Q: Car runs fine but will not go over 45mph the check engine light is not on it was fine until I had to change transmission line

Check the fluid level in the transmission as low fluid will cause all kinds of transmission malfunctions. The problem you are describing could be either a transmission issue or an engine issue and sometimes a combination of both. If you can continue to increase engine speed once you reach 45 MPH but the vehicle won’t actually go faster than 45 MPH, there could be a hydraulic fault in the transmission, a low transmission fluid level, slipping bands or clutches, a torque converter defect and so forth. As far as the engine, inadequate acceleration, or loss of power, can be due to throttle body faults (electronic throttle controls, mass airflow sensor, torn air intake hose), worn out spark plugs, faulty fuel pump or restricted fuel system, and large vacuum leaks (including EGR malfunctions, PCV system, leaking intake manifold, leaking EVAP purge valve). In your circumstance, if your vehicle is 1996 model year or newer, a scan tool would be used to look at real time conditions within the engine (e.g., fuel trim, MAF sensor and TPS sensor outputs, etc.) and identify the root cause. If you want these diagnostic tests performed by a certified Mechanic, dispatched by YourMechanic right to your location, please request a loss of power inspection/diagnostic and the responding certified mechanic will get this resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.