Sent: 3/12/2007 5:16 PM
The ABCs of EGR
You have heard it before: Cars cause smog. Actually, it isn't cars but what happens during combustion, what comes out the tail pipe, and what happens when those emissions 'percolate' in the atmosphere, that causes smog. Perhaps we should first discuss air pollution, define smog and then take a look at how we, as automotive service professionals, can contribute to clean air and driving pleasure.
The Clean Air Act of 1967 (which has been amended in 1970, 1977, and 1990) is the foundation for air-pollution control, and the Environmental Protection Agency (EPA) carries out the requirements of the act. Since it went into effect, there has been a 24 percent reduction in air pollution, yet in the best year more than 50 million Americans (one in five) are still exposed to unhealthy air pollution. In the worst years, that ratio may still be one in three. Many people are sensitive to air pollution because they are very young, elderly, or have respiratory or heart disease.
Since the clean air act amendments were signed into law, an unprecedented number of cities have met the air quality standards. For example, of the 98 areas of the country that were designated as
nonattainment for ground-level ozone in 1990, 55 of those areas now have 'clean' air and 22 have been formally redesignated to 'attainment.' All this happened even as more cars were being driven more miles than any time in history. In 1970, Americans traveled 1-trillion miles in motor vehicles. By the year 2000, we are expected to drive4-trillion miles per year.
What, exactly, is smog?
Some say the term 'smog' originated in England in the early part of the century to describe a mixture of smoke and fog. Today we use the term for either of two types. The first type of smog occurs when the
humidity is high and the air holds suspended particulate matter such as smoke or dust. The second type, photochemical smog, affects areas where there is plenty of sunlight plus high concentrations of volatile organic compounds (VOCs) and nitrogen oxides (NOx). Los Angeles, Mexico City, and Tokyo are famous for their smog problems, but high levels are common in the summer in many U.S. cities.
Photochemical smog is created when sunlight acts as a catalyst with reactive emission ingredients called precursors. Ozone, which is a principal ingredient (and precursor) of smog, is produced when
hydrocarbons (HCs) combine with nitrogen oxides under the influence of sunlight. Ironically, ozone in the upper atmosphere protects us from the sun's ultraviolet rays, but we will reserve that discussion for a story on air conditioning and refrigerants. More than one comedian has suggested using huge fans to blow ground level ozone into the stratosphere to solve two problems at once. Those would have to be some fans!
A bit of chemistry
The 'x' in NOx stands for the various oxides of nitrogen that are created. Among them are: NO (nitric oxide), a colorless, poisonous gas; NO2 (nitrogen dioxide) which is a reddish brown, toxic gas; and
N2O (nitrous oxide) which is commonly known as laughing gas.
In the automotive service industry, smog prevention requires control of nitrogen oxide emissions, as well as gasoline vapors from service stations and storage tanks, and VOCs from body shops' painting
processes. Ninety percent of the carbon monoxide and almost 50 percent of the nitrogen oxides and hydrocarbons come from burning gasoline and diesel fuels in cars and trucks. Nitrogen oxides are only created at very high temperatures (above 2500°F) and pressures, something commonly found in automotive engines.
Your job is to help America keep its air clean or help get it cleaned through conscientious emission control service. And to prevent smog, that means the Exhaust Gas Recirculation (EGR) system service. NOx is only one of the primary contributors to air pollution. As you know, there are many others.
Basic EGR operation
As we mentioned earlier, NOx forms under high pressures and temperatures commonly found in the combustion chamber, so we can control its formation by either reducing the compression or the temperature in the combustion chamber. Emission control engineers chose the latter. How is temperature controlled? By introducing a metered amount of inert gas into the cylinder to partially quench the fire, much like misting your barbecue when it flares. It doesn't put out the fire, but it slows things down a bit. The result is that the fire in the combustion chamber is less intense. EGR exhaust gas occupies space that would otherwise contain air. With EGR, the fire is more like a smoldering pile of leaves than a blast furnace. Exhaust gas was chosen because it is free and plentiful. EGR reduces the formation of NOx up to 60 percent. The cat usually cleans up much of the rest.
