EGR Systems: Operation and Diagnosis
Posted 12/18/1997
By Henry Guzman
Last month, we covered the reasons why exhaust gas recirculation (EGR) systems were created, which is primarily to control and reduce nitrogen oxide (NOx) formation, and secondarily to improve engine performance with ping control. We covered the evolution of EGR systems and ended with a look at how onboard diagnostic (OBD-II) EGR systems operate.
This month, we will detail the operation and testing of EGR systems. It's helpful to remember that if a hard trouble code has been set, the power control module (PCM) has determined that a gross emission failure has occurred. Gross failure is defined as excessive exhaust emissions more than 1 1/2 times what the car was designed to produce, according to the Federal Test Procedure (FTP). This can happen if too little or too much EGR is flowing. In the case of an EGR code, that would be 1 1/2 times the NOx that is allowable. Even if the car appears to run well, excess smog is being produced. An EGR failure will not only cause excess NOx, back up strategies will often entail enriching the mixture or retarding the timing. This in turn increases HC and CO emissions. Even if you don't see it on a 4-gas exhaust analysis test, the emissions are being created and the catalytic converter will not always cover them up.
EGR valves and components
To diagnose EGR systems properly, it's important to understand how they work and what kind of communication they have with PCMs. Knowing this will help you understand a flow chart and wiring diagram, and to come up with a test strategy that you are able to complete with your available tools and equipment.
Let's start with that golden oldie, the single diaphragm EGR valve. It consists of a spring-loaded diaphragm that is connected to a pintle and seat by a slender steel shaft. Normally closed by spring tension, as it receives ported vacuum, the diaphragm rises, which pulls the pintle off its seat and enables exhaust to flow into the valve's chamber and then on to the intake manifold. To test this component, use a hand-held vacuum pump connected to the vacuum nipple to raise and hold the diaphragm. About eight inches of vacuum should do the trick. The valve should hold vacuum and raise the pintle in a linear fashion. When the engine is idling, pumping it up should stall the engine. This type of valve may or may not have vacuum modulation. Remember, vacuum modulation to the EGR is a vital ingredient of good driveability and precise NOx control. This type of EGR valve is used with a thermal vacuum switch and maybe an inline vacuum delay valve.
The positive back pressure EGR valve can be identified by the letter "P" stamped next to the part number and date code. A back pressure valve is easy to spot because its pintle shaft is much thicker than the single diaphragm type. This is so because the shaft is hollow. The hollow design allows exhaust gases to flow into the shaft and push up on it. When positive back pressure in the exhaust system is sufficient, the shaft raises up and seals the built-in control valve. Once the control valve is closed, it allows applied vacuum to pull up on the diaphragm. Without back pressure to lift the hollow shaft and close the control valve opening, the EGR valve will not hold vacuum. It is bled off to the atmosphere. This design thus modulates EGR flow by modulating the applied vacuum. As engine load increases, so does engine back pressure, which causes the control valve inside the EGR to trap vacuum and open up. To test this valve, bring the engine up to 2,000 rpms to create back pressure, then apply vacuum. EGR should open and cause a 100 rpm drop or more. Exhaust leaks or a modified exhaust system can create havoc here. Adding dual exhaust or headers on a car designed for a single exhaust will reduce back pressure and set a Code 32 on GM cars. Positive back pressure EGR valves are used in simple vacuum controlled systems, as well as more complex pulse width modulated applications.
EGR solenoids are used with all types of EGR valves, especially back pressure type valves. The EGR solenoid will have two or more vacuum lines and an electrical connector. The solenoid also has an air bleed and sometimes an air filter. Vacuum is bled off through the filter vent. The PCM uses the solenoid to regulate vacuum to the EGR valve. The vacuum can be manifold or ported vacuum. The solenoid is a vacuum switch with inlet and outlet vacuum ports. The PCM calculates intended EGR flow from various other inputs and then sends a pulsed "on/off" signal to the solenoid. No vacuum flows until commanded by the PCM. This signal turns the vacuum on and off in rapid succession. This is called "pulse width modulation." If the filter becomes clogged, the vacuum cannot bleed off and too strong a signal will be sent to the valve. If that happens, the EGR valve will open too much and cause a driveability problem.
