Hello everyone, first post and it might be long winded but I wanna cover my bases here!
1995 f150 originally a maf 302, last summer I swapped a mildly built roller 351w into it from atk. 4r70w trans. The truck is my baby, I’ve had it for over 10 years now. In between engine swaps, the truck sat for 2.5 years. So when the new engine went in, I put all new motorcraft sensors in it, from the ects, iac, tps, to the ICM. Bosch o2 sensor, Bosch fuel pumps and motorcraft fuel filter. Also flow matched late 90s explorer 4 hole injectors, and a new fuel pressure regulator. The truck starts easy and runs down the road good, but it surges BAD at idle- ONLY in gear, like drive and reverse. It has a nice little lope in park and neutral. It’s bad enough to stall, and the lights will dim when the rpm’s dip making it undriveable in traffic and from going to reverse from drive. The cam that came with the 351 engine is stated by atk to work with the stock stall converter, I thought that initially is my issue but others have said they ran this engine on the same setup fine. No codes present. The only thing I cheaped out on is the distributor, it’s an oriellys brand set at 12° btdc. Stock intake and fuel system, yes I know that’s leaving a lot of room for improvement on the table. Any suggestions? Iac bypass plate?

 

Sounds like a vacuum leak. Use an unit propane torch around gaskets and such.