The Lincoln Mark Viii spark plug has one of the toughest, most crucial jobs in keeping the engine alive and working properly. It produces a hot spark inside the combustion chamber to light up the fuel and air mixture into flames. However, once it starts to wear out, you will experience a sudden drop in engine performance. Ignoring this problem will put a stress on the other components, which will lead to engine failure. If you sense that your vehicle is not working as well as it used to, immediately check the spark plug for any problems such as:
"Shunting" is a condition in which a short circuit occurs from lowered shunt resistance. This usually happens when there's a build-up of metallic deposits on the electrodes of an old spark plug. To check the component for any metallic deposits, remove it from the engine using a plug wrench or a ratchet. Then, look closely at the side and central electrodes for any band of materials forming.
When the engine starts poorly, the spark plug misfires, or the acceleration becomes faulty, then there must be carbon fouling in your system. When you check the spark plug for this condition, there should be dry, soft black carbon on its insulator and electrodes. Carbon fouling is often caused by faulty chokehold over rich air/fuel mixture, delayed ignition timing, bad ignition leads, or cold plug heat range.
When the plug's insulator is extremely white, has small, black deposits, and with premature electrode erosion, it is highly likely that the spark plug is overheating. Its signs include loss of power at high speeds or when carrying heavy load. Check if the plug is not tightened properly as this might be the cause. The engine might also be insufficiently cooled, the ignition timing too advanced, or the plug heat range might be too hot.
While the spark plug is still out, also check for bent or dented electrodes and a broken insulator as these are already considered as mechanical damages. If the central or ground electrode is melted or burned, the plug might have also been loosely screwed. The spark plug will have to be replaced if either of these is the case.