WARNING: This page is about a different car, the 2005 GMC Yukon XL, 2005 Chevrolet Suburban, and 2005 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.
Oil Out the Vent Tube
Checks
Cause
Pump Cover (206)
Cross channel leakage can pressurize the vent area.