After putting the upper intake manifold and valve covers back on my 92 5.0 AOD it is not wanting to idle lower than 1500 and is a pain to get to run. Cranks every time and fires, runs ten seconds stumbles and dies. If I keep at it it will eventually start and run smooth but at a high idle. I have cleaned the IAC and checked it by removing the wiring with the engine running and a very small drop in idle occurs. Cleaned and checked every other wire and vacuum line I can find and tested the fuel pressure regulator as described here with nothing throwing a red flag. I have sprayed around the engine trying to find a vacuum leak to no avail. If I were to get up tomorrow morning and try to start it it will crank but not start without a little pedal and will stumble and backfire. After at least 20 tries it will eventually fire and run at 2000 rpm for 2 seconds and then drop to 1500 and run flawlessly at that idle speed. With it eventually having the ability to start and run without backfires or any misses I was thinking the no start checklist may not apply here. I was thinking that perhaps the previous owner may have "modded" it by setting the idle higher or some other awful thing. Any ideas here? Also it is a catless H pipe now so exhaust smells rich to me, but richer than I would have expected.