Quote:

How does a cracked head cause a misfire?



I guess if there is
a really bad crack
running from valve seat to valve seat across the ridge,
so much gas would leak past the non-sealing valves,
that compression would really low,
and there could be a missfire
even though air, fuel and spark were at spec

Factory Magnum cylinder head cracks
are common across the thin ridge
of the heart shaped combustion chamber.
These cracks are usually tight and shallow,
with the majority not penetrating to the coolant passages.

A "dynamic compression test"
with a noid light connected in that cylinder instead of the fuel injector,
would spot low compression
with the added benefit of
spotting possible valve, spring, and rocker arm problems.

http://www.misterfixit.com/dyncompr.htm

Magnum rocker arms have been known to go bad, where the push rod meets its ball socket.