Would this cause a code for lean condition?

[Zero_chance]

Alright heres the story. Helping a friend in the afternoons at my work. He has codes on his 88 for both O2s reading lean. Before he brought it to me, he bought new Bosch O2 sensors, figuring they were just old, etc. Well he cleared the codes and a few weeks later he pulled codes again for a different problem. The lean codes were there and a code for the TAD solenoid I believe. Maybe the TAB too, he wasnt bvery through in his explanation. I checked the Thermactor system and air is always being sent to the heads, never dumped or downstream. Now would this make the O2s read lean or should we be hunting elsewhere? Hes leaning toward getting rid of the thermactor system and plugging the heads because he doesnt wanna replace the solenoids. Im gonna get a better look at it tomorrow but Id appreciate any input. Never hurts to ask. Thanks.

 

[jrichker]

I would check the wiring.

Here's the tech note for the 41 & 91 codes...

Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.
The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts