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Am wondering how you have determined that water is leaking from the top of the impeller housing while pumping. Some of this “leakage” is normal and expected. Needless to say, excessive leakage from this area will diminish pump output to the powerhead. Excessive leakage is usually caused by grooving/pitting on the top surface of the impeller housing. Worn rubber on top of the impeller, poor impeller fit, or a damaged/distorted SS impeller plate will cause excessive leakage in this area as well. This is an area where it usually best to use OEM parts.
Like you say, there is no conventional seal on top of the water pump on this engine. The “seal” is the fit between the top of the impeller and the aluminum impeller housing. That being said, some leakage in this area is normal and expected.