My car’s temperature gauge only reaches temperature when I don’t have the heater on , after I put it on the temperature gauge drops until I put my foot on the gas . I took it to the mechanic and they replaced the thermostat and flushed the heater core , but it’s still doing it … what could be wrong?
How is your coolant level? How cold has the outside temperature been?
My coolant level is full . It’s been really cold then warm again and then cold here in Pennsylvania.
ahh you’re stuck in the keystone state too! Isn’t it wonderful to bounce from winter, to spring, to winter, to summer, to winter in the course of a week?? Is the coolant full in both the overflow AND the radiator (please make sure the engine is completely cool when checking the radiator!)
Ha ha … yeah it’s absolutely wonderful… lol… Yes the coolant is full in both … I always check the fluids when the engine is always cool . Thank you for the reminder though 
If it’s doing it on the “warmer” days as well as the negative/low temp days, then I wonder if your new thermostat is bad out of the box. The heater core also acts as a radiator, so when it’s really cold turning it on would cool your engine (it’s also why when your car is overheating, it’s recommended to turn the heater on to help pull more heat off the engine while you find a safe place to pull over and shut down. You could try attaching some cardboard to the front of the radiator to see if that helps to keep the temperature up when you turn on the heater (obviously you’ll want to take it off when it warms up outside so you don’t block airflow and overheat)
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Since we like to digress on these threads. In HS I delivered pizzas, the supplied vehicles were 1966 Comet wagons with 6 cylinder engines. Never produced enough heat for evening deliveries in Minneapolis. Even with pizza box in front of the radiator.
I like the suggestion of partially covering the radiator. Back in the late 1940s through the mid 1950s, the owner/operators of the school buses I attended, tied feed sacks over the grilles of the buses to get the engine to warm up more. I am not sure how much good it did as the buses were always cold. This business of covering the radiator is still used. I noticed the other day that the school buses in our district had the grill partially covered.