Open loop/closed loop has to do with whether or not the oxygen sensor is
controlling the engines air-fuel ratio. In open loop the input from the
oxygen sensor is not monitored, the engine just runs on preset controller
programming. In closed loop the controller monitors oxygen sensor input.
After reading the manual it looks like Kuk and I were wrong when we said
that if the engine ran too cool that it ran in closed loop. It'll run in
open loop if the thermostat is opening too soon either because a thermo-
stat with too low an opening temp has been installed or the thermostat is
malfunctioning. In closed loop the O2 sensor adjusts the fuel/air ratio,
which if nothing else will improve gas mileage. The engine will probably
run better too because it won't be too rich. With a free flowing exhaust
and air cleaner closed loop would possibly give more power, but mileage
would go down the tubes. Ya pays yer money and ya takes yer choices.
Larry Elliott, '92, 3.9, 4x4, 5 sp'd, LE
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