* Fuel Delivery Issues: A clogged fuel injector for cylinder #5 is a prime suspect. A faulty injector delivers either too little fuel (leading to lean misfire and excessive heat on the plug) or an inconsistent spray pattern, resulting in incomplete combustion and fouling.
* Ignition System Problems: While less likely if *only* #5 is affected, a weak spark from the #5 ignition coil wire, distributor cap, rotor, or even a failing ignition module could cause incomplete combustion and fouling. A cracked or damaged spark plug wire can also cause this problem.
* Air Intake Restriction: A restriction in the intake manifold or air intake system specifically affecting cylinder #5's intake runner is possible, though less common than fuel or ignition problems. This could lead to a lean mixture and overheating of the plug.
* Valve Problems: A burnt or sticking intake or exhaust valve in cylinder #5 will prevent proper combustion. This would result in incomplete burning of the fuel, leading to fouling. A compression test would highlight this issue.
* Vacuum Leak: A vacuum leak on the intake manifold, especially near #5's runner, could also lead to a lean condition.
* PCV System Issues: A clogged PCV (Positive Crankcase Ventilation) valve or related hoses could lead to excessive crankcase pressure, affecting the combustion process in that cylinder and resulting in fouling.
Why only #5? The engine's design and the way components are laid out could influence a specific cylinder's susceptibility to a particular problem. There might be a slight difference in fuel injector flow, a localized issue in the intake manifold, or even a slight variance in the ignition system components affecting that particular cylinder.
Troubleshooting Steps:
1. Visual Inspection: Carefully examine the #5 spark plug wire for cracks or damage. Inspect the distributor cap and rotor for wear or carbon tracking.
2. Compression Test: Perform a compression test on all cylinders to check for low compression in cylinder #5, indicating a valve or piston ring problem.
3. Fuel Injector Test: Ideally, a fuel pressure test and injector flow test should be conducted to ensure proper fuel delivery to cylinder #5. This might involve specialized tools.
4. Vacuum Leak Test: A vacuum leak test can identify leaks in the intake system.
5. Check PCV System: Inspect the PCV valve and related hoses for proper function and cleanliness.
Addressing the root cause is crucial. Simply replacing the spark plug will likely only provide a temporary fix unless the underlying problem is solved. A qualified mechanic with experience in diagnosing these issues is your best bet for a precise and efficient repair.
