Working on my 1987 E-250 w/ a 302 EFI and AOD trans. Just finished pulling the motor, did head gasket and most accessory parts (water pump, oil pump, distributor cap, etc). I put the motor back in and wired everything up, engine runs okay at idle in park or neutral. When it’s shifted into drive or reverse though it bogs down a bit and makes a “popcorn”ish popping sound at a random interval. It sort of sounds like a backfire (loud) but doesn’t come from the exhaust. Doesn’t seem to be directly tied to rpm but gets worse with load/throttle. Any ideas for what could be causing this? Thanks in advance

 

Would you say it sounds like it is firing in the intake, sometimes described as a cough ?

Did you check the ign. timing?