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GM Tech (Cam)
GM Tech (Cam), Chevy Technician
Category: Chevy
Satisfied Customers: 15201
Experience:  GM Grand Master Technician 2007. 14 years experience.
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I have a 1988 Chevrolet C/K pickup Silverado with a 350

Customer Question

I have a 1988 Chevrolet C/K pickup Silverado with a 350 engine. I am getting a check engine code po404. I would like to know a possible solution to the problem.
Submitted: 1 year ago.
Category: Chevy
Expert:  Roy replied 1 year ago.

good afternoon my name is ***** ***** I will assist

the code is for the egr valve operation. in most cases, there is carbon that builds up on the pindle of the valve and will not allow the valve to seat correctly. removing the valve and inspecting will confirm. it will be obvious. if that is ok, make sure the passages are clear and you will need to replace the valve.


Circuit Description

An exhaust gas recirculation (EGR) system is used in order to lower oxides of nitrogen (NOx) emission levels caused by high combustion temperatures. The EGR 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 in order 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 control module controlled pintle valve. The control module commands the EGR by applying a 12 volt duty cycle to the EGR valve. This can be monitored on a scan tool as desired EGR position.

The control module can monitor the pintle position with the EGR pintle position signal. This sensor uses a variable resistor with a 5 volt reference, a signal circuit, and a ground. The signal circuit monitors changes in voltage on the variable resistor as the pintle moves.

The VCM will set a DTC P0404 if it detects a large enough difference between desired EGR and actual EGR.

Conditions for Running the DTC

The ignition voltage is more than 9 volts.

Conditions for Setting the DTC

The difference between the actual EGR position and the desired EGR position is more than 10 percent for more than 10 seconds.

Action Taken When the DTC Sets

  • The control module illuminates the malfunction indicator lamp (MIL) the first time the diagnostic runs and fails.
  • The control module will set the DTC and records the operating conditions at the time the diagnostic fails. The control module stores the failure information in the scan tools Freeze Frame/Failure Records.

Conditions for Clearing the MIL or DTC

  • The control module turns OFF the MIL after*****trips when the test has run and passed.
  • A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles. A warm-up cycle occurs when the coolant temperature has risen 22°C (40°F) from the startup coolant temperature and the engine coolant reaches a temperature that is more than 70°C (158°F) during the same ignition cycle.
  • Use a scan tool in order to clear the DTCs.

Diagnostic Aids

Excessive carbon deposits may cause a restriction of movement in the EGR pintle. This problem may be compounded in cold weather with moisture freezing in the valve. Check for full range operation in the EGR system with the scan tool.

Check for excessive exhaust back pressure.

An intermittent may be caused by any of the following conditions:

  • A poor connection.
  • Rubbed through wire insulation.
  • A broken wire inside the insulation.

Thoroughly check any circuitry that is suspected of causing the intermittent complaint.

Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

  1. This step will determine if the VCM can control the EGR valve.
  2. Commanding the EGR valve to the specified values determines whether the VCM can control the EGR valve accurately. Production variations may not allow the EGR valve to be commanded to 100%.
  1. There are certain circuits in the VCM C4 connector that could cause a short to voltage to the EGR valve control circuit that may disappear when the C4 connector is disconnected. By checking each individual circuit for a short to the EGR valve control circuit with a DMM will confirm if such a condition exists.
  2. If the EGR valve 5 volt reference is shorted to voltage, the DMM will read battery voltage and additional DTCs may be set.
  1. A problem may exist within the terminals which would not show up in probe type testing. Be sure to check the terminals for being backed out, improperly formed, or damaged.
  1. A problem may exist within the terminals which would not show up in probe type testing. Be sure to check the terminals for being backed out, improperly formed, or damaged.
  1. Clearing the DTCs is a very important step for this diagnostic. The clearing function allows the EGR valve to relearn a new minimum pintle position.