Rough Idle and P0401 Code

[TheEdgeofSanity]

hey guys,
since I got my malibu, it hasn't exactly had the smoothest idle. Recently, it seems like it has gotten worse, especially when the engine is warmed up and the car is in gear. I also have noticed a decline in city fuel economy, but no change on highway mileage.

the other night, my SES light came on for the first time ever and I took it immediately to auto zone. The car had a pending P0401 code which signifies an "EGR system fault". There was also a P0401 in the history.

Well, a new EGR is nearly $80 so I held off and was going to try to clean my existing one. In the mean time, the light has gone off by itself so does this mean whatever malfunction my EGR had has resolved itself?

Also, how common is it for the EGR system to become clogged? my car has 79K on it which to me signifies it should not have a bunch of carbon buildup. I kind of want to still remove the EGR to investigate but I'm afraid I will break something so if anybody has any pointers before I dive into it please let me know

[Cliff8928]

....

Originally Posted by GM

DTC P0401 Exhaust Gas Recirculation (EGR) Flow Insufficient



Circuit Description

The Powertrain Control Module (PCM) tests the exhaust gas recirculation (EGR) system during deceleration by momentarily commanding the EGR valve to open while monitoring the manifold absolute pressure (MAP) sensor signal. When the EGR valve is opened, the PCM should see a proportional increase in MAP. If the expected increase in MAP is not seen, the PCM notes the amount of error that was detected and adjusts an internal fail counter towards a fail threshold level. When the fail counter exceeds the fail threshold level, the PCM will set DTC P0401. The number of test samples required to accomplish this may vary according to the amount of detected flow error.

Normally, the PCM will only allow one EGR flow test sample to be taken during an ignition cycle. To aid in verifying a repair, the PCM allows twelve test samples during the first ignition cycle following a scan tool Clear Info or a battery disconnect. Between nine and twelve samples should be sufficient for the PCM to determine adequate EGR flow and pass the EGR test.

Conditions for Running the DTC

• No TP, MAP, MAF, ECT, IAT, CKP sensor, VSS, injector circuit, Misfire, Idle speed, or EGR pintle position DTCs set.
• The AC status does not change.
• The transaxle range does not change.
• TP angle is less than 1 percent (deceleration).
• Engine coolant temperature is more than 75°C (167°F).
• Engine speed is between 900-1300 RPM.
• IAC position is steady.
• MAP steady between 15-70 kPa.
• Vehicle speed is more than 35 mph during deceleration.
• BARO sensor more than 70 kPa.
• The vehicle will need to be driven above 50 mph, and then allowed to decelerate. When the vehicle is decelerating while meeting all of the criteria listed above, the PCM will enable the test to run. As the test runs, you will see the desired EGR parameter and the Actual EGR position on the scan tool change from 0 to a calibrated value above 0.
Conditions for Setting the DTC

MAP changes monitored during the EGR flow test indicate insufficient EGR flow.

Action Taken When the DTC Sets

• The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.
• The PCM will store conditions which were present when the DTC set as Freeze Frame/Failure Records data.
Conditions for Clearing the MIL/DTC

• The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed.
• The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
• The DTC can be cleared by using a scan tool.
Diagnostic Aids

To verify a repair, clear info with the scan tool and run the EGR flow test keeping the following items in mind:

• The PCM will only run the EGR flow test during a gradual deceleration.
• The PCM will only run the EGR test during a closed throttle condition.
• The PCM will only run the EGR test at vehicle speeds above 30 mph.
• Several deceleration cycles will be necessary to run a sufficient number of EGR flow test samples (9-12).
• The EGR Test Counter displayed on the scan tool can be useful in determining that the EGR flow test is running and to keep track of the number of test samples taken. The counter will increment each time a test sample is taken.
Inspect for the following conditions:

Faulty connection or damaged harness. Inspect the wiring harness for damage. If the harness appears to be OK, observe the Actual EGR Position display on the scan tool while moving connectors and wiring harnesses related to the EGR valve. A change in the display will indicate the location of the fault.

Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

Notice: If the EGR valve shows signs of excessive heat, inspect the exhaust system for blockage (possibly a plugged catalytic converter) using the Restricted Exhaust System Check .

Many situations may lead to an intermittent condition. Perform each inspection or test as directed.

Important: : Remove any debris from the connector surfaces before servicing a component. Inspect the connector gaskets when diagnosing or replacing a component. Ensure that the gaskets are installed correctly. The gaskets prevent contaminate intrusion.

• Loose terminal connection
- Use a corresponding mating terminal to test for proper tension. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.
- Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connection. Refer to Testing for Intermittent Conditions and Poor Connections , and to Connector Repairs in Wiring Systems for diagnosis and repair.
• Damaged harness--Inspect the wiring harness for damage. If the harness inspection does not reveal a problem, observe the display on the scan tool while moving connectors and wiring harnesses related to the sensor. A change in the scan tool display may indicate the location of the fault. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.
• Inspect the powertrain control module (PCM) and the engine grounds for clean and secure connections. Refer to Wiring Repairs in Wiring Systems for diagnosis and repair.
If the condition is determined to be intermittent, reviewing the Snapshot or Freeze Frame/Failure Records may be useful in determining when the DTC or condition was identified.

[MMGT1]

And something I didn't see there was to physically check the EGR Tube for corrosion. Have seen quite a few of those replaced in the last couple of years.