The EGR valve opens during light throttle and warm engine cruising and channels the exhaust gases back into the engine's inlet air. It doesn't take much. EGR accounts for less than 10 percent of the total air/fuel mixture but even this small amount of non-flammable stuff is enough to quench the flame somewhat. When everything is on the money, the EGR lowers combustion temperatures to just under the 2500°F bogey.
NOx production can also be limited by base timing, so make sure it is correct before you troubleshoot the EGR system. And since the cat cleans up over a third of the NOx that leaves the engine, don't overlook it.
As with many other automotive devices, vacuum supplies the power to operate the valve by way of a diaphragm in the majority of systems, but lately the digital and linear systems are entirely electronically controlled. Between the vacuum supply and the EGR valve you may find all sorts of additional controlling devices so that the EGR function
is not introduced when it would upset driveability, such as when the engine is cold or under hard acceleration, or is at idle. Frequently, you'll find a thermal vacuum switch (TVS) to prevent vacuum when the engine is cold.
f there isn't enough EGR, NOx emissions increase, but the only driveability problems are a surging at cruise, a complaint of spark knock or a failed enhanced emissions inspection due to a high NOx reading. If there is too much EGR, or EGR at the wrong time, your clues will be poor engine performance. The symptoms include:
* poor idle
* stalling, especially when starting after cold soak
* hesitation, stumble and rough running during warm-up
* tip-in hesitation or stumble
* surge at cruise, even with warm engine
* poor acceleration
* low engine vacuum.
Description of types
There are currently six types of EGR systems in use. Going from the
oldest (and perhaps the most familiar) they are:
* Ported (late 1960s to present)
* Positive backpressure (1970s to present)
* Negative backpressure (1970s to present)
* Pulse-width modulated (early 1980s to present)
* Digital (electronic) (late 1980-early 1990s)
* Linear (electronic) (early 1990s to present.)
Two types of backpressure EGR valves are normally usedpositive and negative. On GM cars, they are identified by the last letter of the part number stamped on the diaphragm housing on top of the valve. (Prior to 1988, they were not identified and can cause some confusion, so be careful when ordering a replacement.) The letter 'P' stands for
positive backpressure and the letter 'N' for negative backpressure. If there is no letter it operates on ported vacuum.
On the positive backpressure EGR valve, a control valve located in the EGR valve acts as a vacuum regulator valve. The control valve manages the amount of vacuum to the EGR diaphragm chamber by bleeding vacuum
to the atmosphere during certain operating conditions. When the control valve receives a backpressure signal from the exhaust through the hollow shaft of EGR valve pintle, pressure on the bottom of the control valve closes it. When the control valve closes, the full vacuum signal is applied directly to the EGR valve diaphragm which
opens the valve and lets the exhaust gas recirculate.
On the negative backpressure EGR valve, a hose connected to the upper part of the EGR valve supplies a vacuum signal. Manifold vacuum is also applied to the lower diaphragm through an intake port at the base of the EGR valve. When manifold vacuum in the lower chamber isn't strong enough to overcome the spring tension on the lower diaphragm, a bleed valve closes, allowing vacuum in the upper chamber to open the EGR valve. Exhaust flow opens a check valve in the pintle so that vacuum bleeds to atmosphere and the valve rises, but tries to drop
again so it dithers to control EGR flow.
The pulse-width modulated EGR system is controlled entirely by the powertrain control module (PCM). The computer controls the flow rate by sending electrical signals to a solenoid vacuum valve between the PCM and the EGR valve. The solenoid pulses up to 32 times per second. To determine the pulse width, the PCM relies on a ported vacuum signal.
On computer-controlled EGR systems, the ECM controls the vacuum signal to EGR valve via a solenoid valve. The ECM uses coolant temperature, throttle position and manifold absolute pressure (MAP) signals and sometimes other inputs, to compute the vacuum solenoid operation. Whenever the engine is cold or is idling, the solenoid valve blocks vacuum to EGR valve. When the engine is warm, and the rpm is higher than idle speed, the solenoid ground is broken and vacuum opens the EGR valve.
