Open loop at WOT???

[Reiswig, George]

Greetings, experts in the black art of EFI!
	I have a question that I'm hoping someone on the list might be able
to answer.  My vehicle (1990 Isuzu Amigo, 2.6L 4ZE1 engine) has a strange
characteristic: if you floor the gas pedal under load, then back off on the
throttle, you actually feel more acceleration when the pedal is not floored.
A friend of mine monitored his O2 sensor output while this was happening,
and it would appear that, at WOT, the engine is running rich.  
	These engines have a 3-position throttle position sensor, which only
registers closed throttle, partly open, and wide open throttle.  According
to rumors I've heard, it is common for EFI engines to go to open loop mode
at WOT, and therefor run rich.  
	My questions:
	1.  Is that last statement true?
	2.  Why do manufacturers make it so?  What are the advantages?
(e.g. it's cheaper to make the engine run a little rich and prevent pinging
that way than it would be to put in knock sensors and code the ECU to listen
for pinging)

	Since I cannot subscribe to the mailing list, I would appreciate it
if replies were cc'd to my e-mail address.  Many thanks for any light you
could shed on this, particularly truthful light.

George Reiswig