How do you know the exhaust manifold is not corroded internally and leaking into #8 exhaust port? Have you taken it off, looked, and had the manifold pressure tested? This is the most likely cause.
Second most likely cause is a leaky head gasket. A cyl leak down test may be useful, lower readings on #8 can be cause to remove the head.
How do you know the exhaust manifold is not corroded internally and leaking into #8 exhaust port? Have you taken it off, looked, and had the manifold pressure tested? This is the most likely cause.
Second most likely cause is a leaky head gasket. A cyl leak down test may be useful, lower readings on #8 can be cause to remove the head.