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Hey guys, I have a P0401 dtc (Insufficient EGR flow) on my car and just need a bit of reassurance that I properly diagnosed the issue. On this engine there are 3 components that may fail and throw this DTC: Vacuum Modulator, EGR Vacuum Switching Valve, and the EGR valve itself. Another possible reason is clogged passages. I checked all hoses just to make sure there weren’t any cracks.
Anyways, I started off my diagnosing by applying 5Hg of vacuum while the engine is idling. It nearly stalled out, which leads me to believe that the EGR is working properly. Next, I attached a vacuum gauge to the vacuum modulator and revved it to 3000 rpm, and it recorded around 20Hg, which from what i’ve read, indicates a functioning modulator.
Then I tested the VSV, which I’ve heard is a common trouble area. First test I did was I checked the resistance. My DVOM read around 65-75 ohms, whereas the specifications say around 30 +-5 is where it should be. I then checked to see if it could hold vacuum, so I removed the two hoses, connected the vacuum pump to one end and plugged the other end with my finger. I tested it this way about 20 times and not once did it hold vacuum. It would leak every time. I have cleaned any passages yet, so i’m wondering if dirty passages could cause a VSV to fail. I’m pretty sure this is reason enough to conclude that the VSV malfunctioning, but just need clarification.
Thank you, and Happy New Years!
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OK guys, I went out and did another check on all of the tests and the modulator failed. At 3000 rpm it only produced 5Hg of vacuum. That means the diaphragm is either broken or it’s clogged, is that right? Could both the modulator and VSV be broken? Ugh.
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