When to change the engine's impeller? (Tuggers Vol. 65)

Q: When is the best time to change your engine impeller?
A: Before a vane breaks off!

Hjortie's 37VT33 broken impeller after only one year

An engine impeller looks like a gear with rubbery veins that spin and flex as the pump works.  Over time impellers absorb water and are exposed to severe stresses.  This causes the veins to warp, bend, or in the worst case, break off.  Lack of use can be just as damaging as the impeller can dry out and its veins take on a permanent set. Once the impeller is damaged, it cannot pump the full amount of water the engine needs.  In addition, detached vein pieces can easily clog the heat exchanger and cause the engine to overheat.  

It's recommended that the impeller be replaced annually.  Using glycerin when installing the new impeller can help keep things running smoothly.

So, when is the best time to change your engine impeller?  Before it needs it!