Experience has recently brought to light a point where overhauling a water pump unit can cause overheating in Sapphires and Star.

In my case, a new water pump bearing, seal and impellor were fitted and appeared to work satisfactorily when fitted during cold winter months. More recent warm weather has shown a certain amount of overheating, although no actual boiling with its attendant steam displays. Investigations showed the temperature gauge and sensor to be working correctly as was the thermostat and the radiator.

The trouble was actually pinpointed when the radiator was removed for back flushing. The block was similarly treated and the absence of any sediment meant that the trouble was either elsewhere – or the block was so bunged up as to render cleaning impossible. Such a state of affairs would certainly have caused more violent overheating and boiling with more devastating consequences.

At this point, the new water pump bearing and housing was removed from the pump body. It is not possible to check clearances between impellor and pump face with the unit assembled so a piece of plasticine was placed on the edge of one of the impellor blades and the housing refitted to the pump body. On removing the housing, the absence of marks on the plasticine showed that it had not touched the pump face so a larger piece was used and this showed that the space between impellor and face was about three sixteenths of an inch. This was too big and would render the pump virtually inoperative. Further investigation showed that the old impellor was deeper in the blade than the new item, a point I should have noted when fitting the new part.

The old impellor was fitted to the shaft since it was in good condition and all has since functioned properly. Needless to say, the unit was checked with plasticine before the reassembled unit was fitted to the car. The gap of two thousandths of an inch ensured that the pump would be most efficient.

It would appear that there are two types of impellor for these cars although the spares lists for 346 and Star quote only one number. To be certain of having the right part, check it with the old item before fitting – as I should have done! – and all will be well.

Peter Caunt