Coolant boiling after stop in 93 Camry

The car runs fine, and the temp gauge does not indicate overheating. But after the car stopped yesterday, coolant was boiling out of the cap of the reservoir.

How do I diagnose the problem?

The coolant’s old, but full. The oil looks fine. I’ve read that that the problem could be that hot exhaust is leaking into the coolant through the head gasket, or perhaps something simpler, like a bad radiator cap or thermostat. If it’s the head gasket, what other effects would that have?

Thanks for any help.

  1. bad cap?
  2. watery coolant? (too much H20/not enough ethylene glycol)

I doubt (to a certain extent) this is a thermostat thing, because you report normal temp gauge. Does it boil like this all the time, or only in certain circumstances? If only occasionally, is it more likely to do it (a) on hot days, and/or (b) when you had to travel uphill to your destination?

If the coolant’s old, it’s due to be changed anyway. You can, if you want, measure coolant strength befor draining to see if weak antifreeze mixture was playing a part in this. Do that, replace the cap, and see if the problem isn’t solved.