I am returning with a new problem involving the restoration of my Taurus SE 3.0.
In the past year I have replaced the plugs and wires, cap and rotor, fuel pump and filter, fuel pressure regulator, EGR pressure sensor, injectors, MAF sensor, and both heated oxygen sensors.
The engine runs rough and slow and will not idle!
I am getting contradictory OBD1 Codes!
157 MAF sensor circuit below minimum voltage
158 MAF sensor circuit above maximum voltage
172 Lack of heated oxygen sensor (HO2S-1) switches, indicates lean (Bank #1)
176 Lack of heated oxygen sensor (HO2S-1) switches, indicates rich (Bank #1)
I have checked my service manual and Powertrain Control/Emissions Diagnosis Service Manual and can find no instruction for handling these seemingly contradictory OBD1 Codes.
What do I do now?
I Think I’d Be Checking The MAF Sensor, TPS, And IAC For Intermittent Or Erratic Operation.
CSA
Proper diagnosis of those cars required a Break out Box, DVOM, test lights and ideally a digital lab scope. You can back probe the MAF and look for abnormal outputs and inspect the O2 sensors for damaged wiring or a failed heating element or the relay that powers the heating elements. Did you use direct replacement O2 sensors or the generic models that require splicing the wires? If the specific component tests show no problems the ECU is likely failing.
From Ford, discussing the 91 - 99 Taurus -
“MAF sensors can get contaminated from a variety
of sources: dirt, oil, silicon, spider webs, potting compound from the sensor itself, etc. When a MAF sensor gets contaminated, it skews the transfer function such that the sensor over-estimates air flow at idle (causes the fuel system to go rich) and under-estimates air flow at high air flows (causes fuel system to go lean). This means Long Term Fuel Trims will learn lean (negative) corrections at idle and learn rich (positive) corrections at higher air
flows.”
Some of the OBD1 DTCs associated with this:
" • 171, 172, 173 (HO2S11 lack of switching, Bank 1)"
" • 175, 176, 177 (HO2S21 lack of switching, Bank 2)"
" In the past year I have replaced the plugs and wires, cap and rotor, fuel pump and filter, fuel pressure regulator, EGR pressure sensor, injectors, MAF sensor, and both heated oxygen sensors. "
Did the rough and slow running and no idle begin before or after replacing all those items you’ve listed and if it was before replacing them, did any parts cause any change in the car’s operation ?
CSA
I think these DTCs are a symptom, not the root cause.
How’s the battery?
If it’s more than 5 years old, replace it.
Have you checked compression?
How’s charging system voltage . . . nice and steady at 14V - 14.5V
Have you at least cleaned the throttle body? I’ve seen many cars that wouldn’t idle worth a . . . . simply because the throttle body needed a good cleaning.
I see that you replaced the fuel pump, injectors, filter and regulator . . . have you verified that the fuel system is in fact working correctly? Just because something is new doesn’t in fact guarantee that everything is fine and working correctly.
One thing that these DTCs might have in common are voltage problems.
Bad alternator?
Here’s an idea:
Unplug the MAF and start the engine.
If it’s idling well, the new MAF is faulty.
With problems like this, 9 times out of 10 it’s a bad connection someplace… In modern automobiles, some circuits can go through 6 or 8 connectors and junction blocks… After 13 years, the reliability of automotive wiring is not what it once was…