I have a 1975 GMC with 350, auto trans. Have been burning oil. Blue puff at start-up but donut otherwise see smoke. Replaced valve stem seals. No change. Checked, some oil in plugs 1&3. Did cylinder leak test with mechanic. All cylinders ok. Checked (intake) valve seals again, looked fine. Didnpt check exhaust valve seals (though these were replaced 6 mo. ago along with intakes. Cannot figure source of blue cloud. If oil were leaking from gasket below say manifold wouldn;t truck be smoking all the time, not just blue puff at startup....thought it was rings but since leak test checks out fine....I'm puzzled.
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