So I got the fun P0400 code and decided to just replace the entire egr. I pulled out the intake tube also and it was very dirty. So I cleaned it up using maf cleaner and flexible wire. I also had to lift up the fuel injectors a little to get access to the egr bolts. I also replaced the vacuum tube because it was crumbling.

I put it all together with the new egr, and I started the car. But a knocking noise appeared. Using a mechanics stethoscope, it seems like it's coming from the back of the engine. I took off the new egr and replaced it with my old one, and it still makes the knocking noise. The noise ticks faster as the RPMs go higher.

I don't understand what happened. I was very careful to clean the intake tube to make sure there was no debris in it, and I tried covering any exposed holes that debris can get in. The only thing I can think of is that some debris got either into the intake tube or the fuel injector area.

Anyone have an idea what the cause could be? Is there anything I can do to figure it out without having to tow it to a shop?

Here's some video I made to demonstrate the noise.
https://youtu.be/v9utLF6r4C0

Car: 2001 E320 base sedan

 

I appreciate your comment, I really do. Thank you. I already do have a general understanding of what the EGR system does. However, I don't see how doing an indepth study of the EGR system will help me with my problem. Could you clarify what you're trying to point out?

 

Check that the intake is seated properly on the throttle bod Sometimes it may look like it is but it's not. The gasket in there should be replaced too. Check the routing of all the vacuum lines and that one isn't bending the other etc. It seems like a small thing but with vehicles this old Just a bit of movement or pressure against a hose can reveal a leak.. Theres a lot of vacuum lines in that area that can be knocked out or mistakenly re attached. Did you re attach the large vacuum hose that connects to the intake tube near the throttle body?