eec egr scheme

Tue Jun 9 14:27:53 GMT 1998

> 
>  I have a 500 line dissertation on EGR if anyone wants me to
> >post it, lots of tech details
> >
> If ya don't post it PLEASE send me a copy.    Just don't make me
> send some of the CSH, staff over too get a copy, LOL <g> HOHOHAHA......

Here it is

if you send those CSHers over here I will have some men I white coatsi
with butterfly nets waiting for them

are there any of the CSH located in the Toronto area ?

Clive 

Technical Feature--December, 1996

Diagnostic Quarterly:
The ABCs of EGR

   BY Bob Weber

   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&deg;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&deg;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.

   If 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 used&shy;positive
   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.

   The 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.

   Early Ford systems used either a ported vacuum system, a remote
   backpressure transducer or an integral transducer EGR valve. To check
   the transducer action, tee in a vacuum gauge at the EGR valve and
   start the engine. Rev it up to about 3000-4000 rpm and you should see
   a vacuum reading. If you don't, check that hose and the transducer. To
   check the transducer, connect your vacuum gauge to the outlet port,
   but leave the inlet port connected to the manifold vacuum source. With
   the engine running, apply vacuum to the signal port and watch for
   manifold vacuum to appear at the outlet.

   On Fords with computer controls, the EGR is electronically controlled
   and there is an EGR Valve Position (EVP) sensor attached to the top of
   the EGR valve. The most important thing to remember about these
   systems is that the computer relies on the EVP sensor for more than
   EGR flow. It uses the sensor input to compute timing advance and
   injector pulse width. Check the EVP sensor in much the same way you
   check a throttle position sensor using a DVOM or, better yet, a
   labscope. If it is defective, replace the EVP sensor and EGR valve
   together to prevent inaccurate signal voltages.

   The Ford pressure feedback electronic (PFE) system monitors exhaust
   backpressure and sends a 3-5V signal to the PCM. (Less than three
   volts would indicate a vacuum in the exhaust system&shy;a rather
   unlikely prospect.) At idle it should be 3.0 volts and at maximum
   backpressure, almost 5.0 volts. High readings at idle indicate an
   exhaust restriction, low readings an exhaust leak. The backpressure
   hose on these systems has been known to clog up from carbon and
   condensation, which can be a diagnostic devil if it turns to ice in
   the winter. If you must replace the hose, do not use regular vacuum
   hose. It must be able to withstand the high heat.

   Bob Weber, former editor of Super Automotive Service and an ASE-Master
   Technician, writes for the automotive industry. He is based in
   Virginia.