4.6 liter number 8 cylinder has 30 % compression all others are good. Please help.

 

First remove the valve cover and make sure the valves are opening as much as the others. If the valves are opening like they should, you need to pull the head to know for sure what is causing the low compression. A leakdown test could show whether it's the rings/piston, exhaust valve, or intake valve. But, either way you need to rebuild the head and/or engine.

 

A borescope would help.