Cold start: what to listen for and look at

Why a cold engine tells you more

A warm engine hides problems: the oil has thinned, the metal has expanded and the tensioners are under pressure. Worn timing chains and tensioners, cam phasers, hard starting, injector faults and piston or bearing wear all show most on a cold start. That is why some sellers warm the car up before you arrive.

Before you touch the key

• Put a hand on the bonnet and on the engine: both should be cold. If they are warm, the car was run recently. Ask why, and consider coming back another day.
• With the engine off, check the oil level and colour on the dipstick and look at the coolant reservoir. Black, thick oil suggests missed oil changes. A milky film under the oil filler cap, or oil in the coolant, points to a head gasket leak.
• Look under the car for fresh puddles.

The start

• Turn the ignition on and let all the warning lights come up, then start. A petrol engine should start within a second or two, and a diesel once the glow-plug light goes out. A long crank, a sluggish starter or a car that needs two or three attempts points to the battery, the starter, the fuel system or something worse.
• The revs are higher for the first seconds and then settle down to a steady idle. Hunting up and down, stalling or a shaking idle are problems.

What the sound tells you

• A short rattle from the front of the engine in the first 2–5 seconds that then disappears can be a tired timing-chain tensioner or a cam phaser (variable valve timing). If it lasts or comes back at idle, the chain may be stretched. It is common on some engines and expensive to fix.
• A light ticking that fades as the engine warms up is often the valve train (lifters or rocker arms). If it does not fade, let a mechanic listen.
• A metallic knock that grows with the revs is serious: worn bearings. Walk away.
• A knock on a cold engine that goes away as it warms up, together with oil consumption, can mean worn pistons or cylinder walls.
• A squeal for a few seconds is usually a slack drive belt. A whine that rises with the revs can be the alternator, the power steering pump or a bearing.
• A diesel is louder than a petrol engine and clatters when cold. A loud, uneven knock or a rough idle, though, points to the injectors.

What the exhaust tells you

• A little thin white vapour that disappears quickly is condensation and is normal, especially in cool weather.
• Blue-grey smoke with a smell of burnt oil means the engine is burning oil: worn rings, valve seals or a turbo.
• Thick white smoke that does not disappear, often with a sweet smell, means coolant is getting into the cylinders: a head gasket.
• Black smoke means too much fuel or too little air: a faulty injector, a clogged air filter or a sensor fault.

Warning lights and the idle

• All the lights come on at ignition and go out after the start. A light that stays on (engine, oil pressure, battery, airbag, ABS) is a fault. A light that never comes on at ignition may have been disabled: ask why.
• Let the engine idle for a few minutes. The temperature gauge should climb steadily, the fan should cut in later and the engine should stay smooth. Vibration through the steering wheel, uneven revs, or a smell of coolant or burnt oil is a problem. With the engine running, look for bubbles in the coolant reservoir: they point to combustion gases in the coolant.
• Record 30 seconds of the engine on your phone, so you can listen again or play it to a mechanic.

Hybrids and electric cars

A hybrid may not run its petrol engine at first and starts it by itself when it needs it: ask the seller to let it run so that you can hear it cold. An electric car has no cold start; check the battery, the charging port and the charging instead. The rest of the checks are in How to check a used car before you buy it, and the drive itself in the used car test drive checklist.