Symptoms of a bad radiator cap:
* Coolant loss: This is your primary symptom. A bad cap may not seal properly, allowing pressure to build in the cooling system and force coolant out the overflow. This often happens *while* the engine is running and the coolant is hot and under pressure.
* Low coolant level: Consistent low coolant despite not having visible leaks elsewhere indicates a problem with pressure regulation.
* Overheating: If the cap doesn't release pressure properly, the system can overheat. This may be accompanied by the engine temperature gauge rising, steam coming from the engine bay, or even the engine overheating warning light.
* System pressure issues: A pressure test of the cooling system is the best way to determine if the cap is the problem. A mechanic can perform this test.
Other possible causes of coolant being pumped into the overflow tank (and potentially overflowing):
* Head gasket leak: This is a serious problem. A head gasket leak can allow combustion gases to enter the cooling system, increasing pressure and forcing coolant out. You might also notice white smoke from the exhaust, milky oil, or loss of compression.
* Cracked cylinder head: Similar to a head gasket leak, a cracked cylinder head can allow combustion gases or coolant to leak into each other, causing pressure build-up.
* Warped cylinder head: Similar to a cracked head, causing pressure buildup.
* Faulty water pump: Though less likely to directly cause overflow, a failing water pump can lead to poor circulation and overheating, potentially forcing coolant into the overflow due to pressure.
* Radiator leak: A slow leak in the radiator can cause the system to lose coolant, and then it can't build enough pressure to fully utilize the overflow tank to manage the pressure, instead just pumping it out.
What to do:
1. Check the coolant level when the engine is COLD. Don't open the radiator cap when the engine is hot; you risk severe burns.
2. Inspect the radiator cap: Look for obvious damage or corrosion. A simple visual inspection isn't sufficient to determine if it's bad, though.
3. Consider a pressure test: This is the most reliable way to diagnose cooling system problems. A mechanic will use a special tool to pressurize the cooling system and check for leaks. This will help pinpoint the source of the problem, whether it's the cap, head gasket, or something else.
4. Don't just replace the radiator cap without a pressure test: While it's inexpensive to replace, it might not solve the problem if the root cause is something more serious. Replacing it first might mask a bigger issue.
In short, a bad radiator cap is *a possibility*, but you need a proper diagnostic test to be sure. A pressure test is essential before replacing parts. Ignoring this could lead to significant engine damage.
