TYPICAL TROUBLE
BROKEN LEAD: The rubber-coated high-tension (HT) lead that’s fitted onto the top of each spark plug can fracture, resulting in arcing and misfires. When releasing the HT lead or coil pack from the top of the spark plug, avoid pulling on the lead, which can increase the risk of damaging it.
MIXED UP: Always refit the HT leads in their original positions. Mixing them up will result in an incorrect firing order, loss of engine power and misfires. If the HT leads are not clearly labelled, or their lengths don’t indicate where they should be fitted, replace one spark plug at a time.
SEIZED SPARK PLUG: If a spark plug cannot be undone with a 3/8in ratchet, try to spray penetrating fluid around it and leave it to sink in. Avoid using a breaker bar as you may shear the plug and leave the remains inside the cylinderhead. Work the spark plug back and forth to release it.
DAMAGED THREAD: The thread into which a spark plug is wound may be damaged; this may prevent it from tightening, and cause a misfire or allow gases to escape past it. It may be possible for the thread to be helicoil repaired in situ without having to remove the cylinderhead. We had such a problem with CM’S MINI project – see the December 2015 issue.
RUBBER INSERT: Most spark plug sockets have a rubber insert, which helps to grip the spark plug and prevent it falling out when extracting or fitting it into the cylinderhead. The rubber insert can fall out, leaving it attached to the spark plug, which can be annoying when fitting a new one. If this happens, try using a short length of rubber hose to initially fit the spark plug, then finish off with the spark plug socket, but remove the rubber insert first.
HT LEAD OR COIL PACK: If you don’t have a spare length of rubber hose to begin threading a spark plug into the cylinderhead (as described in the previous step), then detach the HT lead or separate coil pack and use this to initially fit the spark plug. After a few gentle turns, use the correct socket, but first extract the rubber insert to ensure that it doesn’t remain attached to the spark plug.