moderately labour intensive and so other wearing components are changed too – water-pump, coolant and tensioners.

Valve damage

You won’t be surprised to know timing belts are often over looked, which is decision many regret.

Timing chains are more durable and less likely to break, in addition a worn chain will protest loudly long before grenading.

Damage caused by a broken belt varies. Not all engines are interference and so non interference engine will likely just require a new belt. However interference engines don’t fare out so good. The valves meet the pistons which causes the valves to bend and sometimes destroys the cylinder head and pistons. A repair is possible but will be expensive.


Remove a top cover and check the belt is in place. Try turning the engine over by hand if possible using a ratchet and socket on the crankshaft.

Less Common Engine Failure Causes

An engine could fail for many reasons, heavily modified engines fail all the time, they basically operating outside their tolerances. Human error is another viable reason for engine failure, repair work poorly executed or loose fasteners ingested by the engine is a common mechanic fail.

I’ve seen ton of them fail for lack of oil or coolant only to discover the underlying issue was something quite simple had the driver listened to the warning signs.

So, here’s a few of the not so common underlying causes.

  • Lack of oil – Faulty turbo charger oil seal starving the oil system.
  • Lack of oil – Faulty blocked oil filter.
  • Engine overheating – Weak anti freeze allowed engine to freeze and crack head.
  • Pre-ignition (fuel mixture igniting early) – Faulty knock sensor, EGR valve, Carbon build up.
  • Detonation (Erratic fuel burn)- Faulty knock sensor, EGR valve, Coolant system fault.

Related Questions

How do I know if my bottom end is bad? Symptoms of a bad bottom end include:

  • Deep hammering sound from the bottom end
  • Radial movement of the harmonic balancer
  • Radial movement of the flywheel/flex-plate
  • Oil leaks at the front and rear crank seals

John Cunningham

John Cunningham is an Automotive Technician and writer on Rustyautos.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of classic car ownership, from tires to roof aerials and everything in between.

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