Yes. By disconnecting the O2 sensor, the computer will see the loss of activity on this input,
revert to Open Loop operation, and also set the SES light. (Assuming you are not in Field Service Mode.)
NOTE: If you do this my expectation would be that in Field Service Mode you should only
see constant, nonstop fast blinks from the SES light, indicating Open Loop operation.
****
TBI OPEN LOOP operation review:
Just a quick recap on how simple the TBI system is conceptually. It is referred to as
a Speed Density system. In it's essence it monitors how fast the engine is turning
(the Reference signal from the dizzy sensor) and how deep it's breathing (by
monitoring how close the intake manifold is to atmospheric pressure via the
Manifold Absolute Pressure {MAP} sensor.)
The computer uses these 2 primary inputs to address the correct spot in the
fueling tables to retrieve just how many milliseconds
** the injector needs to be pulsed
in order to deliver the 'correct' calibrated fuel shot.
1) So, for
passable steady state operation, all that is absolutely needed is the MAF sensor
and the Reference (rpm) signal. You could even have a 'locked out' spark advance set
for a set amount of timing. (Say, 32° total advance.)
2) Of course, I failed to mention that the above setup only works best at a single, specific
ambient (and engine) temperature. So, in order to give this super-simple setup the ability to work as
well in sub-freezing temperatures or in AZ heat, we'll add the ECT (Engine Coolant Temperature)
and possibly a further optimization IAT (Intake Air Temp) in additional tables in order to refine the
base fuel calculation as necessary to compensate for temperature changes.
3) But while OK steady state operation at a single rpm is OK to support a steady load on a generator,
if we take this engine and put it into a car or truck, then the engine will be expected to
run well during the transitions from one rpm and load to a different rpm & load.
4) So in order to optimize the transient response, then the engineers added a TPS. (throttle position sensor)
And after spending maybe 10x as much time & effort in this area, (vs the steady state calibration) we
now end up with an engine that is responsive, even snappy when the human depresses the gas pedal.
For inputs, we're now up to the following inputs telling the computer what's going on:
* Reference Voltage
* MAP
* ECT
* TPS
5) The timing is still locked at 32°. And the computer is *expecting* (but can't directly monitor) the fuel
pressure to be a constant, not a variable on the input side to the injector. As well as expecting the
fuel injectors to work as designed no matter how many millions of cycles that accrue on them over the years.
(2000 rpm x 4 injections per rpm = 8,000 injections per minute. So how many injections occur on a highway
cruise for a single hour? 8,000 x 60 = a cool 480,000 fuel injections. Wow. (!)
Of course, all the sparky types have been reading the above, and stewing that we have yet to
take the locked-down 32° ignition timing and optimize it for everything from starting to cruising
to pulling a long grade while fully loaded. So we are going to start the engine on the base timing
that the mechanic set up by twisting the TBI's HEI distributor.
Once we get over ~400 rpm, then computer starts constantly adjusting the ignition timing in order
to get the Air/Fuel mixture to be expanding the most right when the piston is ~10°
after TDC.
(Top Dead Center) More lookup tables.
6)And the Knock Sensor(s) are also added in order to dial this back a bit to protect the engine if too
much advance is calculated for the conditions / bottom tier gas is being fed to the machine.
7) Of course, there's always one more thing. ~80% of the air that surrounds us is made up of
Nitrogen. And Nitrogen is inert, doesn't burn like Oxygen. But if we subject Nitrogen to combustion
temps in excess of ~700°F, it will break down into Oxides of Nitrogen. (NOx) And this is the main
pollutant of photochemical smog. (The first time I drove into the soup surrounding LA during a
thermal inversion it was a real eye-opener.)
So the engineers came up with a mechanism to dilute the intake charge with a small percentage of
inert gas sourced from the exhaust stream, and this is how we keep our engine's combustion temps
close, but
not over this magic limit.
To recap, the computer is now juggling the following variables:
* Reference Voltage
* MAP
* ECT
* TPS
* Spark Timing / Ignition Advance
* KS
* EGR control
Sure, there's now quite a bit going on. Bur the Reference voltage & the MAP inputs are the key,
and all the rest are just optimizations piled on top. But there's a case for each of them to be
included into your engine bay.
****
Guess what? All this is going on without the O2 sensor connected. (!) Welcome to Open Loop operation.
Just no BLM's. No crossover counts, no worries about exhaust leaks, using the wrong silicone sealers,
or leaking antifreeze into the exhaust stream.
Just like our old carbureted engines. The carb was a mechanical fueling computer, substituting brass jets,
metering rods, air bleeds, and needle & seat for code in the computer and injectors being electrically pulsed.
And of course a vacuum advance can working in parallel with spring-loaded centrifugal weights to alter the
timing advance for the variables of load and rpm.
To reiterate, Closed Loop is how the engineers were able to take a set of emissions controls that might
pass the test for 7,000 miles Open Loop, and by local error detection/compensation get these same parts
to pass emissions for 70,000 miles in order to meet Federal Clean Air Laws. A bit of a oversimplification,
but essentially the concept is valid.
So this is why we want to fix the engine & the sensors so that they will run correctly in Open Loop operation.
And then we want to reconnect the Closed Loop and make it resume operation this 10x better/cleaner/longer
operation. Or if you are watching the crossing-counts, this is equal to how many times per second the engine
is being fine-tuned while you are driving. At 5 crossings per second on the highway, that's 300 tunings per
minute. And that's ~18,000 fine tunings for every hour spent on the highway. (!)
****
So, sure, disconnect your O2 sensor and see if your engine drives alright. Of course we no longer
have the blinky lights to work with...but if you have a OBD1-compatible scan tool we can still
watch the live data. So I guess the plan would be to confirm where your engine is getting the
fueling wrong & see if you can match up any driveability issues to these A/F anomalies.
Then disconnect the O2 sensor and see if this makes your truck drive the same, better, or worse.
And if necessary, we'll eventually take it to the next level using live data. (Seems like you are
experimenting with a phone app for live data?) But the tighter we can define the problem the
easier it will be to use the live data to figure out what part(s) are no longer working to spec.
PS - Here is where we stop and be thankful that we aren't working with the 'even cleaner/tighter'
Vortec setup, adding a MAF sensor, 8 individual injectors, downstream O2 sensors to monitor
the conversion efficiency of the cats, P1345 CMP/CKP issues, failure prone crab cap, etc. :0)
****
**For a point of reference, a single human eye blink lasts anywhere from .1 to .4 seconds.
This is 100 to 400 milliseconds. In comparison, at idle our fuel injectors may be pulsed
from ~1 to 4 milliseconds. Injectors shoot fuel more like camera shutters than human
blinking. Very fast, very precise when all is working to plan. (!)
EDIT: Here's some neat info & injector waveforms from the Snap-On site. (Injector signals)
