[McLin]

Bad radiator cap

The radiator cap does several things. It
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 expansion to vent to the expansion reservoir (pressure seal function)
allows coolant to return to the radiator when the engine cools down (return seal function)
As you may have gathered from the above section, the radiator cap has three seals, any of which may fail independently of the others:
The main seal is the one that seals the cap against the top of the filler neck. Just a rubber gasket that operates just like one on the lid of a pickle jar. Simple and reliable.
A failed pressure seal will allow the coolant to boil at a lower temperature, and coolant will be able to travel freely and foamily to the expansion reservoir. This will cause localized hot-spots inside the engine, which can 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 failed return seal will prevent the coolant from returning to the radiator as the rad cools off, causing a vacuum that can collapse the radiator's hoses. 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, so it's a cheap first step to try before bringing it in. 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.

But anyway, a new rad cap is less than $20. Make a habit to change it every 5 years, just in case. It's pretty important.

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