Edit: Engine stumbling refers to engine response when hand vacuum trigger is pulled, not vehicle drivability.

Recently I had P0401 insufficient EGR flow. I searched internet, but most were throwing parts based on guesswork, including replace EGR, clean exhaust port/path downstream of EGR, replace EGR control valve, etc. I am glad I didn't follow them because my problem was the leaky vacuum reservoir. I hope this post to serve as a general "approach" for average fix-it-yourself-ers like me. Tools needed are a hand vacuum pump and a vacuum gauge. I had a 2-way scanner but it didn't help much.

First, disconnect vacuum hose at EGR valve, and attach the hand vacuum. Apply vacuum. If engine stutters, EGR valve and exhaust gas port/path downstream of EGR are functioning ok. In my case, engine stuttered. If your engine runs normally, either EGR valve is faulty or exhaust gas flow path is blocked. There are many threads and youtube videos for this type of repair.

If engine stumbles, problem location is upstream of EGR valve. (This was my case.) Engine vacuum is somehow not reaching EGR valve. At fault is either EGR control solenoid (EVR) not channeling proper vacuum to EGR or vacuum to EVR is insufficient.
Disconnect vacuum feed hose from EVR (not the one going to EGR) and measure vacuum. If vacuum is healthy (about 20), either EVR is faulty or the computer signal to EVR is faulty (this includes DPFE sensor sending improper signal to computer). To check EVR, open the cap and plug up the vent hole under the cap with finger. If engine stumbles, EVR is ok. Test DPFE (there are multiple threads and youtube videos on this) Scanner makes DPFE testing convenient but a multimeter can do the same. (I took advantage of my scanner)

In my case, vacuum at the EVR feed tube was out of range. I continued to check vacuum level at different hose connections, and found the vacuum reservoir didn't hold vacuum. It had cracks. No one sells the unit. Junkyard pull only.

Thank you.

 

You should reconsider the statement about lack of vacuum and EGR causing a drivability issue.
If the EGR does not open after the second test, a code is set for the fault detected.
When this happens, the PCM ignores the system because it knows it does not work.
The results of this is no drivability issue, but a loss of some fuel mileage.
A non opening EGR is minor to the drivability and not allowed to affect engine operation by Logic, and built into the program by. the software engineer.
EGR is part of Emissions control as a qualification to meet federal law if the vehicle is to be sold to the public.
Under normal operation it. is not allowed to interfere with drivability and let you stranded for it's malfunction.
Now if the EGR were stuck open, then it would have a major effect on engine operation and likely set a 402 code.
Good luck.

 

Thank you for the explanation. I will edit my post to clarify engine stumbling refers to engine response when hand vacuum trigger is pulled, not drivability. PS: You are the reason why I visit this forum. I know my problem will be at least reviewed by you. I am sure many other posters feel the same way.

 

Diagnostics is a lost art for most people. Good job doing yours!

 

Seems there is a missing problem - DPFE could be bad as well ...

 

Thank you for the pointer. Indeed it was missing. OP edited.

 

When I got P0401, I thought it would be an easy repair with a scanner. Finally, I am getting my money back! How wrong I was. I searched for an overview of EGR repair, but couldn't find one. I hope this writeup helps someone.

When I started going over EGR system, I had new parts in hand (DPFE, EGR, EVR, vacuum hoses and connectors) When the problem turned out to be the cracked vacuum reservoir, I returned all parts.

 

Thanks for the write up!