Hey Guys, my 2003 impala 3.4 engine started knocking and running rough on sunday...i borrowed a autozone code reader and po401 came up...(egr) and also multiple cylinder misfire...i cleaned out the EGR valve real good but the knocking did not go away...i then pulled a sparkplug wire off one at a time and when i pulled the middle plug wire (front of engine) the knocking stopped, i then put it back on and knocking started again...can someone please give me some advise as to what might be the problem or what i can further check...thanks in advance...bobbyboyg

the only guess that im gettin but dont hold me to it is spark plug. i would go and get another set of spark plug and wires and see what happens

Hey Talon, i changed the plugs and the wires that same day...i started it a few minutes ago and it sounds like a rat a tat rat a tat sound with a louder thump at the end, it seems to be coming from the engine under the top of the intake manifold cover area...thanks for replying...bobbyboyg

well i couldnt tell you but :bump: you

i really wouldnt know unles i saw the car.
did the CEL come back on? is the coil bad? do you have good oil? is there oil mixed with your coolant and vice versa?

I have recently encountered the code P0401 or Insufficient EGR Flow on my lexus and learned so much about it... Thanks to my mechanic which is also the previous owner of my car... Anyway, the first time that I have encountered it.. I already assumed that there was a problem on my EDITED Specifically in the emission system or the EGR to be exact.. We all know that EGR stands for Exhaust Gas Recirculation. It is part of the vehicle emissions system, and is used to reduce combustion temperature and pressure to control Oxides of Nitrogen. There are generally three parts to the EGR system: the EGR valve, an actuator solenoid, and a differential pressure sensor EGR (DPFE). These things work together to deliver the correct amount of recirculation based on engine temperature, load, etc. The P0401 code means that OBD detected an insufficient amount of EGR. You may notice drivability problems such as pinging (a.k.a. pre-ignition knock) when the engine is under load or the vehicle is at higher speeds. There may also be other symptoms.

A code P0401 most likely means that the DPFE (differential pressure feedback EGR) sensor is faulty and needs to be replaced, there could also be a blockage in the EGR (tube), most likely carbon buildup, a faulty EGR valve or it may not be opening due to a lack of vacuum. In fixing this code, it is quite common for people to just replace the EGR valve only to have the OBD code return. The EGR valve is not always the culprit. Other solutions such as using a vacuum pump and pulling the EGR valve open while monitoring engine RPM's & DPFE voltage. There should be a noticable difference in RPM's with the EGR open... You can also clean out the EGR valve and/or tubing to remove deposits... Checking the voltage at the DPFE can also do the trick, compare to specified values (refer to a repair manual for your specific model)... The resort would be replacing the DPFE sensor or the EGR valve with a good quality / OEM one... Good luck!!!

If you pulled the plug wire off that cylinder, and the knocking stopped the problem is almost always a rod bearing.

To explain why; if the problem were in the valvetrain the knocking would make no difference vs. one cyclinder firing or not. Even a main bearing the difference in sound would be minimal by pulling one plug wire off. Also a valvetrain problem or main bearing would almost certainly cause low oil pressure.

Rod bearings usually make the most noise at idle and upon the beggining of accleration.

If you check the oil you may or may not find metal shavings.

Good luck and for your sake I hope I'm wrong.