There are many reasons why a raw-water-pump impeller in a marine inboard engine can go bad, including improper operating conditions, a poor operating environment, excessive wear or simple old age. Given the relative ease with which most impellers can be inspected, it’s a good idea to examine all of the impellers you have on board periodically.
Removing an old, beaten-up water-pump impeller is easy. After removing the pump’s face plate, you merely use a screwdriver or pocket-knife to pry the darn thing out. Inserting a new impeller, however, is often problematic. Because the cavity into which the new, relatively stiff impeller must be inserted tends to be asymmetrical, it’s often difficult to get the entirety of the impeller’s circular-shaped cluster of rubber fins or vanes crammed in simultaneously.
Going with the following sequence is bound to help. First, lubricate the fins of the impeller and the cavity with some sort of low-temperature grease—this will facilitate insertion and keep the fins or vanes from overheating when they rotate during startup. As luck would have it, manufacturers often supply small tubes of such grease in their impeller replacement kits. Second, prior to the insertion of the new impeller, circumscribe its fins with a tie- or cable-wrap, placing the wrap at a point that is at least, say, one-quarter of an inch from the side of the impeller that will go into the cavity first. Tighten the wrap until the circumference of the impeller is slightly less than the cavity’s smallest transverse dimension. Third, partially slide the impeller into the cavity (and on to the shaft that resides at its center), making sure that any possible fin ‘set’ accommodates the shaft’s direction of rotation. If the set seems to oppose the direction of rotation, simply flip the impeller over, reposition the wrap and continue as before.
Once you’ve got the impeller partially inserted, maintain pressure on it with your hand while you remove the wrap with shears, side-cutters, or some other appropriate tool. Then shove the impeller home, slap on a new gasket, and secure the face plate. That’s it—easy, eh?