Hi !
Ive had several Cadillacs pre northstar, and been wonderful, i picked this 1998 Cadillac Seville SLS, knowing about the possible head gaskets issues, but could not say no, its in amazing , mint condition, super clean, it actually runs smooth, after 20 minutes or so of driving, coolant read hot on display and gauge, and displayed engine overheating, i also lost heat, so i changed the thermostat, heat seemed okay, back on the road, it did the same thing, my mechanic ran the compression test, no leaks, compression ok, also no white smoke from exhaust, no antifreeze in the oil, no leaks, tried driving it again last nite and again gauge went up to hot, display showed engine coolant hot and then eventually displayed showed engine hot..it was usually after 20 minutes, ohh and after i parked and idled i had no heat again.. i dont want to jump the gun and assume its the head gasket, 98k miles and considering the antifreeze level is the same and the compression test came up alright... What else should i do or try. Its a shame this car is so nice, comfortable and in pristine condition.. hate to lose it.. please help !

 

Only other thing I could think of to check is the water pump

 

Thanks Karen, you think water pump although there is no sign of leakage or loss of antifreeze..? Im also considering from all the research here on these wonderful forums is a temp sensor or faulty sending unit.
Today i cleaned out hoses, and flushed coolant, it actually went longer then 20 min about 33 minutes if idling and then a 2 minute drive it did it again, the fans are operating properly when the gauge was in the red, display read engine hot and coolant was to the top, can hear it boiling,, heat was still cranking, no loss of fluid...
Suggestions are welcomed, encouraged and most appreciated..