Any one know why some 240's specify Resistor spark plugs? My 88 does, my 84 does not. Both run fine as far as I can tell with non-resistor NGK BP6ES plugs. What is the intended purpose of a "Resistor" plug?

Thanks, Robert

Even the resistor plugs give some interference. I noticed that my portable phone picked up the ignition firing as I approached the car while it was running. Back in the 70s, my buddy put his not-street-legal bracket racing wires on his AMX...you couldn't come near the car with your own without the staccato in your radio.

"What is the intended purpose of a "Resistor" plug?"

A resistor plug, or resistor wires, damp the high voltage oscillations that would otherwise occur up 'n down the wire each time the plug fires. These oscillations radiate a tremendous amount of radio interference. The resistor wires -- or plugs -- dramatically reduce this source of radiated interference.

I don't know the answer to the question you haven't asked ("Well, if that's so, how come one of my Volvos specifies resistor plugs and the other does not?"). But an educated guess is that Volvo realized that resistor wires, as used on your earlier car, were found to be somewhat fragile and less reliable than the solid core wires used on your later car. By moving the resistors to the plugs, Volvo improved reliability.

You must be one of those attorneys that keeps asking the same question until you get the answer you want to hear......

Leonard