The car expansion cap may leak if tightened too tightly.
Tightening the expansion cap of a car too much can indeed cause leakage. The main reason is that excessive tightening can cause the cap to deform, damage the sealing gasket, or affect the normal function of the pressure relief valve.
Reasons for leakage due to excessive tightening
Cap deformation: The expansion cap is usually made of plastic or rubber. Over-tightening can cause it to permanently deform, destroying the sealing structure, and allowing coolant to leak.
Pressure relief valve malfunction: The expansion cap contains a pressure relief valve to regulate the pressure in the cooling system. Over-tightening can damage the valve or prevent it from opening properly, causing the coolant to leak through gaps when pressure cannot be released.
Sealing gasket damage: Excessive tightening can squeeze the sealing gasket, causing it to distort or rupture, losing its sealing function.
Correct handling method
Moderately tighten: When installing the expansion cap, only tighten it until there is a slight resistance. Avoid over-tightening. Usually, when you hear a "click" sound or feel a significant resistance, stop tightening.
Check and replace damaged parts: If leakage occurs due to excessive tightening, first check if the cap is deformed or the gasket is damaged. If there is damage, replace the cap or gasket with a new one that matches the original vehicle.
Verify the cooling system status: After replacement, start the engine and observe if there is still leakage and ensure the coolant level is within the scale line to rule out other faults.
Daily maintenance suggestions
Regular inspection: Inspect the expansion cap and the cooling system for any leakage marks every few months to ensure the pressure relief valve is not blocked.
Use regular coolant: Avoid adding tap water to prevent corrosion of the system or blockage of the valves by impurities.
Professional repair: If leakage persists, it is recommended to go to a repair shop to conduct a comprehensive inspection of the cooling system to prevent engine overheating and damage.
Detailed working principle of the expansion cap
The expansion cap regulates the pressure of the cooling system internally through the pressure relief valve and vacuum valve. It releases excess gas when the engine heats up and replenishes coolant when it cools down, maintaining the stable operation of the system.
Core functions and roles
Pressure regulation: Control the cooling system pressure within a safe range (typically 0.12–1.2 bar) to prevent pipe bursts due to excessive pressure or air blockage due to insufficient pressure.
Coolant management: Absorb the volume changes caused by thermal expansion and contraction (approximately 5%–8%), avoiding coolant leakage or shortage.
Step-by-step working principle
Engine heating stage (pressure increase)
The coolant expands due to heat, and the system pressure exceeds the threshold (e.g., 0.12 MPa).
Pressure relief valve automatically opens: Releases excess steam and gas to the expansion cap, and the pressure drops after the valve closes.
Function: Prevents water tank or pipe damage, and the radiator starts to cool down.
Engine cooling stage (pressure decrease)
The coolant contracts, and a negative pressure forms in the system.
Vacuum valve automatically opens: Allows the coolant in the expansion cap to flow back to the main circulation, replenishing the system level.
Function: Prevents air blockage (air blockage), ensuring continuous coolant circulation.
Key components and design
Pressure relief valve: Spring or pressure-sensitive structure that opens to release exhaust when subjected to high pressure (>1.2 bar).
Vacuum valve: One-way valve design that opens the return channel when subjected to low pressure.
Sealing system: Rubber gasket and threaded cap ensure airtightness, reducing coolant oxidation.
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