How long should an intake manifold gasket (upper and lower) last?

Unfortunately the 3.1 litre engine has a manufacture flaw. The gasket is plastic and rubber. Over time the plastic crackes due the heat and the rubber seal fails. When you have the lower gasket replaced make sure you insist on a Felpro gasket. Felpro has redesigned that gasket. Instead of being plastic and rubber it is steel and rubber. Any Felpro number that has a T at the end of the part number represents of redsign of the gaskets to improve upon the OEM (original equipment manufacture) design. Hence T for trouble solver. I know it sucks that we are at the mercy of a bad design from the manufacture but it is what it is. I know because I had to change my gaskets at 58,000 miles.

Unfortunately, intake manifold gaskets on most GM V style engines are prone to failure. In most cases they begin to leak in the 60K - to 90K range.