What is the purpose of the radiator's pressure cap?

Radiator pressure cap: critical component that pressurizes the closed cooling system. Functions: (1) PRESSURIZES the system — typically 13-17 psi above atmospheric pressure (varies by vehicle); rating stamped on cap; (2) RAISES BOILING POINT of coolant — each 1 psi of pressure raises water's boiling point about 3°F; at 15 psi, water's boiling point rises from 212°F to about 257°F; antifreeze additive further raises boiling point; pressurization prevents coolant from boiling at high engine temperatures (engine can run at 230-240°F without boiling); (3) PRESSURE RELIEF VALVE — releases excess pressure if system overheats (preventing burst hoses or radiator); pressurized coolant escapes to OVERFLOW RESERVOIR; (4) VACUUM RELIEF VALVE (in newer caps) — when engine cools and coolant contracts, a vacuum could form; vacuum relief allows coolant to flow BACK from reservoir to fill the system; prevents reservoir overflow when system warms again; this is essentially a closed coolant recovery system. SAFETY WARNING: NEVER REMOVE RADIATOR CAP WHEN HOT — pressurized hot coolant can erupt out, causing severe burns. Wait until engine has cooled completely (1-2 hours minimum after running); or if necessary, cover cap with thick rag and slowly turn 1/4 turn to release pressure gradually before fully removing. Cap can lose its seal or pressure rating over time; replace as needed (about $10-20 part); failed cap symptoms: coolant loss with no visible leak (boiling off through relief), white deposits around cap (escaping coolant), overheating without other obvious cause. Aftermarket high-pressure caps for performance applications: 18-22 psi; raise boiling point further; race applications. OVERFLOW RESERVOIR (expansion tank): translucent plastic bottle next to radiator; typically marked with COLD and HOT level lines; receives expanding coolant when engine warms; returns coolant when engine cools; should be at appropriate level. Some modern vehicles use a DEGAS BOTTLE (different design where reservoir is the system's primary fill point, radiator cap may not exist). CHECKING COOLANT LEVEL: ONLY when COLD; check reservoir level mark; if low, add 50/50 premix antifreeze (matching color/type — never mix incompatible coolants). Modern coolants: ORGANIC ACID TECHNOLOGY (OAT, extended life, 5+ years), HYBRID OAT (HOAT), INORGANIC ADDITIVE TECHNOLOGY (IAT — traditional green); using wrong type can cause corrosion. SIGNS of cooling system problems: visible leaks under car; sweet smell of antifreeze; steam from hood; high temperature gauge reading; coolant warning light; oily coolant (head gasket leak); coolant in oil (head gasket leak — milky 'chocolate milk' oil); white exhaust smoke (coolant entering combustion chamber). HEATER CORE: small radiator inside dashboard providing cabin heat; failure causes coolant leak inside cabin (sweet smell, fog on windshield, wet floor); replacement is labor-intensive due to dashboard removal. THERMOSTAT: separate component; valve that opens at specific temp (typically 195°F); stuck closed = engine overheats; stuck open = engine slow to warm up, poor heater. RADIATOR FAN(S): electric fans behind radiator turn on as needed; controlled by ECU based on coolant temp and A/C compressor; failure causes overheating in slow traffic but not highway. WATER PUMP: belt-driven (sometimes timing-belt-driven) pump that circulates coolant; failure causes overheating and possible coolant leak; bearing noise or leaking pulley shaft are warning signs; replacement typically with timing belt service in some engines.