Tester was correct about the 401 being related to low EGR flow. Let me preface my post with this statement ahead of time… I would NEVER challenge Tester to a mechanical knowledge/diagnostic competition. I respect Testers mechanical opinions to the absolute highest degree.
I just want to share a successful repair of a 401 on a Toyota Camry earlier this month.
The 98’ Camry posted a 401 which is low EGR flow. In this vehicles instance the EGR flow was calculated based upon how far open the Vacume Switching Valce (VSV ) was operated…if the vacume switching valve (VSV) did not register the expected result the ecu assumed it also did not open the EGR valve enough as requested (which would be low EGR flow). The EGR valve in this instance was all manual…no electronics/intelligence associated within it, so the only reference to how far open or closed the EGR valve was came from the result of the VSV. Fuel trim and MAP could be factored in I believe but still it posted the 401…
What repaired the problem was a new VSV…which was pulling the EGR valve open when it should not have been. Because it was in a position of being stuck open slightly. Stalling the engine, running very rich, misfires, black sooty exhaust.
What confused me was how can “low flow” make my EGR be open? I believe the answer was that the VSV did not signal as opening enough when requested, thus posting a low flow code. In reality the VSV stuck open slightly and probably registering a “closed” result back to the ECU. So while the ecu thought it wasn’t opening the valve to the value being asked (it wasn’t) it did not know that it was stuck open or hard to move/not being in the closed position.
So… this was just my experience…if I am interpreting the hows and whys of it all incorrectly then I apologize. The solution was the new VSV. After the new valve was installed…it instantly fixed the issue. Before the new VSV, the EGR was half to wide open even at idle…if it would idle… Vacume was constant at the EGR till new VSV was installed yet it registered as a 401 which indeed is low flow. The old VSV was indeed stuck open slightly as tested afterward. The low flow was a result based on how far open the ecu thought it operated the EGR based on the VSV result…which made it think the EGR also did not open as far as commanded. When in reality the EGR was almost wide open.
If anyone can correct my misinterpretation of this successful repair, I am welcome to be properly informed for my own sanity and future reference.