well it happened to my engine wednesday night on the drive home

i keep a religious eye on the temp gauge at all times .

and boom the gauge drops down to halfway between cold and normal operating temperature

AH !! CRAP!! so i quickly pull over and shut the engine down and walk across the street to a

hotel to grab some sleep and have the mazda towed to the dealership in the morning

waiting on word from the mechanic about my extended warranty coverage

 

???

temp gauge dropped down

How or why would you concluded head gasket issue from that?

 

experienced the very same problem with another vehicles engine many years ago

1 of the cylinders with a failed head gasket port was pumping air into the engines cooling system

the easy way to tell is to connect a cooling system pressure tester and let the engine warm up and watch the tester .

the pressure tester needle went to full pressure on the needle gauge

some where around 30 psi if i remember correctly

we also had a cylinder compression tester to the suspected cylinder

the other 3 cylinders held pressure , but #2 kept fluctutating

sure enough , #2 cylinder was pressurizing the cooling system , but no coolant was leaking out or into the engine oil

we tested cooling system pressure with a radiator cap that was defective and would not open at any pressure in the radiator

 

Yes, all of that is fine, but why would the temp gauge go down if air was being pumped into the cooling system?

It should go up

 

weird isnt it

turns out that that the coolant temperature sender requires a solid mass under pressure to properly send coolant temperature reading

water --- solid mass , cannot be mass compressed

air -- not a solid mass , can be mass compressed

several years later a machinist friend of mine and i came up with a easier and less damaging way of testing for a failed head gasket

he simply machined a piece of steel with 1 end threaded with a spark plugs correct threads and then drilled out the stock .

he then machined off the threads of a air chuck adapter and welded the adapter to the piece he machined

simply screw the adapter into 1 cylinder at a time connect a pressure controlled air hose , and watch the cooling system tester for gauge movement