Q: Engine is running rough and P0401, P0171 and P0174 codes.

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Trying to diagnose codes listed above. Tested EGR for plugged or restricted lines. Checked OK. Will stall engine when idling and you cover EGR Control Solenoid vent with your finger. Pulled EGR valve and lines to visually check. All OK. All electrical checks of EGR Solenoid and DPFE are OK. Found vacuum to EGR valve keeping EGR valve open all the time engine is running. When tested with a vacuum guage you have 5 inches of mercury at idle which increases to 18 to 20 when you depress accelerator and drops back to 5 inches when you release accelerator. This happens even when truck is in PARK and engine is running. I think this is causing engine to act like it has a large vacuum leak which I think would cause the P0171 and P0174. I don't know what is causing the EGR Solenoid to allow the vacuum to pass through and open EGR Valve. Any advice? Sincerely, Ron Zara
My car has an automatic transmission.

The first thing to check is to see if the EGR solenoid will release the vacuum when the electrical connector is disconnected. If it still doesn’t release the vacuum then the solenoid should be replaced. The codes need to be cleared and the vehicle should be driven to see if any of the codes come back.

The EGR solenoid has a small foam filter on the vent side and it is there to keep dirt out of the valve. If dirt gets into it, the solenoid will be held open and allow vacuum to pass through it. If you find that the solenoid is not leaking vacuum when the connector is disconnected then you may have a problem with the DPFE telling the ECM to open the valve. From what it seems like, I think you will find the solenoid is going to be the problem. I’d recommend having a certified mechanic check your vehicle out, they will be able to properly diagnose your Check Engine Light codes and make the necessary repair.

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