I think my engine's thermostat stuck old and it takes forever to warm up and it even stays cold on the highway. Is it bad for the engine to drive like this?
Yes this is actually bad. Get a thermostat. For most cars, they don't cost much, unless you own one of those V12 Jaguars, that you've got to take the intake manifold off to change it or a VR8 Volkswagen where you have to pull the heads off to change it.
You'll need to change the thermostat because, if you run it too cold, it's bad on the engine main bearings and they can wear off because they aren't running at the right temperature. The engine has to run on right temperature so that all the metal heats up right. If you need help with this, a certified mechanic, such as one from YourMechanic can replace your thermostat so that you can get your car back on the road safely.
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