Remote vacuum transducers
All manufacturers use them, but they are very popular with Toyotas and other Asian cars. Shaped like a flying saucer with three or more vacuum ports, they modulate vacuum by using manifold and ported vacuum against each other along with an exhaust back pressure input. The result is a carefully controlled vacuum signal to the EGR valve that is mechanically modulated by engine load. Your best bet with these is to study the vacuum diagram on the underhood emissions label. Make sure the vacuum hoses are in good condition and properly routed. Many of these units have air filters also. You can clean them out to prevent too much EGR flow.
The negative back pressure EGR valve is identified by the letter "N" and looks similar to the positive back pressure EGR valve. The valve is opened by a combination of applied engine vacuum to the control valve and negative exhaust system pulses that happen as each exhaust valve closes. As soon as the pintle opens, back pressure is reduced slightly, which opens a control valve vacuum bleed and then the valve quickly closes. In this manner, EGR is modulated by negative exhaust system pulses. To test it, apply vacuum with a hand pump when the engine is off. The valve should open and hold vacuum.
Integrated electronic/mechanical EGR valves
This type of valve has different names with each manufacturer. It is easily identified because it has a single vacuum source inlet and a three-wire electrical connector. Mechanically, it operates like a single diaphragm EGR valve with a twist. It has a pintle position sensor riding atop the EGR diaphragm. This tells the PCM the amount of EGR valve opening as it is actuated. The PCM then commands a pulse width modulated solenoid to apply an appropriate amount of vacuum on-time. GM makes one of these units that has the integral pintle sensor and an integral solenoid with air filter. The only separately serviceable part is the air filter. Ford, Honda and Mazda all use a variation of this design with remotely mounted solenoids. The idea behind the pintle sensor is to give the PCM precise feedback as to exactly where the EGR valve is positioned. The PCM can then modulate the vacuum signal to it accordingly. The pintle position sensor is a potentiometer. Like a throttle position sensor, it is a variable resistor. The wiper arm within the sensor can wear and develop opens in the sensor return signal. A sweep test with a digital volt/ohm meter (DVOM) or scope can be used to test the sensor. The PCM has an internal "map" of where the pintle sensor should be at any given time. If the sensor's voltage reading is too high or low, a trouble code will be set. With Fords, it is the infamous Code 31. This code could be caused by several different factors.
If the pintle position sensor (Ford calls it the EVP sensor) is shorted or open, you could have a code set.
If the EGR valve becomes carboned up and does not seat fully, the EVP sensor gives a high reading and a code is set.
If the diaphragm of the EGR valve is bad, then it, too, is flagged.
The fix
On Fords and Mazdas, the only sure fix is to replace the sensor and valve as an assembly. Sometimes you can get temporary relief by filing down the pintle sensor stem to lower the sensor return voltage to specs. Or you could add a thicker gasket between the valve and sensor. You can spend a lot of time trying to capture the intermittent failure in the act. This is not recommended. The codes are rarely false. Note that there are two interchangeable sensors; one is gray and the other is black. Key-on-engine-off (KOEO) voltage for the gray sensor is 0.40 volt and for the black sensor is about 0.83 volt. Don't mix them up, or Code 31 won't go away.
On Hondas, the fix is sometimes achieved by cleaning out moisture and crud from the EGR vacuum lines with shop air pressure. Disassembly of the vacuum air box is required for access.
