madmanguruman@gmail.com wrote:
> oh yeah ... there is slow bubbliing in the overflow reservoir.

you'll get purging after a flush and refill. come back in a few days
and check again. if it's still doing it, the head gasket's definitely gone.

 

i checked the rad this morning (after an overnight cooldown) and there
definitely was some air in there.

i topped up with some more pre-mix and will recheck tomorrow.

 

just to bring closure to the story:

i noticed a little bit of play in the temperature gauge, but it never
exceeded the midpoint reading throughout the week. the overflow tank
level seemed a little high, and the bubbling did stop after 3 days
worth of driving (a few hundred km).

when i was able to get the car to my usual mechanic, he was able to
quickly diagnose a faulty radiator cap as the cause of the overflow
tank level not being right. he postulated that the overheat events
caused by the 'improvised' thermostat likely contributed to a cap seal
failure, preventing the rad from venting into the overflow tank. there
was no issue with compression, so the head gasket is most likely fine
(as fine as a 16 year old head gasket can be, at least).

i replaced the cap, and everything seems to be in order now. a few
weeks' worth of panic and uncertainty were corrected with some elbow
grease, some internet research and $40 worth of parts.

thanks for the opinions, all!

 

madmanguruman@gmail.com wrote:
> just to bring closure to the story:
>
> i noticed a little bit of play in the temperature gauge, but it never
> exceeded the midpoint reading throughout the week. the overflow tank
> level seemed a little high, and the bubbling did stop after 3 days
> worth of driving (a few hundred km).
>
> when i was able to get the car to my usual mechanic, he was able to
> quickly diagnose a faulty radiator cap as the cause of the overflow
> tank level not being right. he postulated that the overheat events
> caused by the 'improvised' thermostat likely contributed to a cap seal
> failure, preventing the rad from venting into the overflow tank. there
> was no issue with compression, so the head gasket is most likely fine
> (as fine as a 16 year old head gasket can be, at least).
>
> i replaced the cap, and everything seems to be in order now. a few
> weeks' worth of panic and uncertainty were corrected with some elbow
> grease, some internet research and $40 worth of parts.
>
> thanks for the opinions, all!

thanks to you for posting follow-up. resolution is very good for the
archives.