Sorry I didn't read your post more carefully. The extra codes are a definite clue to your problem.
CODE: 31 (KOEO) - EVP circuit below minimum voltage. Vref (5 volt reference voltage supplied
by the computer) missing or broken wire or bad connection in circuit. Use a DVM to check for
5 volts on the orange/white wire. If it is missing, look for +5 volts at the orange/white wire
on the TPS or MAP sensor located on the firewall near the center of the car. Use the black/white
wire for the ground for the DVM.
With the sensor removed from the EGR and still connected, press the plunger and watch
the voltage change on the brown/lt green wire. Pull the passenger side kick panel and
measure the voltage at the computer. You will need to remove the plastic cover over
the wires and probe them from the backside. A safety pin may prove very useful for this task.
Use pin 27, EVR input (brown/lt green wire) and pin 46, signal ground (black/white wire) to
measure the voltage. The orange/white wire is Vref and should always be 5 volts -/+ .25 volt.
Be sure to measure Vref at the EGR sensor to rule out any broken wires or bad connections.
Code 51 Engine Coolant Temperature (ECT) sensor signal is/was too high -
Possible bad ECT sensor, or wiring. Possible missing signal ground –
black/wire wire broken or bad connection. With the power off, measure the
resistance between the black/white wire and battery ground. You should see
less than 1 ohm. Check the same black /white wire on the TPS and MAP
sensor. More than 1 ohm there and the wire is probably broken in the harness
between the engine and the computer. The 10 pin connectors pass the
black/white wire back to the computer, and can cause problems.
Pin 7 on the computer - ECT signal in. at 176 degrees F it should be .80 volts
Voltages may be measured across the ECT by probing the connector from the rear.
Use care in doing it so that you don't damage the wiring or connector.
50 degrees F = 3.52 v
68 degrees F = 3.02 v
86 degrees F = 2.62 v
104 degrees F = 2.16 v
122 degrees F = 1.72 v
140 degrees F = 1.35 v
158 degrees F = 1.04 v
176 degrees F = .80 v
194 degrees F = .61
212 degrees F = .47 v
230 degrees F = .36 v
248 degrees F = .28 v
Ohms measures at the computer with the computer disconnected, or at the sensor with the sensor disconnected.
50 degrees F = 58.75 K ohms
68 degrees F = 37.30 K ohms
86 degrees F = 27.27 K ohms
104 degrees F = 16.15 K ohms
122 degrees F = 10.97 K ohms
140 degrees F = 7.60 K ohms
158 degrees F = 5.37 K ohms
176 degrees F = 3.84 K ohms
194 degrees F = 2.80 K ohms
212 degrees F = 2.07 K ohms
230 degrees F = 1.55 K ohms
248 degrees F = 1.18 k ohms
Diagram courtesy of Tmoss & Stang&2birds
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring
http://www.veryuseful.com/mustang/tech/engine/
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
Code 54 – ACT sensor out of range. Broken or damaged wiring, bad ACT sensor.
Note that that if the outside air temp is below 50 degrees F that the test for the ACT can be in error.
Check the resistance of the black/white wire to battery ground. If it is less than 1.5 ohm,
it is good. If it is more than 1.5 ohm, the black/white wire has bad connections or a broken wire.
Always take resistance measurements with the circuit powered off.
Then check the resistance of the ACT sender located in the #5 intake runner on most 5.0 stangs.
ACT & ECT test data:
The ACT & ECT have the same thermistor, so the table values are the same
Pin 7 on the computer - ECT signal in. at 176 degrees F it should be .80 volts
Pin 25 on the computer - ACT signal in. at 50 degrees F it should be 3.5 volts. It is a good
number if the ACT is mounted in the inlet airbox. If it is mounted in the lower intake manifold,
the voltage readings will be lower because of the heat transfer. Here's the table :
68 degrees F = 3.02 v
86 degrees F = 2.62 v
104 degrees F = 2.16 v
122 degrees F = 1.72 v
140 degrees F = 1.35 v
158 degrees F = 1.04 v
176 degrees F = .80 v
194 degrees F = .61
Ohms measures at the computer with the computer disconnected, or at the sensor with the sensor disconnected.
50 degrees F = 58.75 K ohms
68 degrees F = 37.30 K ohms
86 degrees F = 27.27 K ohms
104 degrees F = 16.15 K ohms
122 degrees F = 10.97 K ohms
140 degrees F = 7.60 K ohms
158 degrees F = 5.37 K ohms
176 degrees F = 3.84 K ohms
194 degrees F = 2.80 K ohms
Code 66 MAF below minimum test voltage. Insufficient or no voltage from MAF.
