Two sides to this coin, first up the plastic impellers dont crack that often, what they do is come loose on the shaft, when cold its a snug fit when the engine warms up it expands slightly and then is free so doesnt move the coolant as it should, metal impellers, now ive NEVER had any problems with these, and always replace water pumps with metal, as the impeller doesnt come free like the plastic
the other side of the coin, if a plastic one comes free it normally just sits there and does nothing, if a metal one came off then yes engine damage COULD happen
There are a couple of reasons to replace a water pump when doing the cambelt change, here are the 2 main reasons
1, you have the belt off so your halfway there
2, it is showing signs of play and or there is a poweredery dust on the underside, a sure sign the pump is letting a little coolant weep
For me the reason i change pumps is because when a belt is fitted you set the belt tension to the correct amount, as the miles stack up the belt slackens off a little, so new belt on at the right tension on the belt and there is now a water pump bearing under pressure again, also if a pump has already done 60k your dreaming if you think it will do another 60k without problems
Get it changed
one other thing, i dont care what anyone say's YES pumps do have a warrenty, so you fit a new OEM 2 yr warrenty pump, after 18 months the impeller comes free, YOU WILL ONLY GET A NEW PUMP, THEY WILL NOT COVER ANY LABOUR, they could even claim it was fitted and tensioned incorrectly and not even cover the water pump
Ive been in the motor trade for 20yrs, ive seen many claims for parts made and very few ever get paid out, use genuine, have it fitted properly and if the pump lets rip, best you can hope for is a new pump, a massive £24 TPS trade, not worth the hassel of finding the invoice TBH
