EGR DIAGNOSTICS

On some vehicles, the OBD II system may monitor the operation of the exhaust gas recirculation system.

The EGR system reduces the formation of oxides of nitrogen (NOX) in the exhaust when the engine is under load.

OBD II may check for a drop in intake vacuum through the MAP sensor when it commands the EGR valve to open.

Some systems also have an EGR valve positon sensor to monitor the opening and closing of the EGR valve.

Code for EGR-related faults include P0078 to P0086, and P0400 to P0409.

An EGR-related fault code does NOT mean the vehicle is polluting. It might be producing elevated levels of NOX in the exhaust if the EGR system is not operating correctly.

The only way to know for sure is to check NOX emissions with an exhaust analyzer.

NOX emissions are highest when the engine is under load, which means you may not be able to get accurate readings unless the vehicle is tested in a "loaded mode" by driving it on a dyno, or using a portable exhaust analyzer while test driving the vehicle on the road.

MORE ABOUT THE EGR SYSTEM

Exhaust gas recirculation is only a part-time function. It should NOT occur when the engine is cold because it acts like a vacuum leak and can cause a rough idle or lean misfire.

EGR also should also NOT occur at idle for the same reason. It should only occur after the engine has warmed up and is running at a speed above idle.

The EGR valve should be fully closed when the engine is off. If it is not, it can make for hard starting when the engine is cranked.

On most OBD II vehicles, the EGR valve is controlled by the powertrain control module. If teh engine has a vacuum-operated EGR valve, the PCM controls a solenoid in the vacuum line to open and close the valve.

The PCM may cycle the solenoid on and off to vary the EGR flow rate.

Inceasing the "on time" of the solenoid holds the valve open longer and increases the flow rate.

"Digital" EGR valves have several solenoids (two or three in the case of GM). Each has a different sized valve (small, medium and large).

The PCM varies the flow rate by energizing various combinations of the solenoids. The solenoids are normally closed, and open only when the PCM completes the ground to each.

If an engine has an electronic EGR valve, the PCM operates the valve by energizing a motor or solenoids.

"Linear" EGR valves use a motor to open the valve. The further the valve opens, the greater the flow rate. Linear EGR valves may also be equipped with an EGR valve position sensor (EVP) to keep the PCM informed about what the EGR valve is doing.

The EVP sensor also helps with self-diagnostics because the computer looks for an indication of movement from the sensor when the it commands the EGR valve to open or close.

The sensor works like a throttle position sensor and changes resistance. The voltage signal typically varies from 0.3 (closed) to 5 volts (open).

GM FALSE EGR CODE P1406

On some 1995 and newer GM vehicles with OBD II and linear EGR valves, the MIL light may come on and set a Code P1406 when the engine has high mileage on it.

The P1406 code indicates an EGR problem, but replacing the EGR valve won't fix it because the real problem is that the OBD II system's self-diagnostics are overly sensitive.

When the computer commands the EGR valve to open, it wants to see a movement signal from the EVP sensor within a certain number of milliseconds (typically 50 ms or less).

But as the EGR valve ages, it may not open as quickly as it once did. It still works and keeps NOX emissions within acceptable limits, but the computer thinks the EGR valve is not opening quickly enough and sets a code.

The cure, in this case, is not to replace the EGR valve but to reflash the PCM with new programming instructions that allow a longer response time from the EGR valve. This requires a Tech 2 scan tool and a software update from GM.

MODE 06 SCAN TOOL DIAGNOSTICS

Most scan tools can access a menu called "Mode 06." You will usually find this by choosing "global" or "generic" OBD II on the scan tool main menu rather than entering the vehicle year, make and model.

When you go to the Mode 06 menu, you can see the OBD II self-test data for all the EGR components and can tell at a glance whether they are operating in or out of range. If a component is acting up, it will FAIL the self-test -- but it may not set a DTC unless all the other failure criteria for the DTC have also been met.

This may take several drive cycles to occur.

See the Mode 06 Diagnostics section for more info.

COMMON EGR PROBLEMS

Common EGR problems include:

• Pinging (spark knock or detonation) because the EGR system is not working, the exhaust port is plugged up with carbon, or the EGR valve has been disabled. The fix here is to inspect the EGR system, remove the EGR valve and check the valve and port for carbon, and remove the carbon from the port if it is plugged.

• Rough idle or misfiring because the EGR valve is not closing and is leaking exhaust into the intake manifold. You may also find a P0300 "random misfire" code.
  
  The fix here is to connect a vacuum gauge to the intake manifold and check intake vacuum at idle.
  
  A normal reading is typically 17 to 22 inches Hg. IF the reading is low, it indicates a vacuum leak.
  
