Code P0401, Check Engine?

1998 BUICK LESABRE
254,000 MILES • 3.8L • 5 CYL • 2WD • AUTOMATIC
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ALPHAA10
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According to my code reader, a P0401 code indicates an EGR flow problem. "Insufficient exhaust gas recirculation, "A" flow indicated."

My reader displayed also "$0010" in the upper right of the P0401 screen.

Already, I have installed a new EGR valve and cleaned EGR passages with an old speedometer cable attached to a drill driver. The cable emerged with sooty smears, but no large particles were seen.

After cleaning was completed, but before the new EGR valve was RE-installed, I ran the engine briefly until the "Check Engine" light began flashing, and I shut it down. Afterward, no soot particles were found anywhere in the engine bay.

Since I had driven briefly with the new EGR valve installed, and to make sure of my cleaning, I left the new EGR valve soaking in WD40 overnight, then reinstalled the valve.

Next, I cleared old codes from the ECM, and drove about 10 miles. During the drive, the MIL light stayed off until the final three miles, for a total of about seven code-free miles.

The engine still starts, idles and accelerates without hesitation. However, I have limited my driving to avoid possible catalytic converter issues.
Jun 2, 2024 at 5:39 PM
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BORIS K
  • Experienced Mechanic
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Hello,

Regarding the P0401 code you still might have a restriction in one or both EGR ports
A quick way to check the EGR passages on the 3800 engine is to take off the EGR valve. Get two long rubber hoses that will fit snugly into the EGR ports. Have the hoses long enough so that you can hold them as you are sitting in the driver's seat. What you will be doing is feeling suction in one hose and air pressure coming out of the other hose which tells you the EGR passages are clear. Take one hose and cover the opening with your thumb. Start the car. If you feel suction, that port is clear. Shut the car off. Now, get the other hose. Cover the hole of the other hose with your thumb. Start the car, with the engine running, feel for air pressure coming out of the hose. If you felt air pressure from one hose and suction from the other hose, your EGR passages are clear.

Also test the new EGR for function. Using a suitable scan tool command the EGR valve fully on. The engine, at idle, should stumble, start idling very rough and stall.

Cheers, Boris
Jun 2, 2024 at 9:11 PM
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ALPHAA10
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Thanks, Boris,

You said, "... test the new EGR for function. Using a suitable scan tool command the EGR valve fully on. The engine, at idle, should stumble, start idling very rough and stall."
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1. "Suitable scan tool" which commands the EGR valve ON sounds like something expensive. I don't recall my reader having a command function. Since my EGR valve is electrical, not pneumatic, is there a test I can make across two of its four conductors, to switch the EGR valve for the same effect?

2. Commanding the EGR valve ON, or open to flow, seems designed to test the proper function of the pintle under signal from the ECM. Would it help-- before the EGR command test-- to RE-soak the new EGR in cleaner overnight, to ensure no residue has re-formed around the pintle?
Jun 4, 2024 at 10:55 AM
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ALPHAA10
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Boris, are you there?
I left two questions for you several days ago. (See above)

1. I did the tube test for the two EGR passages and both are clear. One produces suction, and the other blows engine exhaust, and both increase with RPM.
2. I have no means to command the EGR valve Full On, as you suggested.
My OBD2 scanner cost only $15. It reports and erases codes, with minimal text explanation.
Jun 30, 2024 at 7:56 PM
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BORIS K
  • Experienced Mechanic
  • 836 POSTS
Hello,

Sorry for the late reply, I have been very busy with work.

You could try and clean the EGR valve with suitable solvent.

A good video, for a 5.7 but same principle:
https://youtu.be/6bmwYNuHhJQ?si=KxbJ3WdrIjlTnW_v
and
https://youtu.be/8zLgmL7iBqI?si=SUNl6m9xCE0LXZGJ

How to test:
https://youtu.be/Hf42gtAJN7I?si=0pMItMhIldzBfJmW

trying to simulate EGR opening/closing by hand would be impossible as the EGR would be either fully open or closed, nothing in-between.

Cheers, Boris

Jul 1, 2024 at 9:12 PM
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ALPHAA10
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Thanks, Boris!
And I fully sympathize with being overwhelmed by work-- work must come first.
Thanks for your help with this difficult-to-diagnose problem.
Your new suggestions look promising.
Jul 2, 2024 at 7:28 AM
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