1) Theory — what the radiator (coolant) cap does
- It seals the pressurized cooling system. A spring‑loaded pressure valve lets excess coolant flow to the overflow/expansion tank when system pressure reaches the cap’s set point; that raises the coolant boiling temperature so the engine can run hotter without boiling.
- A vacuum/return valve (small one under the cap) opens as the system cools to draw coolant back from the overflow bottle so air isn’t pulled into the radiator.
- If the cap’s pressure valve leaks or the spring weakens you lose system pressure → coolant boils at a lower temperature, steam pockets form, heater performance/flow drop and overheating can occur. If the vacuum valve fails, the system can suck air back in and develop persistent low‑coolant symptoms and hot spots.

2) Symptoms indicating a bad cap
- Coolant loss to overflow or frequent need to top-up with no visible external leak.
- Persistent air in system, overheating, boiling or spitting at the overflow.
- Coolant level in radiator not returning from expansion tank after cool‑down.
- Visible cap seal damage, corrosion, or spring stuck.

3) Tools/parts
- Replacement cap to OEM pressure rating (match the number on the original cap or vehicle manual — common values 13–16 psi; use the exact spec for the vehicle).
- Clean rag, small brush, coolant if needed, hand‑held pressure tester (optional but recommended for diagnosis), gloves, eye protection.

4) In‑order procedure (with why at each step)
1. Let engine cool completely. Why: opening a hot pressurized system causes scalding steam/coolant hazards.
2. Clean the area around the radiator neck and overflow neck. Why: prevents dirt falling into the system when you remove the cap.
3. Remove the radiator cap (push down and turn counterclockwise on many caps) and inspect:
- Rubber seal for cracks, hardening or deformation.
- Spring and metal parts for corrosion.
- Vacuum valve for isolation and free movement.
Why: visual inspection often reveals the cause.
4. Optional bench test with a pressure tester: attach tester to radiator neck, pump to the cap’s rated pressure and watch for pressure drop or bypass to overflow. Why: confirms whether the cap holds specified pressure and that the valve opens at the correct pressure.
5. Inspect the radiator neck and overflow hose connection for corrosion, pitting, or a collapsed hose. Clean or replace as needed. Why: a damaged neck or hose can ruin a new cap’s sealing function.
6. Fit the correct replacement cap: press down and turn clockwise until it latches per manufacturer procedure. Why: ensures proper seating and spring preload so valves operate as designed.
7. Top up radiator/expansion tank to the correct level with the proper coolant mix. Why: removes air and restores system fluid volume.
8. Start engine and run to operating temperature with heater on, watch for stable temperature, no large pressure spikes, and coolant returning to the radiator from the overflow when cooling down. Why: verifies cap and system are functioning — the cap lets excess flow out under pressure and the vacuum valve lets it flow back in when cooling.
9. After a cool‑down, recheck coolant level and cap seating. Why: to confirm no slow loss or ingress of air.

5) How the repair fixes the fault (concise)
- Replacing a leaking/weak cap restores the designed pressure limit so the coolant’s boiling point is raised and the system keeps coolant in the radiator rather than boiling off. That eliminates steam pockets, restores normal flow through the head and heater core, and prevents overheating.
- A working vacuum valve prevents air being drawn into the radiator during cool‑down, so the cooling passages remain full and avoid hot spots and intermittent overheating.
- Replacing the cap (and fixing a corroded neck/hose) re‑establishes a sealed, pressure‑controlled circuit — stopping loss of coolant, preventing pressure spikes and returning normal thermal behavior.

Safety notes (short)
- Never remove the cap when hot. Use eye/hand protection. Use the OEM cap rating. If uncertain about pressure testing or if you see major corrosion/sleeve damage, have a professional do the repair.
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