The O-ring on the distributor, valve covers or the intake manifold. You can try snugging down the valve covers the hard ones are the two on the passenger rear if its automatic tranny the valve cover bolts are also the studs to hold the tranny dipstic in you need deep metric sockets to pull the bracket off first then snug down the valve cover bolts. It`s not uncommon for the intake manifold to leak out where the heads mate to the block in the valleys is another common leak. What you can try there is take a rag and clean that area real good with a good fast drying solvent.squirt a bead of permatex #2 in the areas all three meet head, block and intake and with your finger squish it in the valleys as best you can. What that does is makes like a plug inside the valley if you do it right. Have you changed your PCV lately if not change it with regular tune ups also change the breather filter inside the air filter housing that will slow down the leak also it keeps pressure from building up in the crankcase.
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