Hi Everybody. First time poster, long time lurker. Just wanted to post this for anybody who runs into the same issue.

I've got an 88 Econoline that was recently converted from a 302 to a 351w after the 302 was toasted. It had an ongoing issue where it ran well under any kind of load but couldn't hold a steady idle. It would rev up and then die down to a stall before revving back up over and over if it didn't stall completely. The mechanic that did the swap made a half-hearted attempt at solving it (replaced the TPS) but after that didn't work, threw up his arms and assumed it must have been some compatibility issue with the 302 ECU.

Here's how it was eventually resolved:

1. Checked thoroughly for air leaks, discovered the upper intake manifold was cracked from being over-torqued. Replaced it but problem persisted.

2. While messing with the intake manifold, noticed that the fuel rail was kinked before the Y. At this point, it was pretty clear that my engine had been running lean as hell so it was no mystery why the 302 had failed. Replaced it but the problem persisted.

3. Replaced the IAC. Problem was somewhat improved when warm but still persisted.

4. Took it to an old pro Ford mechanic. He noticed it was running lean, flushed the injectors (probably pointless since I had them serviced during the engine swap) and also noticed that the IAC O'Rielly's had given me was the wrong one. Now it would idle fine when warm but was still rolling badly when cold. The mechanic mused that it might be the ECT was reading too hot and making it run lean.

5. Pulled it apart this weekend, replaced the ECT, and now it runs better than ever.

Hope that helps for anybody who has a similar issue. Thanks everybody for this forum - it's been a tremendous resource for troubleshooting and choosing the right parts for the swap.