when i get between 45-50 my car starts to cut out
car boggs down on 1993 Mitsubishi Diamante
Many things could cause this without being able to drive the car and experience the problem first hand or being able to do some diagnostic tests its hard to say. Your car has electronic fuel injection which relies on inputs from sensors on the engine to determine how much fuel to inject into the engine and when to supply spark to the spark plug to ignite that fuel. If any sensor is failing or sending an "false" signal the engine's performance will be compromised. I would first start by verifying fuel pressure and volume to see if the fuel pump is failing or fuel delivery is reduced due to a clogged fuel filter. It is also necessary to check the ignition system to see that is supplying adequate spark to allow the engine to run properly.
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When a vehicle has a lack of power at higher speed, like this, there are several things that can cause this. Engine misfiring, plugged exhaust, poor fuel delivery, ignition timing to name a few. This will need to be looked at by someone who can determine the cause of the problem.