If you really mean "underdrive" this means they spin SLOWER. I'd measure the crank and pump pulleys, get back onto the March website, and figure out what you have
If you overheated the thing enough that you now have some head/ gasket leakage, you might be getting combustion pressure injected into the coolant. In a case like this, you will NOT necessarily see anything special in the coolant, but there is a chemical test radiator shops have just for this purpose, it detects combustion products in the coolant.
