Reading a plug is a skill, but the basics are simple: look at the electrode, the insulator and the colour. A plug that is black, wet or melted is telling you something specific.
Remove it safely
Let the engine cool. Clean around the plug before removing it so dirt does not fall into the cylinder. Use the correct socket and turn it by hand until it is seated.
On a deep head, use a rubber insert or a magnet to lift the plug. Do not force a cross-threaded plug.
Read the condition
A light tan or grey colour is normal in many engines. Black soot suggests a rich mixture or a weak spark; wet fuel suggests flooding or no ignition; white or blistered suggests overheating or a lean mixture.
Oil fouling suggests oil entering the combustion chamber. A melted electrode is a serious warning: stop and investigate.
Check the gap and wear
Use a feeler gauge or a wire gauge and set the gap to the specification. A worn electrode is rounded; a plug with a cracked insulator should be replaced regardless of gap.
Do not bend the centre electrode roughly. Use the correct tool and check the gap after adjustment.
Test for spark
Fit the plug to the cap and ground the shell against the engine. Crank and look for a strong, consistent spark. A weak or orange spark may be a plug, cap, coil or battery issue.
Do not hold the plug or the lead while cranking. High voltage can injure you.
Checklist
- Engine cool and plug area clean.
- Correct socket and thread care.
- Electrode wear and insulator condition.
- Gap set to specification.
- Colour and deposits read against the engine's history.
- Spark quality with the plug grounded.
- Plug torque according to the manual.
Questions
Can I clean and reuse a plug?
Sometimes, but a plug is inexpensive. If the electrode is worn, the insulator is cracked or the plug has been fouled repeatedly, replace it.
What does a black plug mean?
Usually a rich mixture, a weak spark or a lot of idling. It can also be a dirty air filter or a faulty sensor. Read it with the engine's behaviour.
Do I need a torque wrench?
Yes, if you have one. Over-tightening damages the head; under-tightening can cause a compression leak.
