From: elliott@cyberspace.com@INTERNET on Wed, Oct 30, 1996 1:01 AM
>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.
All CC computer controlled EFI engines go back to open loop at WOT because
the control system is not fast enough to keep up with the engine. I am not
sure about the magnum engines but my turbo Daytona goes closed loop at
approximately 150 degrees water temperature, I would not expect that the
magnum engine would be much different.
The O2 sensor puts out more voltage as the mixture gets richer in the range
of 0 to 1.1 volts. They also do not work until they reach about 600 degrees,
this is the reason for the heated units now used (three or four wire). The
sensor gets up to operating temperature quicker so the engine can go closed
loop sooner. The ideal air/fuel ratio is 14.5:1 and this equates to a .45
volt output on the sensor. In a closed loop condition the sensor should
continuely cycle around this value but never be steady at .45v. So the
computer knows that the sensor is not ready (verse not working) it applies a
bias voltage of .45 when the sensor is cold and non conductive.
For anyone interested it is easy enough to measure. You will need a volt
meter that has an input impedance of at least 1Meg Ohm per volt (most analog
do not meet this and most digital meters do). I do not recal the color codes
so you will need to probe the connector to find the right wire. Two will be
ground (one for the sensor and one for the heating element), one will be
switched 12 volts for the heating element and the other is the sensor output.
With the engine off and cold the sensor output will be zero. When the
engine is first started the voltage will go to a steady .45 as the sensor
warms up and starts to operate you should see the voltage start to change
initially it will go rich (some value over .5 volts) once the computer sense
the water temperature as in the operating range the computer will switch to
closed loop and the sensor value will start to cycle around .45 (.2 to .7
volt range). You can cause rich and lean conditions by; Plugging the PVC off
should cause a monetary rich condition (greater than .45 volts), pulling the
vacuumn line off the PCV value should create a momentary lean condition
(less than .45 volts). If you see these conditions all is in working order.
As far as engine mods go within reason the computer can compensate for open
exhaust, air intake mods, cams and such at at partial throttle openings when
the system is in closed loop mode. The biggest danger is at WOT when the
computer is telling the injectors to operate at a fixed duty cycle resulting
in a fixed amount of fuel being injected into the engine. The danger is the
mixture will lean out causing engine damage. The MP computer compensates for
this by dumping extra fuel in at WOT conditions. As modifications are made
to the engine there is a point where the injectors can no longer flow enough
fuel also resulting in a lean condition. Adjustable fuel pressure regulators
or higher flow rate injectors can be used.
The first thing I would do for the overheated engine would be disconnect the
battery for a minute or two to reset the computer back to the factory
defaults. Then if poor performance still exists check for proper operation
of the O2 sensor. The sensor could have been damaged.
Dave Clement
89 4x4 LE
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