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Thats interesting that lean burn stays in longer when grille blocked. Like unchosen said, it's likely that for some emissions reason it's possible the ecu may drop out of lean burn to keep the cat or 02 sensors at operating temperature, but I thought that lean burn ran hotter which was why NOx is produced because of the higher combustion temperature.
OBD-2 honda's aren't as good with efficiency as OBD-1 versions, so it wouldn't surprise me if lean burn was kicking out for some emissions reason. I've found/seen increases in mileage and performance in every EK series civic that I've converted to OBD-1 or that my tuning buddy did. Stunning example was my turbocharged 1996 Civic HB. In completely stock trim the car got 28mpg in mixed driving conditions. I turbocharged it, converted it to OBD-1, added 2-1/2" exhaust, and front mount intercooler.
On our tuning runs, we dialed in the 'under boost" map, and then did a highway cruising run and were able to run the AFR to 16.0:1 before it started lean surging. Result was 31 mpg average from there on, requiring premium unleaded however. The converter, evap, and pvc system are all functional and still in place.
My bosses son bought the car and it's going on two years under his ownership just with regular maintenance.
It's obvious to me that OBD-2 sacrificed mileage and performance for something else, but I have no idea what. If you can find a crome pro tuner in your area that really knows their stuff it may be beneficial to convert. The adjustments and changes they can make are amazing, turn things off and on, rpm switches and map changes for nitrous, vtec, propane or water injection...wideband o2 sensor reading etc.
I put my tuned ecu to the test coming home from work one day. I blew a heater hose(different car) and lost ALL of my coolant. I didn't notice until I got home that the temp guage was pinned. The tuned ecu's maps were extended out so far that it had pulled back timing and was DUMPING in fuel to try to keep the motor cool, and I never heard a single "ping" of detonation. I let it cool overnight, fixed the hose and filled it up, and it ran like nothing happened.
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