Cracked manifold affecting 15mph smog test? (CA)

[Dirty Dude]

Some may recall that I just got the Corolla a week ago. Well, I didn't make the PO smog it before buying and I guess that's alright because the car was cheap and I'm still in it for less than I could sell it for if I wanted. Anyway, it failed the first time for high hydrocarbon. I was honestly surprised because the car was well cared-for; I got a big fat folder with service records going back to the day the car was purchased. I went ahead and changed the oil, distributor cap, rotor, air filter, spark plugs, and added a quart of denatured alcohol to the 1/2 tank of gas. I got a pretest this time and the HC came down a lot, but it still failed by ~14ppm. $240 later it has a new 50-state Catco converter as well and passes smog right on the 64ppm HC limit. Carbon monoxide and oxides of nitrogen were well below the limits, indicating no excessive combustion temps and no incomplete burn. This was only on the 15mph test; the 25mph was quite clean in all cases.

The car doesn't use oil that I have noticed yet, doesn't smoke, gets good economy, has no codes, and just generally runs well. The only thing I haven't addressed yet is the cracked exhaust manifold. There is no audible leak, but I did see a shadow of soot on the heat shield. I'm wondering if it's drawing a little air in through the crack and causing the O2 sensor read lean so the ECU adds fuel. What do you all think? Has anyone had that happen?