1996 ford 460 motorhome code 312

Hey guys, looking for a little OBD1 knowledge. I have a 1996 Ford E-superduty, 14000GVW chassis motorhome, 460 engine. It is using way too much fuel at idle/medium load, like obnoxious amounts. When I ran codes, I got a 312.

My understanding of how the self test EEC checks for this is as follows: Open valve, look at O2 for temp change, then close valve, check again. Whats odd about my system, is it appears to have two isolated air injection systems, one for each bank. I'm thinking the codes for the 14000GVW 460 might be slightly different, and 311 is AIR not opening left, and 312 is AIR not opening right. Before I purchased this RV, the PO removed the Y-pipe exhaust and installed isolated true duals, no H or X pipe, no crossover. So, since there's only one O2, and it's only seeing one side now, is it possible that the EEC just doesn't see the other side opening because the O2 isn't in that sides flow? How much would this screw with the fuel tables at idle/medium load? I have verified that the system does in fact function on both sides, but the EEC doesn't think its working. Is it possible that since its not seeing the AIR on that side that its over fueling it? Also of note, at certain times of medium load, I get backfires out of the right bank, the side with the O2 sensor in it.

I'm thinking the O2 sensor may be done, but I can't really afford to just chuck parts at it. When I look at the live data on my scanner, I see STFT (short term fuel trim) at cold idle anywhere from 22% to 35%, never going over 55 even when hot. That seems like a pretty rich setup, even for a big thirsty 460. 

Anyone got some info for me? Thanks in advance!

-J0N

Look for TSBs for it.  I had a 94 460 in a superduty and that code was constant.  I think there was a bulletin about it and the fix was to delete the AIR system.  I think they put a chip in that ignored it and pulled the fuse for it.