Well it's grounded from the negative terminal straight to the cylinder head with 4 gauge battery cable. Maybe I'm wrong but why would it still have to be grounded to the frame? Im thinking it may be corroded where it is bolted on the head or the cable is broken inside the sheathing.
Several reasons..
1. When the car was built it had at least 2 wire links grounding the engine to the frame
2. When the starter is activated a significant current pull (>100 amps) is made from the battery so if there are missing and/or poor grounds the voltage will travel in strange ways
3. A unitized ground system is required for a proper running ignition/electronic plus minimizing electrical noise interference
4. Even more important in todays sophisticated computer car systems
Many users fail to understand how to wire/maintain a reliable electrical system, as long as it works its OK...
Just my $0.02...
