I've had this problem off and on for quite some time. 1998 SOHC car(103k miles). Car runs in the normal op temps while highwaying, guage roughly stays around the 1/2 to 3/4 of normal temp range on the gauge. I will come into town and drive town speeds, 20-25, or sit idling, the gauge will just raise within 30 seconds to the top of normal range, and then sometimes will just flop up to the red caution line on "H". and when i say flip, it will literally just jump to the line within a half second or so. Car doesn't act to be running hot, not puking antifreeze or smelling hot, running rough, nothing like that. The danger dinger will alarm.

If i take off onto the highway, it'll drop like a lead weight back to the top of normal op temp range, then gradually go back down to the 3/4 to 1/2 range. I let it idle and when i observed the needle at the very top of normal range, i looked under hood and saw neither fan operating.

Any ideas???? Im just stumped whether this isa legitimate cooling problem, or just sensor problems.

 

Sounds like air in the system.

 

Alrighty, i'll give purging the system a system...