* Water pushing out of the reservoir cap: This is a classic symptom of pressure building up in the cooling system. A blown head gasket allows combustion gases to enter the cooling system, increasing pressure. This pressure forces coolant out.
* Not excessively hot: While the coolant might not be scalding hot to the touch, the pressure build-up is the key indicator, not necessarily the temperature.
* New pump and thermostat: Since you've replaced these, it's less likely to be a problem with those components themselves.
Other possibilities (less likely given the symptoms):
* Cracked cylinder head or block: This is less likely since the problem is appearing relatively quickly after replacing the pump and thermostat. A crack would usually lead to more substantial and consistent leaks.
* Radiator cap issue (less likely): Though possible, a faulty radiator cap would usually just cause a slow leak or boiling over when the engine is hot, not the forceful expulsion you describe.
What to do:
1. Pressure Test the Cooling System: This is the most important step. A mechanic can perform a pressure test to confirm if there's a leak in the system, and pinpoint its location (head gasket, etc.). This test will definitively answer if your cooling system is holding pressure.
2. Exhaust Gas in Coolant Test: A mechanic can also test your coolant for the presence of exhaust gases. This is a direct way to confirm a head gasket problem.
3. Careful Observation: Keep an eye on the coolant level frequently, and try to note if there's any pattern to when the pressure builds up (e.g., only after driving a certain distance, or when the engine is idling).
Do not drive the vehicle until you have determined the cause of the problem. Continuing to drive with a potential head gasket leak can cause severe engine damage. The pressure build-up can warp the head or block, leading to a far more costly repair.
