From what I've read HHO offers a way more potent form of cumbustion then hydrogen alone
gasoline 76 ft/sec
hydrogen 78 ft/sec
Oxygen 0 ft/sec
HHO 8164 ft/sec
(these numbers are ball park)
Oxygen acts an accelerator
I don't see much advantage in burning only hydrogen
I have found that the only time the EFIE is needed in my 1990 Ford Bronco II, OBD1, is at idle, once the (TPS) throttle position sensor takes over the CPU runs in open loop.
Unplug your 02 sensor, warm the engine up and run it at 2500rpm, check the leads for current, if you find 0.9VDC or greater, your engine is running rich. Less then 0.2 your running lean.
Jager
1990 Bronco II 2.9L v6 (HHO) 22mpg
1994 Camery 2.3L i4 (HHO) 34mpg
1997 Ford Prope GTS 2.5L 24V V6 31mpg