Hello I have a 1991 olds eighty eight with a 3.8Litre engine..........went to replace the coolant thermostat because it would only heat to 150 maybe 160 degrees.........and stay there unless I was sitting still........well after I replaced it.........the temp guage now reads higher, but I notice a fluctuation from slightly over 220 down to 170, 180 maybe(factory guage with numbers).........debating on whether to leave it alone cause the heater inside the car works better now, or search for what causes this fluctuation........My 1986 s10 truck with a 2.8 does this too.....temp guage fluctuates............the gauge movement on both vehicles moves smoothly but like something is shutting off coolant capability so guage will rise to 220 on car and then let coolant do its thing till guage slowly drops down to 170.........I would sure like to know if this is normal or not