Sorry if this has been discussed I searched for a bit and couldn't find any similar experiences. so mostly stock 99 NB Miata, headed into the mountains to visit a buddy, 20 ish minute mountain drive, where I was driving moderately, not redlining it third or fourth gear the whole way up stuck behind some tourist at 35 -45 mph. parked the car and killed it, got out talked for 3-5 minutes, then walked back to my car to get something and noticed I could hear boiling from under the hood, from the coolant overflow tank. It wasn't over heating temp gauge was in the green(i know it doesn't mean much on nb's). Car doesn't eat coolant and the levels in the radiator and the tank are about where they should be. any one have any thoughts? The only thought that I jumped to was maybe I should do a coolant reroute, but i can't see how that would help in this situation. thanks in advance

 

My car does this, however i know its from a coolant leak i cant track. I need to pressure test it or something but im too lazy. It comes from having air inside the cooling system.

 

Check your radiator cap. Low cooling system pressure = lower boiling point. Could be this or a pin hole leak, or as noted above air in the system. A good burp may be the answer as well.

 

worst case like mine, blown head gasket.

 

Higher altitude makes things boil more easily. Could be why you didn't notice it before. Don't know exactly how high you went up, but it does say Colorado. /shrug

 

Sorry to revive this but I had this issue, knew it wasn't overheating but still boiled only after I turned the car off.

The coolant hoses to my EFR have a lower and upper one, lower for the "feed" And upper for the "drain." Most water line threads say "doesn't matter which is which" But my EFR seemed particular. I swapped them around to what would match the normal flow of coolant, and it stopped the boiling issue.

 

Throw on a new quality radiator cap and bleed the system before you do anything drastic like pull the head.

 

Agreed, try a new radiator cap before anything else, I've had the same problem. Didn't help that I was in colorado at the time as well.