  Check all vacuum hose connections for loose or disconnected hoses.
  
  Replace any hoses that are cracked. If there are no obvious vacuum leaks, remove and inspect the EGR valve.
  
  Look for carbon buildup on the EGR valve or the seat that would prevent it from closing. Clean as needed. IF teh EGR valve is stuck open or is damaged, replace it.

• Hard starting because the EGR valve is not closing and is creating a vacuum leak into the intake manifold. See same fix as above.

GENERAL EGR TROUBLESHOOTING TIPS

There are several ways to troubleshoot an EGR system. If the MIL lamp is on and there is anOBD II code that is related to the EGR system, you can refer to the diagnostic charts in the vehicle service manual to isolate teh fault. If you don't have access to the service information, you can use the following procedure:

1. Does the engine have a detonation (spark knock) problem when accelerating under load? Refer to the timing specs for the engine and check ignition timing. The timing may be overadvanced. If the timing is within specs, check the engine's operating temperature.
   
   A cooling problem may be causing the engine to detonate.
   
   If the temperature is within its normal range and there are no apparent cooling problems, other possibilities to investigate include a spark plugs that are too hot for the engine application, a lean air/fuel mixture, low octane fuel or too much compression (due to a buildup of carbon in the combustion chambers or because of pistons or heads that have too much compression for the fuel you're using).
   
   Be sure you've ruled out all the other possibilities before focusing on the EGR system.
2. If the engine has a vacuum-operated EGR valve, use a vacuum gauge to check the EGR valve vacuum supply hose on the intake side of the PCM-controlled EGR solenoid for vacuum.
   
   Got vacuum? Okay, then check for vacuum on the EGR valve side at 2000-2500 rpm when the engine is warm.
   
   There should be vacuum if the engine is at normal operating temperature. No vacuum may indicate a bad solenoid, a fault in the wiring betweent he PCM and the solenoid, or another sensor problem such as a faulty coolant sensor.
   
   Refer to a vacuum hose routing diagram in a service manual or the vacuum hose routing information on the vehicle's emission decal for the location of the EGR vacuum control solenoid.
   
   If the solenoid fails to open when energized, jams in the open or shut position, or fails to function because of a corroded electrical connection, loose wire, bad ground, or other electrical problem, it will obviously affect the operation of the EGR valve.
   
   Depending on the nature of the problem, the engine may have no EGR, EGR all the time, or insufficient EGR.
   
   If bypassing the suspicious solenoid with a piece of vacuum tubing causes the EGR valve to operate, find out why the solenoid isn't responding before you replace it. The problem may be nothing more than a loose or corroded wiring connector.
   
   Check the solenoid's resistance with ah ohm meter. If the solenoid is shorted, open or out of specifications, replace it.
3. Inspect the EGR valve itself. Because of the valve's location, it may be difficult to see whether or not the valve stem moves when the engine is revved to 1500 to 2000 rpm by slowing opening and closing the throttle. The EGR valve stem should move if the valve is functioning correctly.
   
   A hand mirror may make it easier to watch the valve stem. Be careful not to touch the valve because it will be hot!
   
   If the valve stem doesn't move when the engine is revved (and the valve is receiving vacuum), there may be something wrong with the EGR valve. Another way to "test" the EGR valve on some engines is to apply vacuum directly to the EGR valve.
   
   Note: This only works on ported vacuum EGR valves, not backpressure EGR valves or electronic EGR valves. Vacuum should pull the valve open creating the equivalent of a large vacuum leak. This should cause a momentary drop in idle speed and a noticeable increase in idle roughness.
4. Remove and inspect the EGR valve if you suspect a problem. Most failures are caused by a rupture or leak in the valve diaphragm. If the valve is not a backpressure type, it should hold vacuum when vacuum is applied with a hand-help pump.
   
   If it can't hold vacuum, it needs to be replaced. Note: This test does not work on backpressure EGR valves.
   
   Backpressure EGR valves sometimes fail if the hollow valve stem becomes clogged with carbon or debris. This you can see for yourself. It's almost impossible to remove such a clog, so replace the EGR valve.
   
   Carbon accumulations around the base of the EGR valve can sometimes interfere with the opening or closing of the valve. These can be removed by careful brushing or by soaking the tip of the valve in solvent.
   
   Do not soak the entire valve in solvent or allow solvent to get anywhere near the diaphragm. The solvent will attack and ruin the diaphragm.
5. Inspect the EGR passageway in the manifold for clogging. Use a pipe cleaner or small piece of wire to explore the opening for a blockage. Sometimes you can dislodge material that's clogging the opening by carefully poking at it.
   
   Other times, it may be necessary to remove the manifold and have it cleaned.

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