Symptoms
- Coolant loss or overflow with no other leak found
- Hissing sound from the cap area, especially right after shutting off the engine
- Coolant reservoir overflowing repeatedly
- Slightly lower boiling point symptoms, like earlier-than-expected boiling
Likely causes
- CommonWorn or hardened rubber seal on the capThe seal degrades with age and heat cycles, losing its ability to hold pressure.
- CommonWrong pressure rating cap installedUsing a cap with a different pressure rating than specified can cause premature venting or inadequate sealing.
- PossibleDamaged or corroded sealing surface on the radiator neckEven a good cap won't seal if the surface it presses against is damaged or corroded.
- Less commonDebris or mineral buildup preventing a full sealAccumulated grime can prevent the cap from seating fully.
What to do
- Inspect the cap's rubber seal for cracking, hardening, or visible damage once the system is cool.
- Confirm the cap's pressure rating matches what's specified for your vehicle, checking the owner's manual if unsure.
- Check the radiator neck's sealing surface for damage or heavy buildup while the cap is off and the system is cool.
- Replace the cap if the seal looks at all questionable, since caps are inexpensive relative to the problems a bad one can cause.
- Avoid reusing an old cap after a coolant flush or other service just to save a small cost.
- Have a mechanic pressure-test the cap specifically (a common shop test) if problems continue after replacing it.
- Recheck for overflow or loss issues after installing a new, correctly rated cap.
Tools and parts
- Gloves
Radiator cap (correct pressure rating)Search: “radiator cap for [make model year]”
When to call a pro
See a mechanic if replacing the cap doesn't resolve overflow or loss issues, since the radiator neck itself might be damaged.
Safety
Use caution
Only remove or check the cap when the engine and coolant are fully cool; a hot, pressurized system can spray scalding coolant.