I'm assuming you had a coolant leak before you had the water pump changed.
The coolant from the engine block to the water pump goes through passages in the timing cover. Pontiac and Buick engines are known for the gaskets to go bad and many times the water pump is blamed.
To check this area for leaks you need to clean the area around the timing cover as best you can. With the engine at operating temp, again with a mirror, check both sides of the cover where it meets the engine block. You may get a better view of the cover from down below. If you have a regular "radiator cap" you may be able to get a cooling system pressure tester and add pressure to the system to help find the leak. Let me know what you find.
Wednesday, April 1st, 2009 AT 1:09 PM