* Head Gasket Failure: This is a serious and common culprit. A blown head gasket allows combustion gases to enter the cooling system, pressurizing it and forcing coolant out. The pressure builds, causing the steam. This also explains the overheating. A compression test and a coolant leak-down test are needed to diagnose this.
* Cracked Cylinder Head or Engine Block: Similar to a head gasket failure, cracks allow coolant to mix with combustion gases or leak externally. Pressure buildup and overheating would result.
* Radiator Leaks: Even a small leak in the radiator can cause significant problems over time. While the pressure cap might hold it initially, the leak might worsen as the engine heats up, leading to pressure buildup and steaming. Inspect the radiator carefully for any cracks, pinholes, or leaks at the connections.
* Water Pump Failure (despite replacement): While you replaced it, there's a small chance the new water pump is faulty or wasn't installed correctly. It might not be circulating coolant efficiently.
* Thermostat Issues (despite replacement): Again, a faulty new thermostat or incorrect installation could prevent proper coolant flow. It might be stuck closed, preventing coolant from circulating.
* Cooling Fan Problems: If the cooling fan isn't working correctly, the engine might overheat, leading to pressure buildup and steam. Check the fan's operation (both low and high speed if applicable). The fan relay and its wiring are also potential failure points.
* Clogged Radiator: Internal blockages within the radiator itself restrict coolant flow, causing overheating and pressure buildup. A radiator flush might be necessary.
* Low Coolant Level: Even if you've added coolant, a persistent leak (from any of the above sources) will eventually lead to low levels. This can exacerbate overheating.
What to do next:
1. Check for External Leaks: Carefully examine all hoses, connections, the radiator, and the water pump for any signs of leaks.
2. Pressure Test the Cooling System: A professional mechanic can pressure test the system to identify leaks that might not be immediately visible. This will often pinpoint head gasket or crack issues.
3. Coolant Leak Down Test: This test specifically checks for leaks between the cooling system and the combustion chamber, helping diagnose head gasket failures.
4. Inspect the Radiator: Check for obstructions and ensure the radiator fins are not damaged or bent over.
Because overheating can cause serious engine damage, it's crucial to get this issue diagnosed and repaired promptly by a qualified mechanic. Ignoring the problem will likely lead to much more expensive repairs down the road.
