92 Gt High Nox Wont Pass Emissions; Help

How to pass emissions testing:

High NO - high combustion temps - retard timing, check EGR for operation.
High CO – Rich condition - fuel pressure too high, check O2 sensors, replace air filter, Clean MAF element.
High HC – Lean misfire, vacuum leak, common misfire due to worn or weak ignition system components. On rare occasions, an overly rich mixture may be the cause. Do the ethanol/E10 fill up as suggested.
High CO & HC - Cat converters, smog pump, and smog pump controls. Make sure the smog pump has good air output at 1200-1700 RPM

How to pass emissions testing:

1.) Make sure all the emissions gear the car was made with is present and connected up properly. That includes a working smog pump and cats. The smog tech will do a visual check to make sure that all the original equipment is present and connected up.

2.) Make sure that you have fresh tune up with spark plugs, plug wires, cap, rotor, fuel & air filters. An oil & filter change is a good idea while you are at it.

3.) [Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

Here's the way 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.

Post the codes you get and I will post 86-93 model 5.0 Mustang specific code definitions and fixes. I do not have a complete listing for 94-95 model 5.0 Mustangs at this time.

Be sure to turn off the A/C, and put the transmission in neutral when dumping the codes. On a manual transmission car, be sure to press the clutch to the floor.
Fail to do this and you will generate a code 67 and not be able to dump the Engine Running codes.





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.



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.

89 through 95 cars have a working Check Engine light. Watch it instead of using a test lamp.



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.


WARNING!!! There is a single dark brown connector with a black/orange wire. It is the 12 volt power to the under the hood light. Do not jumper it to the computer test connector. If you do, you will damage the computer.

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. This is crucial: the same wire that provides the ground to dump the codes provides signal ground for the TPS, EGR, ACT and Map/Baro sensors. If it fails, you will have poor performance, economy and driveablity problems

Some 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.

Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, and clutch (if present) is pressed to the floor, and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

Your 86-88 5.0 won't have a working Check Engine Light, so you'll need a test light.
See AutoZone Part Number: 25886 , $10




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.


Dumping the Engine Running codes: The procedure is the same, you start the engine with the test jumper in place. Be sure the A/C is off, clutch depressed to the floor and the transmission is in neutral. You'll get an 11, then a 4 and the engine will speed up to do the EGR test. After the engine speed decreases back to idle, it will dump the engine running codes.
Cylinder balance test
Warm the car's engine up to normal operating temperature. Use a jumper wire or paper clip to put the computer into test mode. Start the engine and let it go through the normal diagnostic tests, then quickly press the throttle to the floor. The engine RPM should exceed 2500 RPM's for a brief second. The engine RPM's will increase to about 1450-1600 RPM and hold steady. The engine will shut off power to each injector, one at a time. When it has sequenced through all 8 injectors, it will flash 9 for everything OK, or the number of the failing cylinder such as 2 for cylinder #2. Quickly pressing the throttle again up to 2500 RPM’s will cause the test to re-run with smaller qualifying figures.
Do it a third time, and if the same cylinder shows up, the cylinder is weak and isn’t putting out power like it should. See the Chilton’s Shop manual for the complete test procedure



4.) Post the codes and get help to fix them. Don’t try to pass with codes not fixed. Clearing the computer just temporarily removes them from memory, it doesn’t fix the problem that caused the code to be set.

5.) Be sure to do the testing on a hot engine. Drive for 15-20 minutes prior to taking the test to get operating temps up into the normal range. Do not shut off the engine while waiting for your turn on the test machine. An engine up to full operating temperature puts out fewer emissions.



Some basic theory to clarify how things work is in order…

EGR System theory and testing

Revised 29-Sep-2013 to add code definitions for EGR sensor and EVR regulator.

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. It does this by reducing the amount of air/fuel mixture that gets burned in the combustion process. Less air from the intake system means less air to mx with the fuel, so the computer leans out the fuel delivery calculations to balance things out. This reduces combustion temperature, and the creation of NOx gases. The reduced combustion temp reduces the tendency to ping.

The computer shuts down the EGR system when it detects WOT (Wide Open Throttle), so the effect on full throttle performance is too small to have any measurable negative effects.

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.

The resistor packs used to fool the computer into turning off the CEL (Check Engine Light) off are a bad idea. All they really do is mess up the data the computer uses to calculate the correct air/fuel mixture. You can easily create problems that are difficult to pin down and fix.



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. A defective or missing sensor will set codes 31 (EVP circuit below minimum voltage) or 32 ( EGR voltage below closed limit).

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. A defective EVR will set codes 33 (insufficient flow detected), 84 (EGR Vacuum Regulator failure – Broken vacuum lines, no +12 volts, regulator coil open circuit, missing EGR vacuum regulator.)

To test for blockage of the EGR passages, remove the EGR (2 bolts) and place a length of hose in the EGR passages. The hose has to fit snugly in the passages to seal good. Blow on the other end of the hose. If it passes air with no problems, there is no blockage in the EGR passages. If it blocks one passage and passes air through the other passage, someone has installed a manifold gasket the block the flow. There are two places this can happen. The upper to lower intake manifold gasket and the lower manifold to head gasket.

The upper to lower manifold gasket is easy to check and fix. You remove vacuum lines, throttle cables, coolant hoses, air duct and disconnect the 2 electrical connectors. Then remove the the 6 bolts that hold the upper to the lower manifold together. Two of the bolts are hidden under the 5.0 HO cover plate on top of the upper intake manifold. The upper manifold can be lifted off at that point. the EGR passage is the center passage in both upper and lower intake manifolds. Some gaskets block off the center hole for the EGR passage.

The lower intake manifold uses a similar method to block the EGR passages. You will need Chilton or other service manual to assist in removing & reinstalling the lower intake manifold.



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 or see the EGR test jig drawing below. Connnect the test jig or to directly to manifold vacuum.

Do not connect the EGR test jig to the EVR (Electronic Vacuum Regulator).

Apply 5in vacuum to the valve. Using the test jig, use your finger to vary the vacuum

If the engine stumbled or died then EGR Valve and passage(there is a passageway through the heads and intake) are good.

If the engine did NOT stumble or die then either the EGR Valve is bad and/or the passage is blocked.

If the engine stumbled, connect EGR test jig to the hose coming off of the EGR Valve.
Use your finger to cap the open port on the vacuum tee.
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 the EVR has failed

EGR test jig


To test the computer and wiring to the computer, you can use a test light across the EVR wiring connectors and dump the codes. When you dump the codes, the computer does a self test that toggles every relay/actuator/solenoid on and off. When this happens, the test light will flicker. If the test light remains on the computer or the wiring is suspect.

To check the EVR to computer wiring, disconnect the EVR connector and connect one end of the Ohmmeter to the dark green wire EVR wiring. Remove the passenger side kick panel and use a 10 MM socket to remove the computer connector from the computer. Set the Ohmmeter to high range and connect the other ohmmeter lead to ground. You should see an infinite open circuit indication or a reading greater than 1 Meg Ohm. If you see less than 200 Ohms, the dark green wire has shorted to ground somewhere.