simulations - How do I simulate an ideal gas?

I wish to write a simple simulation of an ideal gas, but my maths and how I should have a model based the equation isn't quite up to it. I need some help understanding the relationship between pressure, the amount of the gas, and the temperature (the volume is a constant), not just at one moment, but as one or more of the variables change...

Right now, I have global variables for $P$, $T$ and $n$, and an update loop in my code where I check to see if $T$ has changed. If so, I can solve the ideal gas equation $P = nRT/V$ as the others are unchanged. The results look ok to me. I can warm the tank, and the pressure rises.

Now, if I add or remove some gas, by changing $n$, I am unsure how to change both $P$ and $T$. If I just recalculate $P$, I can remove gas until the pressure falls to $0$, yet the temp remains constant the whole time. This doesn't seem right, shouldn't there be some temperature drop too?

Can you explain how I should go about improving this simulation?