Dirty MAF element, bad MAF, bad MAF wiring, missing power to MAF. Check for missing
+12 volts on this circuit. Check the two links for a wiring diagram to help you find the red wire for
computer power relay switched +12 volts. Check for 12 volts between the red and black
wires on the MAF heater (usually pins A & B). while the connector is plugged into the MAF.
This may require the use of a couple of safety pins to probe the MAF connector from the back side of it.
There are three parts in a MAF: the heater, the sensor element and the amplifier. The heater
heats the MAF sensor element causing the resistance to increase. The amplifier buffers the
MAF output signal and has a resistor that is laser trimmed to provide an output range
compatible with the computer's load tables.
The MAF element is secured by 2 screws & has 1 wiring connector. To clean the element,
remove it from the MAF housing and spray it down with electronic parts cleaner or non-inflammable
brake parts cleaner (same stuff in a bigger can and cheaper too).
The MAF output varies with RPM which causes the airflow to increase or decease. The increase
of air across the MAF sensor element causes it to cool, allowing more voltage to pass and
telling the computer to increase the fuel flow. A decrease in airflow causes the MAF sensor
element to get warmer, decreasing the voltage and reducing the fuel flow. Measure the MAF
output at pins C & D on the MAF connector (dark blue/orange and tan/light blue) or at pins
50 & 9 on the computer.
Be sure to measure the sensor output by measuring across the
pins and not between the pins and ground.
At idle = approximately .6 volt
20 MPH = approximately 1.10 volt
40 MPH = approximately 1.70 volt
60 MPH = approximately 2.10 volt
Check the resistance of the MAF signal wiring. Pin D on the MAF and pin 50 on the computer
(dark blue/orange wire) should be less than 2 ohms. Pin C on the MAF and pin 9 on the
computer (tan/light blue wire) should be less than 2 ohms.
There should be a minimum of 10K ohms between either pin C or D on the MAF wiring
connector and ground. Make your measurement with the MAF disconnected from the wiring harness.
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)
for help on 88-95 wiring
http://www.veryuseful.com/mustang/tech/engine/
Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
Fuel pump, alternator, ignition & A/C wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
Computer,. actuator & sensor wiring
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
Fuse panel layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif
Vacuum routing
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg
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. Many times the end result is an engine that runs pig
rich and stinks of unburned 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
Testing the O2 sensors
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.
Note that all resistance tests must be done with power off. Measuring resistance
with a circuit powered on will give false readings and possibly damage the meter.
Do not attempt to measure the resistance of the O2 sensors, it may damage them.
Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are
disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer.
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
Replace the O2 sensors in pairs if replacement is indicated. If one is weak or bad, the other one probably isn't far behind.
What to look for to fix your problem:
The 51 & 54 codes both point to a bad computer signal ground. This would allow the
voltage to be higher that it should be normally.The black/white wire is signal ground for
the TPS, EGR, ACT, ECT and BARO signals. The BARO sensor hasn't poped up a
code, so the problem is in the engine wiring harness, most likely in the 10 pin connectors.
Clean the 10 pin connectors with electronic parts cleaner or non-inflammable brake
parts cleaner (same stuff in a bigger can and cheaper too). The white connector center pin
marked SIG-RTN in the diagram is the black/white wire signal ground. Use a test light
with one lead connected to the battery and the other to the black/white wire on
the TPS or ECT. You will probalbly need to disconnect the sensor to get a good connection.
Wiggle the wiring and the 10 pin connectors and watch to see if the light flickers. If it still
flickers after cleaning the 10 pin connectors, you have a broken black/white signal ground wire.
MAF code 66: Do you have a cold air intake system other than the factory stock system?
These can cause problems and the fix is often to rotate the MAF housing to smooth
out the airflow through the MAF.
If you measured the MAF output voltage across the C & D pins like it is supposed to be
done and got .86 volts, you may have wiring problems. Never connect the voltmeter
leads to ground when checking the MAF. Disconnect the MAF connector and then
access the computer by removing the passenger side kick panel and disconnect the computer connector.
There is a 10 MM bolt that holds it in place. Measure the resistance between pin 9
on the compute and pin C on the MAF wiring harness connector. Measure the resistance
between pin 50 on the compute and pin D on the MAF wiring harness connector. Both should
read less than 1.5 ohms. More than than that is an indication of wiring problems.
The following is a view from the computer side of the computer connector.
The code 91 is probably a vacuum leak. However, it may disappear after you fix the MAF problems.