* Water pump failure: The water pump is responsible for circulating coolant through the engine. If it's broken or seized, coolant won't be pumped through the radiator for cooling, resulting in the engine overheating while the coolant in the system stays relatively cool.
* Thermostat stuck closed: The thermostat regulates coolant flow. If it's stuck closed, coolant won't circulate until it reaches a dangerously high temperature, leading to engine overheating. The coolant in the reservoir will remain cool until the engine reaches a critical point.
* Blocked radiator: A clogged radiator (due to rust, debris, or scale) prevents proper heat dissipation. The engine gets hot, but the coolant may not be fully circulating through the radiator to release its heat.
* Air in the cooling system: Air pockets in the cooling system can prevent proper coolant circulation and heat transfer. This is less likely to cause such a dramatic temperature spike so quickly, but it's still a possibility.
* Head gasket failure (less likely, but serious): While a head gasket failure *could* cause overheating, it usually manifests differently. A blown head gasket often results in coolant leaking into the engine oil or exhaust, leading to white smoke from the exhaust, oil contamination, or loss of coolant. The lukewarm coolant suggests this is less likely in this scenario but needs to be ruled out.
What you should NOT do:
* Don't ignore this. Driving with an engine at 260°F will cause severe damage very quickly. Stop driving immediately.
What you SHOULD do:
* Let the engine cool completely before attempting any inspection. Hot coolant can cause serious burns.
* Check the coolant level: While the description suggests it's likely not a low-coolant issue, it's always a good first step.
* Inspect the radiator for blockages. Look for obvious obstructions.
* Consult a mechanic: This is a serious problem that requires professional diagnosis. Trying to fix this yourself without experience could lead to further damage or injury.
The combination of a very high engine temperature and lukewarm coolant points to a circulation problem within the cooling system. Get it checked by a professional as soon as possible to prevent catastrophic engine failure.