The digital EGR valve allows the precise amount of EGR flow without using manifold vacuum. The valve controls EGR through three different size orifices for seven different combinations of EGR flow. When the PCM energizes a solenoid, the swivel pintle is lifted to open the orifice.
Some engines have a linear (electronically controlled) EGR valve. It has a control solenoid and EGR valve position (EVP) sensor. The sensor works on the same principle as a throttle position sensor. The return voltage signal ranges from 0.3 volts when it is closed up to 5.0 volts when it is fully open. The PCM controls EGR flow by pulsing the signal to the EGR solenoid. This provides better regulation of EGR flow than with conventional vacuum controlled EGR valves.
THERE HAS TO BE AN EASIER WAY
We only touched on the techniques of testing EGR systems in our feature and, as you can see, it can become time consuming and complicated. This has become a major frustration for smog check technicians in California who must do a functional check on every EGR system that comes through their facility. Doing a functional check on Ford PFE or DPFE systems can be a long, involved process. That translates into a financial loss on each car when you factor in the meager inspection fee they can collect.
There is an easier way. Leave it to American ingenuity and a fellow named Nick Smith. An automotive technology professor and department coordinator at Mott Community College in Flint, MI, Smith invented (and patented) a tool that does a functional test of the EGR system in less than a minute at idle. The Smithtronics ST-5300 Universal EGR Tester checks the EGR solenoid(s), position sensors and valves, including digital and linear types on all domestic makes. The tester uses a divide-and-conquer strategy, as Smith puts it, by exercising all the controls and getting feedback on the system integrity. Buttons activate up to three solenoids and lamps above the buttons indicate that the circuit was completed. No light, open solenoid. (The engine also usually stumbles if the solenoid activates EGR flow.) Position sensors are checked as a segmented bar graph displays the sensor voltage signal. The tester, the only such device in the world and available only from Smithtronics at 1-800-760-8822, comes with 13 interchangeable test leads for direct connection into EGR solenoids, sensors and valves.
The California BAR has equipped all 50 of its referee stations with the device.
Symptoms of EGR malfunction:
The EGR system is often misdiagnosed or blamed for problems that may not be its fault including hard starting, stalling and hesitation during warm-up, rough idle, missing, spark knock, backfiring and loss of power. Sure, the EGR system can cause these symptoms, but so can other components and systems. Don't jump to any conclusions until you have checked the basics. Don't overlook carbon buildup in the combustion chamber for spark knock, for instance. Also, don't overlook vacuum leaks for hard starting and hesitation. Don't overlook the
ignition or fuel systems as the cause of missing. Remember, when it is operating properly, the EGR valve only opens when the engine is at operating temperature under light to moderate throttle, steady state cruise. So, problems (except spark knock and surging) at highway cruise are probably caused by something other than the EGR system.
The EGR system can malfunction in four ways:
* problems with the passages
* problems with the EGR valve
* problems with the vacuum control system
* problems with the computer control system.
he exhaust is full of moisture, carbon, and other stuff that can plug up the passages or the valve itself. The two most common problems with EGR systems are stuck valves or plugged passages. When we get into computer controls, the solenoids and vacuum hoses are things to suspect.
For years, we've been told that we can test an EGR valve by manually opening it with the engine idling. With a glove or shop towel to protect your fingers, lift up on the valve diaphragm and see if the engine stumbles or stalls. But what if the passages are clogged? Moving the valve by hand will have no effect. Nor will you be able to see any problems. Revving a warm engine up to 2000-3000 rpm while watching the EGR valve stem for movement doesn't tell us if the passages are clogged or the valve is 'carboned' up. (Most computer controlled EGR systems have a park/neutral lockout, so the vehicle has to be in gear. On the other hand, you won't know if some EGR could
even be flowing at idle because the valve is not seating.