Digital EGR valves are unique in several ways. Only GM uses them. They are completely electronic, controlled solely by the PCM. They come in two or three solenoid models, depending on the application. Part of the valve is open to exhaust flow at all times. When the solenoid pulls the pintle open, exhaust leaves the EGR valve chamber and directly enters the intake manifold. This method is different from all other EGR valves. All other EGR valves open to allow exhaust to enter their chamber first, then circulate through the valve on to the intake passage. The benefit of the digital EGR valve is speed and accuracy. It meters EGR flow 10 times faster than a vacuum modulated system. The valve is actuated by an individual quad driver from the PCM for each solenoid the valve has. Battery power is fed through terminal D when the key is turned on through a 15-amp ignition fuse. When the PCM grounds a solenoid, a magnetic field is created that causes the armature to lift open the pintle. The PCM uses this system to actuate each solenoid in increments. The increments are displayed on a scan tool as percentages of total flow. With a bi-directional scan tool, the digital EGR valve can be commanded open in a variety of increments. Don't despair if you do not have a bi-directional scan tool. You can still work with this! It's easy. Simply unplug the four-wire connector. Run a fused 12-volt wire to terminal D and alternately touch each of the other terminals to ground with a test probe. This will cause each solenoid to pull open. You can do this test with the engine idling and check for an rpm drop as you ground each solenoid. If you don't get a good rpm drop on this or any other EGR valve, you may have plugged or restricted EGR passages, which can cause a code to be set.
The linear EGR valve is a high-tech system. It uses a closed loop method for the utmost in EGR control and driveability. All electronic, its built-in pintle-position sensor allows the PCM to continuously monitor "actual pintle position" and adjust it to the "desired pintle position" as a percentage. A generic scan tool will display these parameters just as it does "actual rpm" and "desired rpm." This feature is a boon to troubleshooting. For example, I had a '92 Chevy half-ton drive into our shop running terribly. The Malfunction Indicator Light (MIL) was on steady. My aftermarket scan tool pulled up a Code 32. A quick look at the data stream showed that actual and desired pintle position did not match - at idle, "desired pintle position" was zero; "actual" was about 40 percent. This told me the PCM was trying to close the EGR valve but could not. Twenty minutes later, I had the EGR valve out of the vehicle and found a large chunk of carbon stuck between the pintle and its seat. Spring tension was holding it tightly and, of course, the PCM was squeezing it as it tried to close the valve. I pried the carbon out, reinstalled the valve, and all was well.
Repeat failure
Repeat failure is a common problem. You can recommend a top engine clean to the customer and attempt to clean loose carbon from the upper intake. GM has noted the problem and come up with a software update for the PCM. Essentially, what the update does is periodically command the EGR to 100 percent opening to prevent or flush out carbon chunks. The new prom numbers were in a "special policy" procedure bulletin and not a regular TSB. The special policy number is 96067(A). It refers to '92-'94 S/T, M/L and C trucks with the 4.3 V6 engine and linear EGR valve. The bulletin number is 67-65-38. It refers to '95 C, S/T and M/L trucks with the 4.3 V6 engine and linear EGR valve.
EGR/PCM strategy
GM's Code 32 has been around a long time and can be caused by a variety of reasons. Every three to five years, the PCM strategy on this code changes. Make sure you review the proper flow chart any time you work on one on these. The strategy is slightly different depending on which engine, transmission, body type or year the car is. The most common strategy entails the PCM looking for fuel integrator counts to decrease momentarily when the EGR is commanded open. Why? There is no oxygen in the inert EGR gas, so the integrator subtracts fuel to compensate. For this to happen you must have a good working oxygen (O2) sensor. O2 sensor checks are usually not in the Code 32 flow chart, so be aware. Newer models and other makes look for a change in manifold absolute pressure (MAP) when EGR is flowing, which is a more reliable method.
Mechanical failures
In closing, I'll leave you with a pattern failure seen in '85-'92 Nissans, models 240 SX and Stanza, and some Hondas about the same vintage. They have individual EGR passages running to each cylinder. Eventually, some of these passages (one or two) will plug up with carbon while others stay open. The remaining passages receive 4-runner EGR volume, which is far too much and causes a misfire on the affected cylinders. It happens above idle and under load. Sometimes you can unscrew Allen head access plugs to clean out the passages, other times you must remove the upper intake plenum chamber to do the repair.
