* Thermostat Failure: A stuck-closed thermostat prevents coolant from circulating properly, causing the engine to overheat. When it eventually opens (perhaps due to extreme pressure), there's a sudden release of hot coolant, leading to the temperature drop. This is a likely scenario.
* Coolant Leak: A leak in the cooling system (radiator, hoses, water pump, etc.) would cause a loss of coolant, leading to overheating. The temperature drop could be due to the loss of coolant causing the engine to eventually be starved of the coolant's ability to absorb heat. Once the coolant is gone, the temp reading may even drop, as it measures coolant temperature.
* Water Pump Failure: A failing water pump won't circulate coolant effectively, causing overheating. Similar to a thermostat failure, this could result in the temperature spike followed by a drop, once any remaining coolant has been exhausted or the temperature has reached a point where a safety system activates.
* Head Gasket Failure: While less likely to directly cause the immediate temperature drop, a blown head gasket can allow coolant to mix with engine oil or enter the combustion chamber. This could lead to overheating initially, and while the drop might seem sudden, it could be related to the system losing pressure and the engine reacting in a way that causes the gauge to drop. The damage could also be causing the warning light to activate.
* Sensor Malfunction: While less probable to cause the temperature fluctuation itself, a faulty coolant temperature sensor could provide inaccurate readings, leading to the warning light and a seemingly erratic temperature gauge behavior.
It's crucial to:
* Do NOT drive the car. Continuing to drive with these symptoms risks severe engine damage.
* Get it towed to a mechanic. This is not something you should attempt to diagnose and fix yourself.
* Describe the problem exactly as you have here to the mechanic. The sequence of events is important for accurate diagnosis.
The sudden temperature drop is a particularly worrying symptom because it suggests a critical loss of coolant, or a pressure release. Ignoring this could lead to catastrophic engine failure. Professional help is required immediately.
