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the only wire that could possibly be the sti has constant 12v
the only wire that could possibly be the sti has constant 12v
koeo test problem
- Thread starter jssblues
- Start date
the only wire that could possibly be the sti has constant 12v
If the wire has 12 volts, it is the power for the under hood light. The plastic connector shell is dark brown. It is next to a black connector shell that joins one wiring harness to another wiring harness. If you try and use it to dump codes, you may damage the computer.
The STI has a gray connector shell and a white/red wire. It comes from the same bundle of wires as the self test connector.
Dump the codes and see what the computer says is wrong…Codes may be present in the computer even if the Check Engine light isn’t on.
Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great. You watch the flashing test lamp or Check Engine Light and count the flashes.
See Troublcodes.net Trouble Codes OBD & OBD2 Trouble Codes and Technical info & Tool Store. By BAT Auto Technical
If your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.
89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.
Do not jumper anything to the single dark brown connector. It is the power for the under the hood light. It is in another bundle of wires near the windshield wiper.
What to expect:
You should get a code 11 (two single flashes in succession). This says that the computer's internal workings are OK, and that the wiring to put the computer into diagnostic mode is good. No code 11 and you have some wiring problems.
Codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.
Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.
Alternate methods:
For those who are intimidated by all the wires & connections, see Actron® for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.
Or for a nicer scanner see EQUUS DIGITAL FORD CODE READER (3145) – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $30.
The test connectors are in the picture I posted. The connector drawing is as close as possible to the actual connctor shapes. The connectors are normally stored in a plastic cover on the driver side fender.
Code 13 - Key on Engine off - ISC did not respond properly (extends to touch throttle then retracts for KOEO) – ISC
Key on Engine running - Idle Speed Control motor or Air Bypass not controlling idle properly (generally idle too high)
If your idle is above 725 RPM, the computer will set this code. Normal idle speed is 650-725 RPM. Higher
than that may mean that you have a vacuum leak or someone has mechanically set the idle speed by use of the idle speed screw.
This effectively disables to computer’s ability to control idle speed.
Code 21 – ECT sensor out of range. Broken or damaged wiring, bad ECT sensor.
Note that that if the outside air temp is below 50 degrees F that the test for the
ECT can be in error.
The ECT sensor has absolutely nothing to do with the temperature gauge. They are
different animals. The ECT sensor is normally located it the RH front of the engine in
the water feed tubes for the heater.
The ACT & ECT have the same thermistor, so the table values are the same
ACT & ECT test data:
Use Pin 46 on the computer for ground for both ECT & ACT to get most accurate
readings.
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.
Voltages may be measured across the ECT/ACT by probing the connector from
the rear. A pair of safety pins may be helpful in doing this. Use care in doing it
so that you don't damage the wiring or connector.
Here's the table :
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 Mustang FAQ - Engine Information
Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg
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 1.5 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 2-5 in 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 2- 5 in 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 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.
Code 41 is a RH side sensor,
code 91 is the LH side 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. The O2 sensor
voltage should switch between .2-.9 volt at idle.
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.
If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you
are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives.
Replace the tubing and check the PVC and the hoses connected to it.
A secondary problem with only a code 41 is for cars with an intact smog pump and cats. If the tube on the
back of the heads clogs up the driver’s side, all the air from the smog pump gets dumped into one side.
This excess air upsets the O2 sensor calibration and can set a false code 41. The cure is to remove the
crossover tube and thoroughly clean the insides to that there is no carbon blocking the free flow of air to
both heads.
Code 67 - clutch not depressed (5 speed) or car not in neutral or park (auto) or A/C in On position
when codes where dumped. Possible neutral safety switch or wiring problem. This code may prevent you
from running the Key On Engine On tests. You can generally ignore this code, since it has no effect on
engine performance.
The computer wants to make sure the A/C is off due to the added load on the engine for the engine
running tests. It also checks to see that the transmission is in Neutral or the clutch depressed (T5, T56,
Tremec 3550 & TKO)). This prevents the diagnostics from being run when the car is driven. Key On Engine
Running test mode takes the throttle control away from the driver for several tests. This could prove
hazardous if the computer was jumpered into test mode and then driven.
The NSS code 67 can be bypassed for testing. You will need to temporarily ground computer pin 30 to the
chassis. Computer pin 30 uses a Lt blue/yellow wire. Remove the passenger side kick panel and then
remove the plastic cover from the computer wiring connector. Use a safety pin to probe the connector
from the rear. Jumper the safety pin to the ground near the computer.
Be sure to remove the jumper BEFORE attempting to drive the car!!!
