A petrol engine only starts when it has four things: fuel, air, a spark and compression. When it refuses, one of them is missing. Find which one, in order.
ACT
First, listen to what happens when you crank the engine. It's the fastest way to sort things out, and it takes no tools.
- Nothing, or a single sharp click. The starter doesn't turn: the problem is electrical. Very dim warning lights, or lights that go out: flat battery. Normal lights but a click: a loose or corroded terminal, a relay or the starter. Clean and tighten both battery terminals before anything else, it's the most common fault — see Repairing a cable, a plug, a contact.
- The engine turns, but slower and slower. A weak battery, often because of the cold. Let it rest for a minute between attempts, or jump-start it (see ADAPT).
- The engine turns at normal speed but won't catch. The battery and starter are fine. It's missing fuel, a spark or air: keep going.
- The engine spins faster and more easily than usual, with an even sound. Stop right away. It's the classic sign of a broken timing belt: on many engines, every new attempt can bend the valves.
Fuel.
- Check the real fuel level, not the gauge. A gauge can lie, especially after a vehicle has stood for a long time.
- Listen for the pump. On most fuel-injected engines, an electric pump hums for a second or two when you switch the ignition on. Total silence: fuse, relay or pump.
- Look at a spark plug. After a few attempts, unscrew one. Wet and smelling of petrol: fuel is getting through, so look at the spark, or the engine is flooded. Dry: fuel isn't getting through.
Spark.
- Test the ignition with a spark tester fitted between the lead and the plug, if you have one: it should flash clearly while the engine turns. No flash: coil, lead, sensor or ignition supply. Without a tester, a plug that's black, oily or has worn electrodes gets replaced before you look any further.
Air.
- Follow the air's path, from the filter inlet to the exhaust pipe. A waterlogged filter, a disconnected intake hose, or an exhaust blocked by mud, snow or a rag is enough to stop an engine from starting.
UNDERSTAND
A combustion engine burns a mix of air and fuel in a closed cylinder, and turns that push into rotation — the full workings are in The combustion engine — how it all works. To start, all four conditions have to be there at the same time: the right mixture, compressed, and ignited at the right moment.
The starter only gets that cycle going. As long as it turns at normal speed, the battery and starter are cleared, and the problem lies in one of the four conditions. That's why the order matters: you rule out what's simple and common first, and keep what needs taking apart for last.
A diesel is different. It has no spark plugs: the heat of strongly compressed air ignites the fuel. Its starting problems come from the cold (glow plugs), from air getting into the fuel system, or from diesel that has gelled in freezing weather and clogged the filter.
ADAPT
The engine is flooded. A strong smell of petrol, wet plugs. Wait ten to fifteen minutes for the excess to evaporate. On many fuel-injected cars, cranking with the accelerator pressed all the way down cuts the injection while starting and helps clear the flood.
It's a diesel. Wait for the glow-plug light to go out before cranking, and repeat two or three times in cold weather. After running dry, the air has to be pushed out of the system: many filters have a small hand priming pump to press until it feels hard.
It's very cold. A battery loses much of its power in deep cold, and the oil thickens. Bringing the battery indoors overnight is often enough for the morning start.
It's a small engine (lawnmower, generator, chainsaw). The same four conditions apply. The most common culprit is petrol left in the tank for months: it loses its most volatile parts and gums up the carburettor. Drain it and refill with fresh fuel.
You have a second battery. Connect the leads in this order: red to the + of the flat battery, red to the + of the good battery, black to the − of the good battery, and last, black to a bare metal part of the dead engine, away from its battery. Start the working vehicle, then the other one. Disconnect in reverse order.
You have none of that, and a manual gearbox. Push-start it: ignition on, second gear engaged, clutch pressed. At a jogging pace, release the clutch sharply, then press it again as soon as the engine catches. This doesn't work with an automatic.