My Grand Prix has the 3.1L. I haven't had a leaky intake manifold gasket, but in both of the engines I've had in it the rubber O-ring on the cap that covers the distributor hole in the intake manifold has leaked. I believe that's what caused a spun bearing in the orignal engine, because the oil leaked out and I didn't know it until I was going around a corner a couple times and the oil pressure dropped to zero momentarily. In my case it's $3 worth of parts and 30-45 minutes of work to fix it. This problem is common in all early to mid 90's 3.1L GM engines. Go to
http://www.w-body.com/ if you want to know more.
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