How to make an object keep move when pressing a button?

I wan't it so that whenever i press a button my car will turn left and it does do that currently, but only for one frame. It doesn't matter if i hold down the button it still only goes one frame. I wan't my game to be for Android so i didn't use the OnClick function instead i used the "Pointer Down" function from the event trigger. I have tried this with many different functions so i am certain that this is because of the script.

Here is the script (i have marked the function i used in the event trigger (it's at the bottom) )

using UnityEngine; using System.Collections;

public class Car2DController : MonoBehaviour {

 public float power = 3;
 public float maxspeed = 5;
 public float turnpower = 2;
 public float friction = 3;
 public Vector2 curspeed ;
 Rigidbody2D rigidbody2D;
 ScoreManager scoreManager;


 // Use this for initialization
 void Start () {
     rigidbody2D = GetComponent<Rigidbody2D>();
     scoreManager = GameObject.FindGameObjectWithTag ("ScoreManager").GetComponent<ScoreManager> ();
 }


 void FixedUpdate()
 {

     rigidbody2D.AddForce(transform.up * power);
     rigidbody2D.drag = friction;

     curspeed = new Vector2(rigidbody2D.velocity.x,    rigidbody2D.velocity.y);

     if (curspeed.magnitude > maxspeed)
     {
         curspeed = curspeed.normalized;
         curspeed *= maxspeed;
     }
         
     if (Input.GetKey(KeyCode.Space))
     {
         transform.Rotate(Vector3.forward * turnpower);
     }

     noGas();
 }

 void noGas()
 {
     bool gas;
     if(Input.GetKey(KeyCode.W) || Input.GetKey(KeyCode.S))
     {
         gas = true;
     }
     else
     {
         gas = false;
     }

     if (!gas)
     {
         rigidbody2D.drag = friction * 2;
     }
 }


 void OnTriggerEnter2D(Collider2D hit)
 {
     if (hit.CompareTag ("Goal")) 
     {
         scoreManager.scoreCount += 1;
         power += 3;
         maxspeed += 3;
     }    
 }


     //FUNCTION I USED
 public void Turn()
 {
     transform.Rotate(Vector3.forward * turnpower);
 }

}