In a petrol engine, everything starts with a spark lasting a few thousandths of a second. The plug that makes it also tells you, by its colour, how the engine is doing.
Its job
A petrol engine draws in a mix of air and fuel, compresses it, then ignites it at the right moment: the spark plug does that, dozens of times a second. How the whole engine works is in The combustion engine — how it all works.
A diesel engine has no spark plugs. Diesel ignites on its own from the heat of compression. Its "glow plugs" are small heaters that warm the chamber to help cold starts: a completely different object.
What's inside
- The terminal, at the top, where the lead or coil connects.
- The insulator, in white ceramic: it stops the current escaping before the tip.
- The metal shell, with its thread and hex: it screws into the cylinder head and makes the earth.
- The centre electrode and the ground electrode, separated by a small space, the gap, about 0.6 to 1.1 mm depending on the engine.
The ignition coil turns the battery's 12 volts into 20,000 to 40,000 volts. At that voltage, the current jumps the gap as a spark and ignites the mixture.
Materials decide how long it lasts: a standard plug with a nickel electrode lasts about 20,000 to 30,000 km; a platinum or iridium-tipped plug, 60,000 to 100,000 km.
The heat range says whether the plug sheds heat fast ("cold" plug, for high-performance engines) or slowly ("hot" plug). Always fit the one the manufacturer specifies: a plug that's too hot can melt a piston.
Reading a plug
Unscrewed, a plug shows how the engine burns.
| What the tip looks like | What it means |
|---|---|
| Light grey to tan | All is well |
| Black, dry, powdery (soot) | Mixture too rich, clogged air filter, weak spark, too much idling |
| Black, wet and oily | Oil is getting into the chamber: worn rings or seals |
| Wet with fuel | Flooded engine: too much fuel and no spark when starting |
| White, blistered, eroded electrodes | Overheating: mixture too lean or plug too hot. Fix it fast |
| Glassy yellow or green deposits | Additives or poor-quality fuel |
A mechanic's tips
- Checking the spark: unscrew the plug, reconnect it to its lead, rest its shell against a metal part of the engine, and crank the starter. A blue, crisp spark: ignition is fine. Yellow and weak, or nothing: coil, lead or plug. Hold the lead by its insulating boot, never the plug with a bare hand: the shock hurts.
- Flooded engine (smell of fuel, it coughs but won't catch): on a modern car, floor the throttle while cranking and the injection cuts fuel to clear the excess. On a mower or chainsaw, unscrew the plug, dry it, pull the starter several times with the plug out to blow out the fuel, refit.
- Tightening: by hand until it seats, then about half a turn with the wrench for a new plug with a gasket. Too tight and you strip the thread in the aluminium head; too loose and it overheats and works loose.
- Start the thread by hand for several turns so it doesn't cross-thread. A short piece of rubber hose pushed onto the plug helps guide it in straight at the bottom of a deep plug well.
- A spare plug and its wrench belong in the kit of every chainsaw, brush cutter or trail bike. On a two-stroke engine they foul much faster.
- The gap is adjusted by gently bending the ground electrode, and measured with a feeler gauge. In a pinch, a bank card is about 0.76 mm thick.
An engine that won't start is diagnosed step by step in An engine that won't start.