Have you used the recommended sealer?
The users manual says "Use GM Engine Coolant Supplement (sealer) with
any complete coolant flush and refill"
Cooling System Seal Tabs , part 1#3634621 are recommended for all
water coooled engines and aluminum blocks. The original vegetable
formula is designed for use in radiator systems to seal and prevent
leaks due to casting porosity in engine components.
