Code 33 - Insufficient EGR flow detected.
Look for vacuum leaks, cracked vacuum lines, failed EGR vacuum regulator. Check to see if you have 10”
of vacuum at the EGR vacuum connection coming from the intake manifold. Look for electrical signal at
the vacuum regulator solenoid valves located on the rear of the passenger side wheel well. Using a
test light across the electrical connector, it should flicker as the electrical signal flickers. Remember
that the computer does not source any power, but provides the ground necessary to complete the
circuit. That means one side of the circuit will always be hot, and the other side will go to ground
or below 1 volt as the computer switches on that circuit.
Check for resistance between the brown/lt green wire on the EGR sensor and pin 27 on the computer:
you should have less than 1 ohm.
See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host)
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
http://www.veryuseful.com/mustang/tech/engine/images/88-91eecPinout.gif
EGR test procedure courtesy of cjones
to check the EGR valve:
bring the engine to normal temp.
connect a vacuum pump to the EGR Valve
apply 5in vacuum to the valve.
if engine stumbled or died then EGR Valve and passage(there is a passageway through the heads and intake) are good.
if engine did NOT stumble or die then either the EGR Valve is bad and/or the passage is blocked.
if engine stumbled, connect vacuum gauge to the hose coming off of the EGR Valve
snap throttle to 2500 RPM (remember snap the throttle don't hold it there).
did the vacuum gauge show about 5in vacuum?
if not, check for manifold vacuum at the EGR vacuum valve.
if you have manifold vacuum then connect vacuum gauge to the EGR valve side of the vacuum valve and snap throttle to 2500 RPM.
should read about 5in vacuum
The operation of the EGR vacuum regulator can be checked by using a test light applied across the
wiring connector. Jumper the computer into self test mode and turn the key on but do not start
the engine. You will hear all the actuators (including the EVR vacuum regulator) cycle. Watch for
the light to flicker: that means the computer has signaled the EGR vacuum regulator successfully.
Code 34 Or 334 - EGR voltage above closed limit - Failed sensor, carbon between EGR pintle valve and seat holding the valve off its seat. Remove the EGR valve and clean it with
carbon remover. Prior to re-installing see if you can blow air through the flange side of the EGR by
mouth. If it leaks, there is carbon stuck on the pintle valve seat, replace the EGR valve ($85-$95).
If the blow by test passes, and you have replaced the sensor, then you have electrical ground problems.
Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white
wire on the EGR and the same wire on the TPS. It should be less than 1.5 ohm. Next check the
resistance between the black/white wire and the negative battery post. It should be less than 1.5 ohm.
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.
Let’s put on our Inspector Gadget propeller head beanies and think about how this works:
The EGR sensor is a variable resistor with ground on one leg and Vref (5 volts) on the other. Its’ resistance
ranges from 4000 to 5500 Ohms measured between Vref & ground, depending on the sensor. The
center connection of the variable resistor is the slider that moves in response to the amount of vacuum applied.
The slider has some minimum value of resistance greater than 100 ohms so that the computer always
sees a voltage present at its’ input. If the value was 0 ohms, there would be no voltage output. Then the
computer would not be able to distinguish between a properly functioning sensor and one that had a
broken wire or bad connection. The EGR I have in hand reads 700 Ohms between the slider (EPV) and
ground (SIG RTN) at rest with no vacuum applied. The EGR valve or sensor may cause the voltage to
be above closed limits due to the manufacturing tolerances that cause the EGR sensor to rest at a
higher position than it should.
This will affect idle quality by diluting the intake air charge
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.
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
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.
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
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 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.
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.
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.
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
Probable cause:
Code 51 and code 34 are both for sensor voltages above the perscribed limit. That generally means
the signal ground is not working correctly.
How it is supposed to work:
The black/white wire (pin 46) is signal ground for the computer. It provides a dedicated
ground for the EGR, Baro, ACT, ECT, & TPS sensors as well as the ground to put the
computer into self test mode. Since it is a dedicated ground, it passes through the computer
on its way to the computer main power ground that terminates at the battery pigtail ground.
It should read less than 1.5 ohms when measured from anyplace on the engine harness with
the battery pigtail ground as the other reference point for the ohmmeter probe. What sometimes
happens is that it gets jumpered to power which either burns up the wiring or burns the trace
off the pc board inside the computer. That trace connects pins 46 to pins 40 & 60. Only an
experienced electronics technician can open the computer up & repair the trace if it burns
up and creates an open circuit.
How to test the wiring :
Measure the resistance between the black/white wire on each of the following sensors: TPS,
ECT, ACT, EGR and MAP/BARO. If you find one that is greater than 1.5 ohms, measure between that
sensor and pin #1 of the white 10 pin connectors. Pin #1 is the center pin and is labeled
sig-rtrn on the diagram