If the system is clogged, NOx emissions will go up, but there usually aren't any driveability complaints other than spark knock. If the car fails an emissions test due to high NOx while the other readings are within specs, remove the valve, clean all the passages and reinstall the valve using a new gasket if the crud is not too thick. Although you could waste your time (and customer's money) cleaning a heavily coked valve, you would both be better served by replacing it.
The most common way an EGR valve malfunctions is that it hangs open. Carbon is usually the culprit. A bit of carbon lodged between the pintle and seat could prevent the valve from closing. This is quite common with the GM linear valve. Remember the classic symptoms are poor idle, stalling or stumble after a cold start, and so on. If the
problem is a bit of carbon, remove it.
Vacuum signal problems come in many forms, but have the same result: With too little vacuum the valve doesn't open, with too much it doesn't close or opens too soon, causing hesitation. Look for loose, broken, pinched or missing vacuum hoses. Make sure those hoses are routed properly by comparing them to the decal under the hood. If the valve works when you test it with your hand-operated vacuum pump, but does not seem to be getting vacuum from the engine, you must dig a little deeper. The calibration spring in very high mileage cars can lose its tension and allow the valve to open too soon, resulting in tip in hesitation and highway cruise surge.
Before you knock yourself out testing all the vacuum controls, look for a restriction in the vacuum hose to the EGR valve. In the olden days, EGR was accused of causing everything from stalling and pinging to insomnia and impotence, and tampering with the vacuum source was not uncommon. Ball bearings and BBs were often wedged into the vacuum hose. Believe it or not, such tampering (which is illegal) still happens today. Make sure vacuum is able to reach the valve.
Now, begin checking all those components between the EGR valve and the vacuum source. If the system uses a vacuum amplifier, it may malfunction, allowing vacuum to hold the EGR valve open all the time.
Since we don't want EGR during warm-up, there is usually a thermal vacuum switch (TVS), or vacuum control solenoid, through which the vacuum flows. It should not allow flow when the engine is cold.
Backpressure EGR valves rely on a specific exhaust backpressure, so any restrictions in the inlet will make the valve misbehave. In addition, if the exhaust system doesn't flow properly (either too little caused by a clogged cat, or too much caused by a modified, low-restriction system), the EGR valve will misbehave.
Lately, everything from the engine to the transmission to the lighted vanity mirror is being controlled by onboard computers. EGR systems haven't escaped and are being controlled by solenoids that meter the vacuum. The PCM commonly controls the EGR by regulating the pulse-width of the on/off strategy much as it did on feedback
carburetors. Along with controlling the EGR system, the PCM gets feedback from the EGR Valve Position (EVP) sensor. It behaves much like a throttle position sensor with the key on/engine off (KOEO) voltage less than 1.0 volt, and nearly 5.0 volts at wide open. The EVP sensor sits on top of the EGR valve, connected to the stem to measure its position. This provides the computer with pintle position so the PCM can trim EGR flow.
Some systems, such as Ford pressure feedback EGR (PFE) and differential pressure feedback EGR (DPFE) use a sensor in the exhaust stream that reports back to the PCM how much exhaust gas is actually flowing. With PFE, the computer uses its internal formulas to estimate the EGR flow; in the DPFE the computer actually gets a report on the flow by measuring the pressure above as well as below the EGR valve. The computer then adjusts the EGR vacuum regulator (EVR) to optimize the EGR flow under various conditions.
Testing and service
Although we cannot cover all systems on all makes and models, lets take a brief look at how to test and service some common systems found on domestic engines. The imports work similarly, so if you get the idea from these examples, you will be able to troubleshoot anything with the right shop manual.
Conventional wisdom says that EGR systems most often fail due to carbon buildup (coking) in the exhaust gas passages. According to Nick Smith, the inventor of the Smithtronics universal EGR tester, that just ain't so. Sure, it does happen occasionally, but Smith's experience is that most malfunctions are in the control system. That
includes the vacuum solenoids, position sensors, hoses and wiring or connectors.
