[nmader]

Alright, the first thing you should do is have a vehicle engine variable, in my case I use:

pawn Код:

new vehEngine[MAX_PLAYERS];

At the top portion of the script.
Then I would create a timer when a player gets into a vehicle:

pawn Код:

SetTimerEx("Total", 1000, true, "i", playerid);

And then forward it with all your other forwards:

pawn Код:

forward Total(playerid);

Then I would create the public for that.

pawn Код:

Public Total (playerid) //This is probably not right, I'm typing it in this textbox, not the compiler.
{
    new VehHealth = GetVehicleHealth(playerid);
    if (VehHealth <= 300)
    {
        SetVehicleParams(vehicleid, 0, -1, -1, -1, -1, -1);
    }
    return 1;
}

Also be sure to make it to where the "Total" timer stops when the player exits the vehicle.

Note that this is probably buggy as I havn't scripted in a while, lemme know of any bugs or issues and I will gladly fix them.