It won't accelerate over 20 mph

So the Thermostat does 2 things it gives you heat from the engine and cools off the engine. Check your fluid level and squeeze the upper hose. DO THIS WHILE ENGINE OS COLD I could not stress more to you to have the engine off for an hour maybe two before you open. The radiator. If it sounds like water moving around you have a leak and a bubble in the system so check the thermostat or replace it they are cheap. Then. Learn on youtube how to flush your radiator, heater core, and get those bubbles out. Each vehicle has different ways to go in doing this depending on the system yours has. Now if this did not work consult a mechanic.