Diagnosis
The EGR system is monitored by the PCM in order to detect when a failure occurs that causes emission levels to rise more than one and one-half times normal levels. If the PCM detects that the a system failure has occurred due to a restriction, a DTC P0401 will be set. If the PCM detects that it cannot control the EGR valve as it has commanded, a DTC P1406 is set. Exhaust Gas Recirculation (EGR) System Diagnostic will check normal operation of the EGR system. Refer to the applicable table for diagnosis.
Results of Incorrect EGR System Operation
Too much EGR flow at idle, cruise, or cold operation may cause any of the following conditions to occur:
| • | Engine stalls after cold start. |
| • | Engine stalls at idle after deceleration. |
| • | Vehicle surges during cruise. |
| • | Rough idle. |
| • | DTC P0300. |
| • | DTC P1406. |
Too little or no EGR flow may allow combustion temperatures to get too high. This could cause:
| • | Spark knock (detonation). |
| • | Engine overheating. |
| • | Emission test failure. |
| • | DTC P0401 or DTC P1406. |
| • | Poor fuel economy. |
Refer to
EVAP and EGR Controls
.
Circuit Description
An Exhaust Gas Recirculation (EGR) system is used to lower Oxides of Nitrogen (NOx) emission levels caused by high combustion temperatures. It accomplishes this by feeding small amounts of exhaust gases back into the combustion chamber. When the air/fuel mixture is diluted with the exhaust gases, combustion temperatures are reduced.
A linear EGR valve is used on this system. The linear EGR valve is designed to accurately supply exhaust gases to the engine without the use of intake manifold vacuum. The valve controls exhaust flow going into the intake manifold from the exhaust manifold through an orifice with a PCM controlled pintle. The PCM controls the pintle position using inputs from the Throttle Position (TP) and Manifold Absolute Pressure (MAP) sensors. The PCM then commands the EGR valve to operate when necessary by controlling a ground signal through the PCM. This can be monitored on a scan tool as the Desired EGR Position . Ignition voltage is supplied to the valve through a fuse.
The PCM monitors the results of its command through a feedback signal. By sending a 5 volt reference and a ground to the EGR valve, a voltage signal representing the EGR valve pintle position is sent to the PCM. This feedback signal can also be monitored on a scan tool and is the actual position of the EGR pintle. The Actual EGR Position should always be near the commanded or Desired EGR Position .
Diagnostic Aids
| • | Due to the moisture associated with exhaust systems, the EGR valve may freeze and stick in colder weather at times. After the vehicle is brought into a warm shop for repairs, the valve warms and the problem disappears. By watching the Actual EGR and Desired EGR Positions on a cold vehicle with a scan tool, the fault can be verified easily. View the freeze frame data to determine if the DTC set when the vehicle was cold by viewing both the engine coolant and intake air temperatures. |
| • | The Decel EWMA (Exponentially Weighted Moving Average) value can be a great aid in determining if a problem exists and to verify repairs. The Decel EWMA is an average of the difference in the expected MAP change and the actual MAP change caused by opening the EGR valve during a deceleration, and is used to determine when the MIL is illuminated. By driving the vehicle up to approximately 60 mph (97 km/h) and decelerating to 20 mph (32 km/h), it can be determined if the EGR system is OK, partially restricted or fully restricted. |
| • | A more negative number (less than -3) indicates that the system is working normally whereas a positive number indicates that the system is being restricted and that the expected amount of EGR flow is incorrect. A number that falls between negative 3 and positive 2 indicates that the system is partially restricted but not restricted enough to fail. The Decel EWMA value should always be a -3 or lower. If the Decel EWMA number becomes more positive (towards 0 or more), then the EGR system is becoming restricted. Look for possible damage to the EGR pipe or for a restriction caused by carbon deposits in the EGR passages or on the EGR valve. |
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
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The Powertrain OBD System Check prompts the technician to complete some basic checks and store the freeze frame and failure records data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored on the scan tool for later reference.
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Commanding the EGR valve open determines whether the EGR system can control the EGR valve accurately. The Actual EGR Position should follow the Desired EGR Position very quickly. If the Actual EGR Position appears to have to catch up to the Desired EGR Position , the valve is considered lazy or slow.
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If the EGR ignition feed circuit is open, the EGR valve will appear lazy and slowly follow the Desired EGR Position . The EGR valve is still receiving ignition voltage at the valve, so the fault may be either in the circuit between the PCM and the ignition feed circuit splice or the terminal connection at the PCM. If the circuit opens to the EGR valve, a DTC P1406 is set.
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This step verifies if the fault is present and also verifies if a repair corrected the problem. Refer to Diagnostic Aids for an explanation on EGR EWMA. If the EGR EWMA value stays near 0 or a positive number after several tests have been run, then a small restriction may still exist. Be sure to check the EGR pipe for damage or dents and the EGR valve for any excessive carbon build up.
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The resistance reading given is for a valve that has not been heated excessively. If the valve is hot to the touch, allow the valve to cool before taking the resistance reading.
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Visually and physically inspect the EGR passages and valve for excessive carbon deposits or damage.
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The replacement PCM must be reprogrammed. Refer to the latest Techline procedures for PCM Reprogramming.
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Be sure all gasket material is removed from the EGR mounting surface. Even a small amount of material may cause a DTC P0401 to set.
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If no faults have been found at this point and no additional DTC's were set, refer to Diagnostic Aids for additional checks and information.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to | |
Does the Actual EGR Position follow the Desired EGR Position slowly? | 25%, 50%, 75%, 100% | |||
Check the EGR ignition feed circuit to the PCM for an open or high resistance and repair as necessary. Was a repair necessary? | -- | |||
|
Caution: Perform this step ONLY in an area where the vehicle can be operated as described without risk to other vehicles or passengers.
Is the Decel EWMA value greater than the specified value? | 10 seconds 55 mph 20 mph 0 | |||
Is the resistance within the specified values? | 6-13ohms | |||
Was a repair necessary? | -- | |||
7 | Replace any restricted or damaged part that cannot be repaired. Is the action complete? | -- | -- | |
Replace the PCM. Is the action complete? | -- | -- | ||
Replace the EGR valve. Is the action complete? | -- | -- | ||
Does the Actual EGR Position follow the Desired EGR Position slowly? | 25%, 50%, 75%, 100% | System OK Refer to Diagnostic Aids |
