92 740...stuck open-loop?

  • Thread starter Thread starter geronimo
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geronimo

I replaced the O2 sensor as I was getting a code for bad O2
sensor. After disconnecting battery to reset computer, then all
trouble codes were cleared. I am getting a "service engine" light for
the first couple of minutes of operation...seems to be until the eng
gets up to temp....but there is no CHK ENG lite, and no codes set.
Before replacing the O2 sensor I was not getting this SERVICE ENG
lite, I had ONLY the CHK ENG lite.
Before changing sensor I was getting 16 mpg, and after it is still
getting 16 mpg! So my theory is that the car is still running
open-loop all the time. I vaguely remember checking dwell on a 81
Chevy years ago, you could tell when the car had gone closed-loop
because the dwell would change from a fixed value to a varying value.
So to confirm my idea, I would like to get a cheap dwell meter and
check what is going on. But how do I connect it to this engine? For
the CHevy, I think it connected to some signal wire on the
distributor.
 
geronimo said:
I replaced the O2 sensor as I was getting a code for bad O2
sensor. After disconnecting battery to reset computer, then all
trouble codes were cleared. I am getting a "service engine" light for
the first couple of minutes of operation...seems to be until the eng
gets up to temp....but there is no CHK ENG lite, and no codes set.
Before replacing the O2 sensor I was not getting this SERVICE ENG
lite, I had ONLY the CHK ENG lite.
Before changing sensor I was getting 16 mpg, and after it is still
getting 16 mpg! So my theory is that the car is still running
open-loop all the time. I vaguely remember checking dwell on a 81
Chevy years ago, you could tell when the car had gone closed-loop
because the dwell would change from a fixed value to a varying value.
So to confirm my idea, I would like to get a cheap dwell meter and
check what is going on. But how do I connect it to this engine? For
the CHevy, I think it connected to some signal wire on the
distributor.
Click to expand...

You want a digital multimeter, or even better, an analog VTVM, but you don't
see those much anymore. Connect right to the signal wire on the O2 sensor
and it should start bouncing once it goes closed loop.
 
Oh....OK.
I am not sure about doing that.... I just spliced in the new O2
sensor, got the wires nicely covered with black heat-shrink, and don't
want to mess with them...like to "y-in" a wire on the signal wire.
Guess I could open up the harness back along the firewall and y-in to
signal wire there instead.
Well, I was at some Volvo part online vendor a couple of days ago,
and they were offering a rich/lean gauge suitable for 740s. Don't
recall the name of co. now. But it is a $60 .00 gadget. It works by
reading the o2 sensor signal. Wish I had some idea of what the
signal value is when it is at optimum ratio! Maybe radio shack still
sells a cheapie micronta that is analog? The other thing to considder
is that if the circuit is high impedance, you couldn't put a
low-impedance analog meter on on it, as it would load the signal way
down. I think it is high-impedance. Geronimo
 
....noticed you did say "VTVM", not "VOM". Haven't seen one of those
hi-Z critters in ages. Geronimo
 
geronimo said:
...noticed you did say "VTVM", not "VOM". Haven't seen one of those
hi-Z critters in ages. Geronimo
Click to expand...


Yes, a VOM will put WAY too much load on the O2 sensor. I use a Fluke DMM
with an analog bargraph.

A standard O2 sensor doesn't have a specific voltage output for stoich, but
rather a "knee" if you graph the output. Lean of stoich you'll get close to
0v and rich of stoich you'll get around 0.8v. In correct operation it will
oscillate with the average being something around 0.5v.

Even with the O2 sensor disconnected you should get better than 16 mpg. Is
the coolant temp sensor ok? Bad sensor or wiring will make it run pig rich.
 
Haven't ever replaced or checked that. What does it read at ambient
temp if good?
 
geronimo said:
Haven't ever replaced or checked that. What does it read at ambient
temp if good?
Click to expand...


One that I picked up from a junkyard was bad, it read open circuit. The car
had a straight body in pretty good shape so I suspect that's what landed it
in the scrapper. Good one should read a few kilo ohms at room temp as I
recall and should respond significantly to temperature change. When they
fail, they usually go open.
 
I replaced the O2 sensor as I was getting a code for bad O2
sensor. After disconnecting battery to reset computer, then all
trouble codes were cleared. I am getting a "service engine" light for
the first couple of minutes of operation...seems to be until the eng
gets up to temp....but there is no CHK ENG lite, and no codes set.
Before replacing the O2 sensor I was not getting this SERVICE ENG
lite, I had ONLY the CHK ENG lite.
Before changing sensor I was getting 16 mpg, and after it is still
getting 16 mpg! So my theory is that the car is still running
open-loop all the time. I vaguely remember checking dwell on a 81
Chevy years ago, you could tell when the car had gone closed-loop
because the dwell would change from a fixed value to a varying value.
So to confirm my idea, I would like to get a cheap dwell meter and
check what is going on. But how do I connect it to this engine? For
the CHevy, I think it connected to some signal wire on the
distributor.
Click to expand...

Hey you guys he said it was the service light, it's a 740 the service
light comes on at pre set intervals and has to be reset. Theres a
little plug on the dash that comes out and you will need to press the
switch with a pen or pencil and that will turn off the light till the
pre-determined interval turns it on again, reminding you to service
the car.

B. Franklin
 
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