Power line filters are helpful but never underestimate the value of using a very heavy ground strap and making a solid, clean connection to the firewall. Poor ground connections are always a major nuisance in any electrical application (automotive or otherwise) and they are frequently overlooked. A wide, flexible braided strap should always improve the situation.
The spark plugs generate AC noise from DC-20 MHz, just like lightning in the sky. The heavier the ground strap, the lower its inductance (and resistance). The lower the inductance, the more effectively the noise is grounded to the return path (back to the battery).
Inductance is not measured with an ohm meter but it effectively creates resistance as frequency (ie: AC) is increased. Resistance is not dependent upon frequency so simply connecting a wire will not always guarantee a good ground.
Sorry, but you have an electrical engineer here...:snakeman: It is not as painful or complicated as it sounds but physics is not always easily explained. :evil2:
