From Mainly Macro by Simon Wren-Lewis:
I have complained before about IS-LM being the first macromodel most students encounter, when no major current central bank fixes the money supply. The textbook version of Mundell Fleming (TMF)  is the first, and often the last, short run open economy model students are taught, and it shares the same deficiency. However the problem with TMF is even greater. It is inconsistent with Uncovered Interest Parity (UIP), and if we use modern macro as our yardstick, this makes it simply wrong.
Lets take a topical issue: the impact of a temporary increase in government spending. We should be immediately worried that TMF makes no distinction between temporary and permanent increases. It says both have no impact on output. So every student learns that fiscal policy is ineffective under flexible exchange rates. For a temporary increase in spending this is incorrect.
The logic of the TMF proposition is usually demonstrated by shifting various curves, but it is in fact trivial. In TMF money demand must equal a fixed money supply. If money demand depends on prices, output and interest rates, and the first is fixed in the short run and the last is tied to world rates, then output cannot change either. This complete crowding is achieved through an appreciation in the real exchange rate.
But why should domestic interest rates equal world interest rates? UIP tells us they need not. A temporary increase in government spending will raise output, which given a fixed money supply will raise interest rates. This will lead to an appreciation, but the temporary nature of the shock means that the long run exchange rate is unchanged. So the current appreciation implies an expected depreciation, which offsets the additional return offered by higher interest rates. The result is a short run equilibrium where output and domestic interest rates are higher. There is partial crowding out through an appreciation but not full crowding out.