* Faulty radiator cap: The most common culprit. The pressure cap is designed to release pressure at a specific point; if it's malfunctioning (either failing to release pressure or releasing it at too low a pressure), pressure builds and forces fluid out the easiest escape route – the vent. Replace the radiator cap first.
* Head gasket failure: A blown head gasket allows combustion gases to enter the cooling system. This increases pressure and can cause leaks and overheating. This is a serious problem requiring professional attention. Look for signs of white smoke from the exhaust, milky oil, or coolant in the oil.
* Clogged radiator: A clogged radiator restricts coolant flow, causing it to overheat and build pressure. This is less likely to leak from the vent, more likely to cause overheating symptoms, but it can contribute to overall system pressure.
* Water pump failure: A failing water pump can impede coolant flow, leading to pressure buildup.
* Thermostat stuck closed: A stuck closed thermostat prevents coolant from circulating, causing overheating and pressure buildup.
* Blocked hoses or passages: Restricted coolant flow anywhere in the system will raise pressure.
* Cooling system pressure sensor issue: Though less likely to cause a visible leak from the vent itself, a malfunctioning pressure sensor could cause the system to behave erratically and build up pressure.
Diagnosis steps:
1. Check the radiator cap: Replace it with a new one of the correct pressure rating (check your owner's manual). This is the cheapest and easiest fix, and often solves the problem.
2. Check for overheating: Monitor your temperature gauge. Consistent overheating strongly suggests a more serious underlying issue.
3. Inspect the hoses and connections: Look for any visible cracks, bulges, or leaks in the hoses and connections.
4. Pressure test the cooling system: A mechanic can pressure test the system to pinpoint leaks or pressure issues. This will help diagnose a blown head gasket, a cracked block, or other more serious problems.
5. Check the coolant level: Low coolant can also contribute to overheating and pressure build-up.
If replacing the radiator cap doesn't solve the problem, you must have a mechanic diagnose the cause. Ignoring a persistent leak and overheating can lead to serious engine damage. A head gasket failure, for instance, is a costly repair.
