EGR - How does it work? Is mine broke?

yes since the EGR uses vacuum to move it the diaphram must hold the valve open when you close the vacuum port. If it snaps right back then the diaphram is bad and that makes your EGR bad.

Allen

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. A custom chip or Tweecer tune is the only way to disable the EGR in the computer’s programming.

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.

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 http://www.latemodelrestoration.com/iwwida.pvx?;item?item_no=M12071N302 1&comp=LRS for more details

The EGR valve is like a balloon that deflates when vaccum is applied and the EGR opens to recirculate the exhaust gasses. If you have a hole in the "balloon" it will not deflate and the EGR will stay closed. I have a used EGR I can sell you cheap if you want it or just go to autozone.
BTW If your valve is bad you probably have a vaccum leak.

it takes almost no vacuum to open it, you can do it, put a piece of tubing on thur and suck in, you will see it move. it you take off the white thing on the back of the egr, the sensor that tells the computer everything, it should be closed, meaning the spring should not be compressed, if it is, push it back closed(push it in) leave the white thing unplugged for a couple of days until you get a new unit, dont forget to plug the vac line!!

Not to hijack, but...

Suppose your ports on the back of your heads were clogged and your metal tubing from them got broken. Suppose you turned that metal tubing around and bolted it back on to the heads effectively sealing the ports on the back of the heads and then you removed the other tubing, etc. You plugged the vac lines and left the EGR sensor and vac hooked up. What would happen?

If if the check engine light isn't throwing a code, it would seem as though the EVR would still tell the EGR to work and then the computer would advance the timing, correct? If this did, in fact, happen, wouldn't the car ping or have driveability issues?

I'm still confused and I've tried to learn this half-assed several times. Does the thermactor metal tubing bolted to the back of the heads carry exhaust gasses from the heads? So then how exactly does it get back up to the EGR spacer? The only lines I see running to the EGR spacer are the rubber hoses from the lower intake to the EGR spacer and then from the EGR spacer to the heater pipe, which appear to be coolant lines to cool the exhaust gasses that mysteriously appear in the EGR spacer

And how does the exhaust get to the metal tubing on the back of the heads? I realize there are ports in the heads, but is there a little port by the exhaust valve so when the exhaust valve opens, the gasses go into the little port and then also out through the headers? Are the ports all the way through the heads or just on the rear two cylinders (#4 and #8)?

I think I'm confusing the EGR with the smog equipment. I can't be the only one confused here, so lets get this straighted out in a somewhat comprehensive and straight-forward manner and maybe make this a sticky

The EGR and Thermactor Air System are two different and toatally separate systems.

Thermactor Air System
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.

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.

EGR system:
The EGR system uses the passages cast into the heads and intake manifold to route exhaust gas to the EGR control valve. The computer uses the EGR sensor to control how much vacuum is applied to the EGR valve. More vacuum opens the EGR valve to allow more exhaust gas recirculation, less vacuum means less exhaust gas recirculation. Read my post on EGR theory and operation on this page.