1989 geo metro

i have an 89 metro with an incorrectly operating temp gauge. gauge stays below Cold except on hot days when it moves almost up to Cold. using my Chilton manual i checked the system. i replaced both temp sensors [1 for gauge and 1 for computer]. i changed the themostat [195 deg.] the gauge test in which you ground the wire which attaches to the temp sensor and watch the gauge was good. WHAT NEXT ?

Can you tell if the problem is just with the gauge, or does the motor seem to be running lukewarm as well? Does the heat feel really hot when it’s as warmed up as it gets?

Assuming the engine is warming up just fine, your probably have some corroded wiring between the sender and the gauge. Because the temp gauge is actually just a resistance meter that responds to the sender reducing the resistance as it gets warm, the gauge will also reflect the increased resistance due to old or damaged wiring, making it read colder than the sender is indicating. I’d start by changing the last few inches or so of wire and the connector to the sender, since they are closest to the hot engine and are the most likely to be damaged.