Radiator cap-What does it do
The radiator cap does several things:
- seals the system against the outside world (main seal function)
- keeps the system pressurized when needed, so as to raise the boiling point of the coolant
- allows excess pressure and coolant to be able to vent to the expansion reservoir (pressure seal function)
- allows coolant to return to the radiator when the engine cools down (return seal function)
The radiator cap has three seals, any of which may fail independently of the others:
- Sealing the cap against the top of the filler neck is the job of the main seal. This seal keeps the whole system sealed and is considered the most important.
- A leaking pressure seal will allow the coolant to boil at a lower temperature, and coolant will be able to travel freely to the expansion reservoir. This could result in localized hot-spots inside the engine. This may lead to premature head warpage, and may hasten head gasket failure. It will also cause the rad coolant level to be low, just like a failed head gasket.
- A faulty return seal may prevent coolant from returning to the radiator. This may lead to collapsed radiator hoses due to the excessive vacuum. This will prevent the coolant from circulating if the hoses don’t re-expand as the engine warms up.
A bad rad cap can cause similar symptoms to a failed head gasket.
If you replace the rad cap and you still have bubbles in the coolant (or foam in the reservoir), then suspect the head gasket. If the engine starts to overheat at idle, or in heavy traffic, and the gauge goes down when you rev it, the coolant is probably low.
Moreover, a neglected cooling system can load up the cap with crud and corrosion, preventing proper coolant flow in and out through it. Peel the seals back with your fingernail to check for goop. If you find any, a blast with a garden hose and probing with a toothpick should clear most of it out.