Use some carb cleaner and spray around possible vacuum leak areas, one that comes to mind is around the intake manifold gasket possibly from the head gasket work. A change in idle as you're spraying should help isolate the leak. Anouth possibikity is an unplugged vac line somewhere less obvious.

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)