To change the valve seals, the heads don't have to come off. It does take a special tool to remove the rockers, and you need air pressure to hold the valves closed. I've done them in about 5 hours. Here's a link that shows how to do it.
UBB Error - Crownvic.net
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The EGR passage does plug up on the older ones, especially when they use oil. That's not a big job either.
