LS1 Water pump question?
 

So I'm in the process of trying to sort out an odd issue in my '98.

The car doesn't overheat, but when idling the temp gauge does creep up. The engine is equipped with a 160 degree thermostat, and the fan points have been set to come on at the appropriate times. The fans are running, and in good shape. The thermostat has been replaced and coolant flushed to see if that was the issue.

The odd bit is that with no other change, aside from adding about 500 rpm via slightly hitting the gas, the temperature begins to drop like a stone. At that point the radiator begins to shed heat and both the hoses become hot. Before that, the air from the radiator is lukewarm at best and the hoses just warm to the touch.

The water pump in the car is the older LS1 type, with the integrated thermostat/housing setup. I'm planning on replacing the radiator, which while not leaking is definitely on it's last legs and the auto trans cooler has been completely bypassed to an aftermarket cooler and left open to the elements. I do want to correct those issues, as well as properly mount a new transmission cooler, and I'm wondering if it would be worth my time to install a new style water pump while I'm in there.

Does the newer style pump flow better, especially at lower speeds? There had to be some reason they changed the design.

Not what you asked but sounds airbound or maybe the rare instance of impeller damaged/corroded.

Pretty reasonable certain it's not airbound, did the whole uphill idle with the cap off until flow deal, and topped everything off.

I'm thinking the impeller is FUBAR.