Van is an '02 with 379K on it (ex-Pepsi truck) but engine was replaced not too many miles ago with what appears to be a used one. Seems to run quiet and smooth except a misfire on #3 - only at idle, when warm. Cool it doesn't so it, and when up to temp the misfire count on my scanner slows and stops by 1,200 RPM but the lope at idle says it's a dead miss when idling. The plug on the right is the offending cylinder - a little darker but nothing alarming. I've seen the exact same symptom in a Vortec 350 with a bad intake gasket, but it generated massive crankcase pressure because the leak was to the inside. That's not the case on this one, nor does it respond to carb. cleaner from the outside. Any advice? Head gasket or vacuum leak? Haven't done a compression check yet - that's gonna be time consuming given the vehicle it's in.