* Head Gasket Failure (Most Likely): A blown head gasket between cylinder #4 and the coolant jacket is the most probable culprit. Combustion gasses are leaking into the cooling system, pressurizing it and causing overheating. The orange coloration on the spark plug is coolant leaking into the cylinder, which then burns, leaving the orange residue.
* Cracked Cylinder Head: A crack in the cylinder head around cylinder #4 could also allow coolant to leak into the combustion chamber. This is less common than a head gasket failure but still a possibility.
* Other Combustion Issues (Less Likely given the orange plug): While less likely given the orange coloration, problems like a severely burnt valve, piston damage, or a hole in the piston could also contribute to overheating. However, these would likely show other symptoms like low compression in #4 cylinder or excessive oil consumption.
Why your initial repairs didn't fix the overheating: Replacing the thermostat, coolant temperature sensor, radiator, and water pump addresses cooling system *efficiency*, not the source of the excessive heat being *generated*. Your cooling system is struggling because of excessive heat from the combustion process in #4 cylinder, not because of a lack of cooling capacity.
What you should do next:
1. Compression Test: Perform a compression test on all cylinders. A significantly lower compression reading in cylinder #4 confirms internal engine damage (head gasket, cracked head, piston/valve problems).
2. Leak Down Test: This test helps pinpoint the source of the leak if a compression test shows low compression in #4. It identifies whether the leak is in the valves, piston rings, or head gasket.
3. Coolant Pressure Test: Check for coolant pressure buildup in the cooling system when the engine is cold. A pressure increase could indicate the presence of combustion gases.
4. Visual Inspection: Carefully inspect the head gasket area for any signs of leakage or damage. This is challenging but may reveal visible evidence.
In short: Don't focus on the cooling system further until you've diagnosed and repaired the underlying combustion issue causing the overheating. The orange spark plug strongly points to a head gasket or cylinder head problem. A professional mechanic should be consulted to perform the diagnostic tests mentioned above and make the necessary repairs. Ignoring this could lead to serious engine damage.
