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Just wanted to post this in case others have the same problem I did with the 3800 V6 in Impalas and other GM vehicles.

The EGR insufficient flow code P0401 can be a bit misleading, at least on the 3.8L. Just because "EGR" is in the title of the code, that shouldn't be the first thing to replace. If the EGR valve itself actually fails, you would probably get a different code, so save EGR replacement for last resort if these other remedies don't work.

A couple "non EGR valve" things to check if you get this code:

1. Check the EGR passageways: The EGR valve is a variable restriction valve between the exhaust manifold and the intake manifold. A stepper motor inside the EGR valve moves a pintle which creates the variable restriction of flow, from completely closed to wide open. If the stepper motor or feedback in the EGR is bad, you will likely get a different code. If you remove the EGR valve, you will see two ~1/2" holes. One leads to the exhaust, one to the intake. Sometimes, one or the other gets clogged, restricting the flow even when the EGR valve is open causing the code. It seems lots of other people have had this as the cause, but it wasn't the case for me. The easiest way I have found to verify passageways are open (try this at your own risk) is to remove the EGR valve on a cold engine, cover up the two holes with your fingers, and have a friend start the engine. There will be a little suction from the intake side and a little pressure from the exhaust side. If you remove your finger from the exhaust side, you should hear exhaust pretty loud. When you remove your finger from the intake side you should hear a sucking sound and the engine should cut off as the added airflow makes it very lean. If you don't hear much exhaust or the engine runs normal with the intake side open, clean out whichever passage is clogged.


2. Clean the IAC and / or throttle body: If the passage ways are clear from above and you are getting the "insufficient EGR flow" code, it could simply be crud in the IAC and / or throttle body. This was the case on my car. I would suggest trying the IAC first since it is the easiest to get to. I cleaned both of mine at the same time but in hindsight, it was probably just the IAC. To clean the IAC, remove it, clean the plunger and where it seats in the throttle body and see if that does the trick. Hopefully that will do it for you. Since there isn't actually a "flow meter" in line with the EGR, I think the computer uses the IAC position, TPS, and RPM vs. MAP and EGR position to determine if there is sufficient EGR flow, so "insufficient EGR flow" could actually be a cruddy IAC. Save cleaning the throttle body for last since if you end up removing it for cleaning, it could get messy and more involved. The molded rubber o-ring type seal between the throttle body and manifold will probably leak coolant if you don't put a new one in and/or the old plastic manifold (likely brittle by now if it is original) could crack, based on my experiences anyway.

Good luck and hope this saves someone the cost of an EGR valve and tons of troubleshooting time!
 
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