Code 84 EGR Vacuum Regulator failure – Broken vacuum lines, no +12 volts, regulator coil open
circuit. The EVR regulates vacuum to the EGR valve to maintain the correct amount of vacuum.
The solenoid coil should measure 20-70 Ohms resistance. The regulator has a vacuum feed on the bottom
which draws from the intake manifold. The other vacuum line is regulated vacuum going to the EGR valve.
One side of the EVR electrical circuit is +12 volts anytime the ignition switch is in the run position.
The other side of the electrical circuit is the ground path and is controlled by the computer.
The computer switches the ground on and off to control the regulator solenoid.
Removing the EGR without having a custom tune to delete it from the computer's program Is not a good
idea. You can end up running lean at cruse and a resultant ping or spark knock.
Some basic theory to clarify how things work is in order…
The EGR shuts off at Wide Open Throttle (WOT), so it has minimal effect on performance. The addition of
exhaust gas drops combustion temperature, increases gas mileage and reduces the tendency of the engine
to ping. It can also reduce HC emissions by reducing fuel consumption. The primary result of EGR usage is a
reduction in NOx emissions.
The EGR system has a vacuum source (line from the intake manifold) that goes to the EVR, computer
operated electronic vacuum regulator. The EVR is located on the back of the passenger side shock strut
tower. The computer uses RPM, Load. and some other factors to tell the EVR to pass vacuum to open the
EGR valve. The EGR valve and the passages in the heads and intake manifold route exhaust gas to the EGR
spacer (throttle body spacer). The EGR sensor tells the computer how far the EGR valve is open.
Then computer adjusts the signal sent to the EVR to hold, increase or decrease the vacuum.
The computer adds spark advance to compensate for the recirculated gases and the slower rate they burn at.
Troubleshooting:
There should be no vacuum at the EGR valve when at idle. If there is, the EVR (electronic vacuum
regulator) mounted on the backside of the passenger side wheelwell is suspect. Check the vacuum line
plumbing to make sure the previous owner didn’t cross the vacuum lines.
Diagram courtesy of Tmoss & Stang&2birds. (the diagram says 88 GT, but the EGR part is the same for 86-93 Mustangs)
The EGR sensor is basically a variable resistor, like the volume control on a radio. One end is 5 volt VREF
power from the computer (red/orange wire). One end is computer signal ground (black/white), and the
middle wire (brown/lt green) is the signal output from the EGR sensor. It is designed to always have some
small voltage output from it anytime the ignition switch is the Run position. That way the computer knows
the sensor & the wiring is OK. No voltage on computer pin 27 (brown/lt green wire) and the computer
thinks the sensor is bad or the wire is broken and sets code 31. The voltage output can range from
approximately .6-.85 volt.
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 2-5 in 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 2-5 in vacuum
Late Model Restoration has the Ford Racing M-12071-N302 kit with the EGR valve & sensor along with the ACT & ECT sensors for $45.
See 86-93 SENSOR KIT, 5.0L EFI, INCLUDES EGR VALVE & SENSOR, COOLANT TEMP SENSOR, & AIR CHARGE TEMP SENSOR MUSTANG for more details
Key on Engine running - Idle Speed Control motor or Air Bypass not controlling idle properly (generally idle too high)
If your idle is above 725 RPM, the computer will set this code. Normal idle speed is 650-725 RPM. Higher
than that may mean that you have a vacuum leak or someone has mechanically set the idle speed by use of the idle speed screw.
This effectively disables to computer’s ability to control idle speed.
Code 21 – ECT sensor out of range. Broken or damaged wiring, bad ECT sensor.
Note that that if the outside air temp is below 50 degrees F that the test for the
ECT can be in error.
The ECT sensor has absolutely nothing to do with the temperature gauge. They are
different animals. The ECT sensor is normally located it the RH front of the engine in
the water feed tubes for the heater.
The ACT & ECT have the same thermistor, so the table values are the same
ACT & ECT test data:
Use Pin 46 on the computer for ground for both ECT & ACT to get most accurate
readings.
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.
Voltages may be measured across the ECT/ACT by probing the connector from
the rear. A pair of safety pins may be helpful in doing this. Use care in doing it
so that you don't damage the wiring or connector.
Here's the table :
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 Mustang FAQ - Engine Information
Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif
Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif
Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg
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 1.5 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 2-5 in 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 2- 5 in 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 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.
Code 41 is a RH side sensor,
code 91 is the LH side 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. The O2 sensor
voltage should switch between .2-.9 volt at idle.
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.
If you get only code 41 and have changed the sensor, look for vacuum leaks. This is especially true if you
are having idle problems. The small plastic tubing is very brittle after many years of the heating it receives.
