Yes, the IDM was bad. I traded him my bad one for his good one, and gave him $100.
The truck runs great now.
'88 F-250 6.9 IDI ZF-5 4x4 Standard Cab ?? Miles
'94 F-250 7.3 PSD ZF-5 4x4 Standard Cab 374k Miles
'67 Mustang 289 C4 "Who Cares?" Miles