Not familiar with the cooling on the Nissans but assume it is like others.
On the Johnny rude 9.9 - 15hp when you re-install the lower unit you have to be careful when lining up the copper pickup tube with the rubber grommet on the impeller housing. Sometimes the tube can pinch the grommet causing leakage there which could possibly be enough to divert water thru the leak as RPM's increase? If I remember correctly there were 2 different grommets in the OMC impeller kit - slight improvements made in newer models so it would be important to use the right one for your year model.
Does it have the wedge shaped key that locks the impeller onto the shaft? Although forgetting to install that key would probably prevent any water from reaching the tell tell and/or water jacket at any RPM? The shaft would just spin inside the impeller causing it to not pump water.
One other thing, has it been run in saltwater? If so there could be salt deposits/corrosion blocking the water passages. This would especially be a concern if run in saltwater and the p.o. was not diligent about flushing after use.
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1994 Wellcraft V21
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