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What is the difference between the two pumps in the picture other than that raised bit with black rubber around it, and does it matter which one you fit?
water-pump-639x478-93.jpg
 

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The raised bit with the rubber on it is is an 'evaporation chamber' and was introduced fairly early on in the life of the K series engine.

It came about after Rover zgroup started getting an unusually high number of warranty claims for leaking water pumps on new cars. The pump seal is lunbricated by the coolant and as such the seal is designed to allow some coolant to seep between it and the pump shaft. When the engine is left standing for some time, the seal dries ou a bit and allows some seepage externally, and can be enough to drip off the bottom of the enginedamp patch that this leaves on the outside of the pump seal was making people think that the seal had failed and the pump was leaking when in fact that was not the case.

The evaporation chamber was introduced to catch this slight seepage and allow it to evaporate before any drips caused another false warranty claim.

All the Rover 25 and MG ZR would have been fitted with an Airtex water pump with the evaporation chamber when built, but to be honest, it doesn't really matter which one you fit. Some people prefer to fit the non-evaporation chamber one because they think a failed water pump seal may be hidden by the chamber collecting the leakage (although the chamber is so small, that any leakage from a worn/failed seal is likely to overwhelm it fairly quickly).
 
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