* Failing Thermostat: A stuck-closed thermostat prevents coolant from circulating properly, leading to overheating. This can cause pressure buildup in the system, forcing coolant and its dissolved minerals (the "gunk") out through any weak points, including around the radiator cap.
* Coolant System Pressure: A pressure buildup in the cooling system, regardless of the cause, will push out coolant and leave behind the mineral deposits as it cools. This can be caused by:
* Head Gasket Leak: A very serious problem where combustion gases leak into the coolant system, increasing pressure. This usually also results in white smoke from the exhaust and potentially milky oil.
* Cracked Cylinder Head or Engine Block: Similar to a head gasket leak, cracks allow pressure to build.
* Radiator Cap Failure: A faulty radiator cap may not relieve pressure properly, exacerbating the problem.
* Cooling System Leaks: Small leaks can cause the system to lose coolant, concentrate minerals, and overheat.
* Corrosion: Corrosion can contribute to the problem in several ways:
* Rust in the Radiator: Internal corrosion can restrict coolant flow, leading to overheating. The rust particles themselves can contribute to the gunk.
* Corrosion in the Cooling System: This can lead to leaks and pressure buildup.
* Low Coolant Level: Simply running low on coolant will concentrate the minerals and increase the likelihood of overheating and pressure buildup.
What to do:
1. Don't drive the vehicle until you've investigated further. Overheating can cause serious engine damage.
2. Check the coolant level. Is it low? Top it off (carefully, with the engine cold) with the correct 50/50 mix of coolant and distilled water. Never use just water.
3. Inspect the radiator cap. Is it damaged or corroded? It's inexpensive to replace.
4. Inspect all hoses and connections. Look for any leaks or cracks.
5. Have a mechanic check the system for pressure. They can use a pressure tester to identify leaks or other pressure-related issues.
6. Have a mechanic perform a complete cooling system inspection. This should include a pressure test, checking the thermostat, and inspecting the radiator and hoses. A visual inspection of the coolant itself can also provide clues (milky appearance suggests a head gasket leak).
The "gunk" is a symptom, not the problem itself. Addressing the underlying cause is crucial to preventing further damage and ensuring safe operation of your vehicle. Ignoring this could lead to a catastrophic engine failure.
