If I disconnect the O2 sensor and bring the car to full operating temperature, should the ECM set a code? If not, does this indicate a faulty ECM or something else (the SES light works).

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1990 IROC 350
Mods: Too busy trying to make it run right to mod it.
Airfoil, Dynomax cat-back, MSD coil, 180 t-stat, Bald Eagle tires,
Hypertech fan switch, Accel 23# injectors, Holley module, ported plenum,
Ported Daytona Yellow stock base, Moroso valve covers, other stuff,
Streetdampr, Ruger P95DC, hot wife, new oil filter, !cats, !TBC, !AIR.
18.0 @ 85MPH since I'm one big-a$$ MF
"It's better to have and not need than to need and not have."

 

It sets the SES light on all the time on my '89. And does set a code.

May also try disconnecting the TPS as well, see if that lights up and if a code sets too.

 

Jethro,

If that is happening on your car, you either have a failing ECM, or you have a wiring problem that is allowing current leakage to ground in the O2 circuit. Enough dirt, water, and conductive "junk" on the connectors might bleed enough voltage to act as a valid signal.

Since the nominal voltage set at the sensor is about 450 mV, that voltage level is seen as a fully rich condition. When the sensor is exposed to oxygen, it becomes conductive and bleeds some of the bias voltage to ground, effectively lowering the bias voltage. When there is less oxygen present, the sensor is less conductive and the voltage increases.

If the ECM sees a varying voltage somewhere around the 150-900mV range, it will assume that the sensor is working. Voltage outside that range are still acceptable, but are at the fringes of the tolerance of the ECM.

It is because the sensor circuit is so sensitive, and the data is so critical to mixture control, that it is important to test or change the sensor on a regular basis.

In your case, you might be able to test this theory by measuring resistance to ground in the circuit with all components disconnected. If you read infinite resistance, I'd begin to suspect the ECM.

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Later,
Vader
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Justice and Freedom will Prevail

 

I kind of figured the ECM may be on its way out, thanks for the input guys. It will set a code with the MAP disconnected, but the car only acts up when it begins to warm up, making me suspect a closed-loop operation malfunction so I tried disconnecting the O2 and it ran exactly the same. I'll check out the circuit some and then likely be replacing the ECM.

------------------
1990 IROC 350
Mods: Too busy trying to make it run right to mod it.
Airfoil, Dynomax cat-back, MSD coil, 180 t-stat, Bald Eagle tires,
Hypertech fan switch, Accel 23# injectors, Holley module, ported plenum,
Ported Daytona Yellow stock base, Moroso valve covers, other stuff,
Streetdampr, Ruger P95DC, hot wife, new oil filter, !cats, !TBC, !AIR.
18.0 @ 85MPH since I'm one big-a$$ MF
"It's better to have and not need than to need and not have."

 

FYI - The ECM was on its way out. I replaced it with an Advance Auto Reman. unit and it runs much better now when fully warmed. I have been chasing this problem for some time now, so hopefully I'm through fooling with it for awhile. Thanks for the help, guys.