The can is an '86 Monte Carlo LS with a 305 and computer controlled
Quadrajet (TPS + MCS). The O2 sensor reads at idle -2000 RPM ~.04v - .07v. I
don't know if that is incorrect or not, but my feeling is that it should be
reading higher given the sensor has a range of 0v - 1v. If I pump the
throttle several times very quickly it will read .9v - 1v, which is correct.
At idle and a raised RPM when it reads under .1v the mixture is definitely
rich. It smells pig rich out the exhaust, and with the sensor disconencted
while I was testing it even smoked a bit from being rich. The car does run a
bit smoother with the sensor plugged in though and doesn't smell as rich.
Either the sensor is reading false lean and the computer is compensating by
richening it up or the carb is not adjusted properly. I have gone through
the carb and it should be very close.
Is the sensor bad?? It looks like with the rust it's gonna be a pain to get
it out. I'm afraid I'll strip it or something. In any case, what should the
O2 sensor be reading for a normal mixture and for a somewhat rich mixture
and is my sensor bad?
I've done some more searching and found that stoichiometric mixture should
read .45 volts on an O2 sensor. So if the exhaust smells pig rich the O2
sensor should be reading higher than .45 volts. Since it reads under 1v
unless I pump a crapload of fuel down the carb with the accelerator pump I
would conclude the O2 sensor is bad. Would this be correct?
Not a car guy; just an old retired electrical guy who
Having forewarned you - my Haynes manual says (and I've
proven it) that you can easily check your o2 sensor.
Warm the car to normal operating temp; then shut it off.
Unplug the sensor. Connect a high impedance voltmeter to
the sensor. Postive to the sensor, negative on ground.
Set the meter to the closest you can come to 1 volt
(1000 mV) full scale.
Start the engine. At idle, the voltmeter should jump
around almost constantly and randomly between .1 volt
and 1 volt.
I replaced the sensor, and now it seems like the car runs even richer than
with the old sensor. I don't understand why this is. I read the new sensor
with my multimeter and it reads more appropriately like it should. It goes
back and forther above adn below .45 volts (.45 is stouch), though often
stays above .45 for a little while, bouncing around at the higher boltages
before dipping back down and coming back up.I'm struggling with this, I
don't understand why it is nto working properly.
As mentioned in my other reply, check the canister control valve.
Just for grins, also check and make certain that the air pump is
not dumping into the exhaust manifolds once the engine is fully
warmed. (closed loop)
Also, make sure the thermostat is working properly and the engine
isn't running too cool.
How does the coolant temperature sender work on this car? Is it an
open/closed on/off switch? Or does it cover a range of temperatures with
resistance? The gauge in the dash never moves, so I'd like to test the temp
sender to be sure it's working properly. The gauge stays all the way on the
low side. Also, the oil pressure gauge is pegged on the high side and never
moves. Is there a way I can make the computer think the engine is at
operating temp to ensure the sender is not an issue? I was gonna replace
today it since it's onyl a few bucks but I was afraid I would break it off
at the threads. It didn't want to come loose. I just let it soak in WD-40 so
if I need to replace it it will hopefully come out easier.
Also, the engine runs more roughly without the O2 sensor plugged in, so I am
assuming the computer is going into closed loop.
I will check the air pump and canister control valve.
Thanks a lot for all your help, you certainly know a lot about these cars. I
really appreciate it.
If it's got a gauge, it's variable resistance, if it's got a
warning light, it's an open/closed switch.
The gauge sender and the coolant temperature sensor are two
The gauge sender is a one wire sender and is mounted in one of
the cylinder heads between two spark plugs (either 1&3 or 6&8
The coolant temperature sender is a two wire sensor and is
mounted in the thermostat housing.
Verify coolant temperature with a thermometer or backprobe the
yellow wire to the coolant sensor with a DVOM with the engine
warmed up (driven for ten miles), the voltage should be appx .5 -
.7 volts before swapping parts.
If you feel that you have to swap a part , replace the thermostat
with a good quality 190* stat.
I wouldn't assume.
I was the driveability tech in a Chevy dealership the entire time
that GM used these carbs in the 80s.
Even after having rebuilt hundreds of them, I wouldn't attempt an
overhaul without having the proper gauges to set them up
properly, they -are- that finicky.
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