General Motors uses ported, positive, or negative backpressure EGR valves in its various models. In the positive type, the pintle is hollow and exhaust gas flows through it to close off a port allowing vacuum to lift the diaphragm and open the poppet. In the negative backpressure type, manifold vacuum, controlled by the EGR solenoid,
opens the valve while a backpressure transducer attempts to let it close. This dithering maintains proper EGR gas flow.
Ported vacuum EGR valves are the most common, not only with GM but Ford, Chrysler and many imports as well. Testing them is simple. You don't even have to start the engine. Just connect a hand held vacuum pump and pull a vacuum. It should lift the EGR valve and maintain vacuum indefinitely.
Here's how you can check a positive backpressure valve. Put a restriction in the tail pipe. (A 1/2-in. drive socket held in place with a C-clamp or locking pliers works well.) Connect a hand held vacuum pump to the EGR valve. Pull a vacuum on the valve, which should hold indefinitely unless the diaphragm leaks. Then, start the engine and put the transmission in gear. The engine will stall when the exhaust backpressure builds up enough to open the EGR valve at idle. EGR doesn't normally flow at idle.
To check a negative backpressure valve, replace the vacuum hose at the EGR with a hand held pump and pull a vacuum while you feel for diaphragm movement with your finger. It should move up and hold vacuum indefinitely. When the engine is cranked the diaphragm should drop, closing the valve.
With computer control, the EGR system usually has a vacuum control solenoid controlled by the PCM. To see if it is working properly, 'tee' a vacuum gauge into the hose at the EGR valve. Warm the engine, put the tranny in gear, apply the brakes and accelerate. You should get a vacuum reading on your gauge. Now, disconnect the electrical
connector at the solenoid and vacuum should vent off returning the gauge to zero.
The GM Electronic Vacuum Regulated Valve (EVRV) comes in two types and three generations. Early versions had a round solenoid and vacuum sensing switch (1984-86) on the EGR-side of the solenoid. If the computer allows vacuum to pass the solenoid, it lifts the valve and closes the switch to check its operation. It only sees vacuum signal, which is no indication the valve is actually working.
The next generation (1987-'88) was similar and came with a vent filter, which looks similar to a pleated paper Rochester carb filter. A restricted filter causes idle problems or tip-in hesitation. If the filter is restricted, it traps the vacuum instead of venting it and holds the EGR valve open. Removing the filter for inspection often destroys it, but replacement filters are available. Keep plenty on hand. (The vent filters also work on Ford EVRV systems.) When the PCM energizes the solenoid, it closes to keep the vacuum from venting and keeps the EGR valve open. The third generation (1988-present) looks identical to the second, but has a vacuum switch. How do you tell the difference? Look at the connector. If it has four wires, it has a switch; three wires means no switch. Don't interchange them and be careful to get the right one when you order.
You can check the second and third generation valves the same way. Cover the solenoid vent with your finger. The engine should stumble or stall. If it doesn't, the vent solenoid is probably defective.
Chrysler, and most import EGR systems have a backpressure transducer and here is how they work. There is a hose from a vacuum source and a second hose from just below the EGR valve poppet leading to the transducer. Backpressure and vacuum have a tug-of-war in the transducer. The EGR valve opens, then tries to close, then tries to open again. Once again, this dithering controls the EGR flow. Other controls often include coolant temperature and manifold inlet temperature. If the system is computer-controlled, restrict the exhaust flow at the tail pipe, start the engine, and unplug the solenoid. The engine should stall.
This may help you.
P.S. SMOG INFORMATION
Regarding the 1905 carburetor that is installed on your Chrysler engine. The original thermoquad carburetor relied on the vacuum amplifier to operate the EGR Valve. Using the new Edelbrock carburetor, the vacuum amplifier is not needed.
The EGR can be rerouted using the (port ?D?) ported vacuum port. The throttle positioner may also be removed since it is not applicable to the new carburetor. This was how Edelbrock tested the vehicle when they were granted the VC27156 exemption.