Spark plugs go unnoticed while everything is fine. But after a wet autumn or damp winter, condensation collects on them and in their wells, and worn plugs cause hard starting, hesitation under acceleration, higher fuel use and misfires. Ignoring these signs can lead to costlier repairs to coils or the catalytic converter.
A check takes about half an hour and needs no special skills: just the owner's manual, a plug socket and a clean cloth.
What you will need
Important: Start only with a cold engine: plugs and cylinder head are very hot after a drive, and aluminium threads are easily damaged on a warm engine.
Step 1
Locate the plugs and check the service interval
Use the owner's manual to find where the plugs are, what type they are and how often they should be replaced. If the mileage is close to the replacement interval, there is no point cleaning the old ones; fit new plugs straight away.
Remove the engine cover. If you have to disconnect hoses or wires, label them so they go back in the right place.
Step 2
Remove the coils and unscrew the plugs
Pull off each coil or plug cap holding it as close to the base as possible, straight out, without yanking the lead. Blow out or wipe the well so dirt does not fall into the cylinder.
Unscrew the plug with the plug socket. If it is very tight, do not force it: apply a little WD-40 Specialist Penetrant to the thread down the well, wait a few minutes and try again. Work on one plug at a time.
Step 3
Assess the plug's condition
A light brown or grey insulator means normal running. Dry black soot points to a rich mixture or lots of short trips, oily deposits to oil getting into the cylinder, and a white insulator to overheating. If in doubt, show the plug to a mechanic.
A cracked insulator, badly eroded electrode or damaged thread means the plug must be replaced. Check the gap with a feeler gauge against the manufacturer's figure. Iridium and platinum plugs should not be re-gapped or brushed.
Step 4
Clean and dry the plug
Wipe a healthy plug with microfibre to remove dirt from the body and insulator. Apply a thin coat of WD-40 Multi-Use to the outside and leave it for a few minutes. It displaces moisture and leaves an invisible protective film against corrosion.
Wipe off the excess so there are no runs on the insulator.
Step 5
Refit the plugs and clean the contacts
Screw each plug in by hand until it seats, then tighten with the socket to the torque in the manual. Spray the coil connectors with WD-40 Specialist Contact Cleaner: it removes oxidation and condensation and evaporates quickly.
Refit the coils in their places, replace the cover and start the engine. It should run smoothly with no misfire.
Tip: Lay the plugs on a cloth in cylinder order. If one looks different from the rest, the problem lies in that cylinder, which makes the mechanic's job much easier.
What next
Inspect the plugs once a year, ideally in autumn before the cold, and replace them at the scheduled interval. After deep puddles or an engine wash, dry the plug wells with WD-40 Multi-Use.
These uses come from the experience of WD-40 users in many countries. Always follow the instructions and warnings on the product label.
More guides in «How to use».