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Hey Guys, this is my first time posting a question on here. I need some insight on this ongoing issue related to this P0401 "Exhaust gas recirculation flow insufficient detected" issue I've been having for months. I have 2001 Impala 3.8L with 120K miles, and I changed out many old parts for new parts, but this is the only code that keeps coming back. Just to give you a rundown of what has been replaced:
  1. New Air Filter
  2. New Air Change Temp Sensor
  3. New Throttle Position Sensor
  4. New Idle Control Sensor
  5. New MAF sensor
  6. Throttle Body has been throughly cleaned with no carbon buildup
  7. New Throttle Body Gasket
  8. New Intake Manifold
  9. New EGR with a new gasket
  10. Clean Tube to EGR
  11. Clean Lower Plenum and Replaced Gasket and Re-sealed (No Leaks)
  12. New Upper and Lower Elbows
  13. New MAP sensor
  14. New PCV valve with upper and lower o-rings and housing (came with Intake Manifold)
  15. New Spark Plugs and Wires
  16. New Ignition Coil Packs (All Three)
  17. New Purge Valve Solenoid
  18. New Pressure Regulator Vacuum and Vapor Canister Purge Harness
  19. New Catalytic Converter
  20. New Resonator
  21. New Exhaust Pipe
  22. New Muffler (No Exhaust Leaks, sound quiet)
  23. New Fuel Lines
  24. New Fuel Filter
  25. New Vapor Canister Vent Valve
I recently performed a Vacuum Gauge test, at idle it shows 21in Hg. But, when I rev the engine, the needle drops to about 7in Hg then goes above the "Normal Motor" section and then idles back into the same 21in Hg. I know the car is over 20 years old, and it doesn't look the the EGR or it components are the problem. Is it safe to say it's the exhaust manifold causing this issue? I have not replaced the front or rear exhaust manifold, or the crossover. I want to know if this is why this code keeps appearing once the engine gets to operating temp. Or, when the vehicle speed drops below 50mph the check engine light pops on. It's the same code every time. If this is the case, is there a way to clean the exhaust manifold without removing it? I don't see any physical damage on the outside.

Thank you for any insight you can provide to this ongoing issue.
 
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