Replace the tubing and check the PVC and the hoses connected to it.
A secondary problem with only a code 41 is for cars with an intact smog pump and cats. If the tube on the
back of the heads clogs up the driver’s side, all the air from the smog pump gets dumped into one side.
This excess air upsets the O2 sensor calibration and can set a false code 41. The cure is to remove the
crossover tube and thoroughly clean the insides to that there is no carbon blocking the free flow of air to
both heads.
Code 67 - clutch not depressed (5 speed) or car not in neutral or park (auto) or A/C in On position
when codes where dumped. Possible neutral safety switch or wiring problem. This code may prevent you
from running the Key On Engine On tests. You can generally ignore this code, since it has no effect on
engine performance.
The computer wants to make sure the A/C is off due to the added load on the engine for the engine
running tests. It also checks to see that the transmission is in Neutral or the clutch depressed (T5, T56,
Tremec 3550 & TKO)). This prevents the diagnostics from being run when the car is driven. Key On Engine
Running test mode takes the throttle control away from the driver for several tests. This could prove
hazardous if the computer was jumpered into test mode and then driven.
The NSS code 67 can be bypassed for testing. You will need to temporarily ground computer pin 30 to the
chassis. Computer pin 30 uses a Lt blue/yellow wire. Remove the passenger side kick panel and then
remove the plastic cover from the computer wiring connector. Use a safety pin to probe the connector
from the rear. Jumper the safety pin to the ground near the computer.
Be sure to remove the jumper BEFORE attempting to drive the car!!!
Code 84 EGR Vacuum Regulator failure – Broken vacuum lines, no +12 volts, regulator coil open
circuit. The EVR regulates vacuum to the EGR valve to maintain the correct amount of vacuum.
The solenoid coil should measure 20-70 Ohms resistance. The regulator has a vacuum feed on the bottom
which draws from the intake manifold. The other vacuum line is regulated vacuum going to the EGR valve.
One side of the EVR electrical circuit is +12 volts anytime the ignition switch is in the run position.
The other side of the electrical circuit is the ground path and is controlled by the computer.
The computer switches the ground on and off to control the regulator solenoid.
Removing the EGR without having a custom tune to delete it from the computer's program Is not a good
idea. You can end up running lean at cruse and a resultant ping or spark knock.
Some basic theory to clarify how things work is in order…
The EGR shuts off at Wide Open Throttle (WOT), so it has minimal effect on performance. The addition of
exhaust gas drops combustion temperature, increases gas mileage and reduces the tendency of the engine
to ping. It can also reduce HC emissions by reducing fuel consumption. The primary result of EGR usage is a
reduction in NOx emissions.
The EGR system has a vacuum source (line from the intake manifold) that goes to the EVR, computer
operated electronic vacuum regulator. The EVR is located on the back of the passenger side shock strut
tower. The computer uses RPM, Load. and some other factors to tell the EVR to pass vacuum to open the
EGR valve. The EGR valve and the passages in the heads and intake manifold route exhaust gas to the EGR
spacer (throttle body spacer). The EGR sensor tells the computer how far the EGR valve is open.
Then computer adjusts the signal sent to the EVR to hold, increase or decrease the vacuum.
The computer adds spark advance to compensate for the recirculated gases and the slower rate they burn at.
Troubleshooting:
There should be no vacuum at the EGR valve when at idle. If there is, the EVR (electronic vacuum
regulator) mounted on the backside of the passenger side wheelwell is suspect. Check the vacuum line
plumbing to make sure the previous owner didn’t cross the vacuum lines.
Diagram courtesy of Tmoss & Stang&2birds. (the diagram says 88 GT, but the EGR part is the same for 86-93 Mustangs)
The EGR sensor is basically a variable resistor, like the volume control on a radio. One end is 5 volt VREF
power from the computer (red/orange wire). One end is computer signal ground (black/white), and the
middle wire (brown/lt green) is the signal output from the EGR sensor. It is designed to always have some
small voltage output from it anytime the ignition switch is the Run position. That way the computer knows
the sensor & the wiring is OK. No voltage on computer pin 27 (brown/lt green wire) and the computer
thinks the sensor is bad or the wire is broken and sets code 31. The voltage output can range from
approximately .6-.85 volt.
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 2-5 in 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 2-5 in vacuum
Late Model Restoration has the Ford Racing M-12071-N302 kit with the EGR valve & sensor along with the ACT & ECT sensors for $45.
See 86-93 SENSOR KIT, 5.0L EFI, INCLUDES EGR VALVE & SENSOR, COOLANT TEMP SENSOR, & AIR CHARGE TEMP SENSOR MUSTANG for more details
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