1) Quick theory (why the cap matters)
- The radiator/pressure cap closes the coolant system so it can hold pressure. Higher pressure raises coolant boiling point (by about 3°C per 0.1 bar), preventing local boiling and maintaining coolant flow through the head, block and heater core.
- The cap is a two-way valve: a spring-loaded pressure relief valve vents excess to the overflow/expansion tank when pressure exceeds its rated value, and a vacuum/return valve lets coolant return from the tank as the system cools, preventing air being sucked in. A failed cap can let coolant boil off, leak to overflow, or allow air into the system — all causing overheating, poor heating, coolant loss and possible head gasket failure.
- Diagnosing/replacing the cap restores correct system pressure and vacuum behaviour, which restores boiling point, stops abnormal coolant loss and prevents air pockets, so proper coolant flow and heat transfer resume.

2) Tools / parts needed
- Correct replacement cap (OEM or equivalent) with the exact pressure rating specified on the old cap or in the service manual.
- Hand pressure tester (cooling-system pressure tester) for checking cap and system (optional but recommended).
- Coolant to top up, gloves, rags.
- Safety: engine cold, eye protection.

3) Procedure (ordered, with why each step)
1. Ensure the engine is fully cold. Why: opening a hot pressurised system causes scalding steam/coolant injuries.
2. Locate the cap. ZD30DD and KA23DE: the cap may be on the radiator neck or on a remote header/expansion tank — use the visible filler neck/cap assembly. Why: some systems put the cap on the tank; function is the same.
3. Inspect externally for obvious damage (cracked plastic, torn rubber seal, corroded spring). Why: visible damage is immediate cause of failure.
4. If you have a pressure tester: remove cap and fit tester to the filler neck, pump to the cap’s rated pressure (use rating on cap or manual). Observe:
- Does pressure hold steady for several minutes? If pressure drops, there is a leak somewhere (cap or system).
- When you pump past the cap rating, does the cap open and dump to overflow? If it doesn’t, the relief valve is stuck. If it opens below rating, spring is weak. Why: this tests both sealing and relief function so you know whether cap or system is at fault.
5. If no tester: a simple sign a cap is bad — coolant overflow at low temperature, persistent small coolant loss, boiling/steam at lower temps, or collapsing/ingressing of the overflow tank on cooldown (vacuum valve stuck) — replace the cap.
6. Remove the cap correctly: with the engine cold, depress the center if required and turn/unscrew as per cap design. Wipe the filler neck seating surface clean. Why: debris or warped neck prevents a seal.
7. Inspect the neck: check for corrosion or a deformed landing surface that would prevent sealing. Why: a damaged neck can mimic a bad cap.
8. Install the new cap: use the cap with the correct pressure rating; seat it fully and secure by hand until latched. Why: a wrong-pressure cap changes boiling point and pressure relief point — too low causes boil/over; too high can overpressurise and damage hoses or gasket.
9. If you replaced the cap or opened the system, top up coolant to the proper level in the radiator or expansion tank (and use the correct coolant type). Why: correct level is required for the cap to function with the overflow system.
10. Bleed air from the system:
- Start with the heater on max so heater circuit fills.
- With the cap off (or following manufacturer bleeding procedure), run engine until thermostat opens and air is purged — squeeze upper radiator hose to help expel trapped air and keep topping up.
- Replace cap when bleeding complete and coolant level stable. Why: trapped air reduces heat transfer and can cause overheating even with a good cap.
11. Run the engine to normal operating temperature, watch temperature gauge, check for leaks, re-check overflow tank level once cool and top up if necessary. Why: confirm that the system now holds pressure, doesn’t leak, and the cap performs as intended.

4) How this repair fixes the fault (mechanics)
- A proper cap re-establishes the correct maximum pressure. That raises the coolant boiling point and prevents localized boiling/steam pockets that interrupt coolant flow and reduce heat transfer — this directly cures overheating caused by cap failure.
- The relief valve prevents excessive overpressure (protecting hoses/gaskets), while the vacuum valve allows coolant return from the expansion tank during cooldown. Restoring both functions stops coolant loss to overflow, prevents tank collapse on cooldown, and prevents air ingress — eliminating symptoms like repeated top-ups, heater failure, or erratic temperature.
- If the cap was the cause of pressure loss, replacing it removes a path for air entry and coolant escape, so the system can purge air and hold pressure; once bled, circulation and thermostat operation return to normal and temperatures stabilise.

5) Quick diagnostic summary (no-nonsense)
- Visual damage or weak spring → replace.
- Pressure tester: cap must hold its rated pressure and open only at that setpoint; otherwise replace.
- After replacement: fill, bleed, run to operating temp, check for stable temps and no unexplained coolant loss.

6) Final safety notes (brief)
- Never remove the cap hot. Use the correct rated cap. Dispose of old cap properly. A failed cap can be a symptom of other faults (leaks, head gasket) — if overheating or pressure keeps appearing after a correct cap and properly bled system, investigate further (pressure-test the whole system).

End.
rteeqp73

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