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I bought a "93 Merc grand marquis one year ago it has 56k on the 460v eight. when cold it misses and backfires until warm. I have replaced plugs, plug wires,fuel filter. seems to perform ok when hot.
http://www.autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/04/ee/9a/0900823d8004ee9a.jsp
You need to check the timing and the timing chain for wear. The above link will tell you how to inspect the chain to see if it's loose.
Big block ford engines have a habit of wearing things out quickly Re: rings and timing chains, If they are not maintained properly.
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