O2 sensors and code 41 & 91

your smog pump (air pump) is obviously driven by the pulley... but goes to a thermactor air bypass valve that opens with vacuum .... and is controlled by the tab vacuum solenoid ... then the air moves through the tubing to a second valve .... the second valve is a diverter ... thermactor air diverter and is also controlled with vacuum via the tad vacuum solenoid... these two solenoids are located on the inner fender wall under the hood on the passenger side, facing the fire wall... anyways when vacuum is applied in the first case it opens the valve and allows the air to flow through ... when it is closed it gets let out to the atmosphere... the diverter works the same way except it just diverts the air... to the heads or to the exhaust ...... anyways so it is important that you do not have a vacuum leak... because this could cause one of these things not to work... Some people have got about 16 in vacuum and that is supposedly good ... I am getting about 22.. I replaced my vacuum lines with a kit from mustangs unlimited for about 29.99. a bunch of different sized yellow vacuum lines and they come in a bunch of different colors.. allot of people forget to replace worn out cracked sometimes 20 year old vacuum lines. Vacuum gauges are cheap too!! So rule out that vacuum leak before moving on. Late Model Restoration also sells a kit with new o2's, tab, tad, and egr vacuum solenoids as well as relays etc. for around 89.99.ish

I would like to know how the exhaust gas recirculation valve works.. I know that there is a egr vacuum solenoid that opens and closes the egr valve that is located right above the tad and tab solenoids. When the valve opens... I know the exhaust gas is mixed with the air that is going through the intake... via the egr spacer... and I am assuming that is is coming from the heads somewhere.. anyone know where exactly... is it in the same place as where the tad diverts the air from the smog pump that connects to somewhere at the back of the engine??

humaxxx: If you have a vacuum leak, unmetered air is entering the intake manifold. That makes a lean mixture, because the computer cannot calculate the correct air/fuel mixture.

AnthonyR23 said:

I would like to know how the exhaust gas recirculation valve works.. I know that there is a egr vacuum solenoid that opens and closes the egr valve that is located right above the tad and tab solenoids. When the valve opens... I know the exhaust gas is mixed with the air that is going through the intake... via the egr spacer... and I am assuming that is is coming from the heads somewhere.. anyone know where exactly... is it in the same place as where the tad diverts the air from the smog pump that connects to somewhere at the back of the engine??

Click to expand...

Some review of how Theractor Air System (smog pump) works...

The Thermactor air pump (smog pump) supplies air to the heads or catalytic converters. This air helps burn the excess HC (hydrocarbons) and CO (carbon monoxide). The air supplied to the catalytic converters helps create the catalytic reaction that changes the HC & CO into CO2 and water vapor. Catalytic converters on 5.0 Mustangs are designed to use the extra air provided by the smog pump. Without the extra air, the catalytic converters will clog and fail.

The Thermactor air pump (smog pump) puts air into the heads when the engine is cold and then into the catalytic converters when it is warm. The Thermactor control valves serve to direct the flow. The first valve, TAB (Thermactor Air Bypass) or AM1 valve) either dumps air to the atmosphere or passes it on to the second valve. The second valve, TAD (Thermactor Air Diverter valve or AM2 valve) directs it to the heads or the catalytic converters. The air serves to help consume any unburned hydrocarbons by supplying extra oxygen to the catalytic process. The computer tells the Thermactor Air System to open the dump valve at WOT (wide open throttle) minimizing engine drag. The dump valve reduces the parasitic drag caused by the smog pump to about 2-4 HP at WOT.



Computer operation & control for the Thermactor Air System
Automobile computers use current sink technology. They do not source power to any relay, solenoid or actuator like the IAC, fuel pump relay, or fuel injectors. Instead the computer provides a ground path for the positive battery voltage to get back to the battery negative terminal. That flow of power from positive to negative is what provides the energy to make the IAC, fuel pump relay, or fuel injectors work. No ground provided by the computer, then the actuators and relays don't operate.

troubleshhooting
One side of the any relay/actuator/solenoid in the engine compartment will be connected to a red wire that has 12-14 volts anytime the ignition switch is in the run position. The other side will have 12-14 volts when the relay/actuator/solenoid isn't turned on. Once the computer turns on the clamp side, the voltage on the computer side of the wire will drop down to 1 volt or less.

In order to test the TAD/TAB solenoids, you need to ground the white/red wire on the TAB solenoid or the light green/black wire on the TAD solenoid.

To test the computer, you can use a test light across the TAB or TAD 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.


Some basic theory to clarify how the EGR system works

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