addiction2boost

Vacuum leak around intake manifold gasket? Maybe manifold itself if its a cheap knock-off.

s2klariat

This. Check to see if previous owner installed Hondata intake manifold gasket, I've heard of these going bad and causing huge vacuum leaks.

mc-integra111

Thanks for all the help and suggestions, I appreciate it a lot. There are many replies, so let me address a couple of questions first, but I believe a friend of mine and I got it figured out.
- Cleaned the IACV with brakeleen, rag, and a little air. Had no effect (either good or bad).
- Good idea with the intake manifold gasket. I forgot to include this in my list of tests. It checked out OK.
- I did not clean the MAP hole path, but I will now, thanks. After I found out I could use the Haltech MAP sensor instead, I had been doing that. I have since went back to the original MAP sensor and everything is fine, so I think it was fine. Your idea is a good one for maintenance anyway.
- There were no codes thrown, so the ECU was no help. I did want to try a known good factory ECU, but instead just got the Haltech (I'll need it later when it's a race car, so i figured what the hell).
- It did turn out to be cylinder specific (see below).

Problem identified: My friend suggested pulling each individual coil off the plug while the engine idled and see what happens. When I pulled the coil for Cyl #3 the engine was steady (it missed, duh, but was steady). When I pulled any other one, it was very unsteady. Conclusion, cylinder 3 was intermittently missing. With it "unplugged", a steady miss. With any other, you got a steady miss and an intermittent miss. It turned out that the #3 spark plug ceramic was cracked (worse than any I've seen). A new spark plug has since solved the problem.

My friend and I have 2 possible theories. He goes for 2, I think it's 1.
1) Since one of my very first attempts at a solution was new spark plugs, either one of them was bad from the factory or I "broke" it without realizing it (dropped it, too tight, etc). I always give new parts a cursory examination, but my theory is the initial crack was very small and difficult to find, but as soon as the engine warmed up, the crack developed. Since I had convinced myself the new spark plugs were good, I was chasing a ghost until my friend basically started from scratch.
2) My friend thought the likelihood of a bad spark plug without realizing it is rare, so his thought is that one of my other tests was the actual problem. Before I was able to find the exact problem, the problem condition contributed to cracking a new spark plug. Now, replacing the bad spark plug solved the problem, since the other problem has been resolved. I think this is a sound theory, but unless I am willing to undo or swap parts to try and "break" the engine (I'm not gonna do this), I can't prove this. His personal thought is the crank sensor was bad and with that now replaced, the "new" new plug will not crack.