ODE solver running continuously. Why?

I cannot figure out why my code (to solve for a system of ODE's) is running continuously. Every time I run it, I have to use CTRL+C to stop it. Please help! The code is:

clear
clc

%Error in ODE solver, will not "stop" solving. 
fprintf('\nQuestion 3\n');
tspan= [0:.05:20];
F0=[2.5; 3; -1; 1];
[t,fn]=ode45(@fn_feq3,tspan,F0);
y=fn(:,1);
x=fn(:,2);
plot(t,x,t,y);
xlabel('Time (sec)');
ylabel('x and y');
legend('x vs t','y vs t');
title('#3 Plot');

function dF=fn_feq3(t,f)
dy=f(1);
dx=f(2);
dz=(2*f(2))+(-6*f(1))+(-3*f(1)*f(3));
dw=14-2*f(4)-2*f(2)*f(1);
dF=[dy;dx;dz;dw];
end

Answer 1

[Walter Roberson]

One of your outputs is heading to -infinity. When it reaches about -1E12 then MATLAB needs to take a lot of smaller steps to try to maintain integration tolerances.

If you switch to ode15s then you will fairly quickly get a final result around -9E17

[Antoine Mora]

So I had this problem in a final exam today where we were instructed to use ode45 (as that was the only ode solver we learned) so how would I have fixed this so that it would run using ode45?

Thank you!

[Walter Roberson]

You could either just let it run until it finishes or you could pass in an options structure that permits less accurate integration tolerance

[Antoine Mora]

I'll try that out. Thank you