One of the problems I have found with w/pumps, in different brands of engines, is that there is too much gap between the impeller vanes & the housing. This gap leaves a space for the liquid to escape, instead of being captured by the vanes & pumped through the engine. Obviously, the larger this gap is, the more liquid escapes & the less efficient the cooling is. I weld a thin donut shape piece of steel to the impeller to close off the gap & this always gives a noticeable reduction in temp.
