P0401 puzzle (1 Viewer)

[Imogene]

Thank you Ih8mud users for all of the P0401/EGR posts, I've made extensive use of the posts in my P0401 code chase (even though the lack of emissions testing in my Colorado county makes this process purely for "fun". )

My Code Chase has included the following:

1. New EGR tested with vacuum tool to confirm proper operation
2. Cleaned and tested Vacuum Modulator
3. Dorman VSV replacement
4. Replacement of EVERY vacuum tube
5. Cleaned out the "pass through" from VSV to EGR
6. Pulled and Cleaned out bent ports E, R and P on throttle body

The puzzle I can't solve which appears to be causing the P0401 code is the lack of Vacuum coming into the VSV from the throttle body. There is a very strange passageway which passes through the gasket to get around the butterfly. I've tested for leaks and blockage but can't see any vacuum.

Any ideas???

Thank you

 

[inkpot]

Can you blow thru it to insure the passage is open?

 

[Imogene]

Yes, I used a vacuum pump and pulled air through with no pressure. Then I attach the pump and run the car and no vacuum is being pulled (this would be the vacuum that runs to the VSV)

 

[izzyandsue]

I fixed mine after years of having it off and on. The fix was to remove the TB, disconnect the hoses but not the cables. Then clean out the tiny passages that are inside. There are 2 that I remember, and you may need a needle to poke them and spray some cleaning fluid until it cleared the obstruction and gunk that collects in it.

 

[Malleus]

Thank you Ih8mud users for all of the P0401/EGR posts, I've made extensive use of the posts in my P0401 code chase (even though the lack of emissions testing in my Colorado county makes this process purely for "fun". )

My Code Chase has included the following:

1. New EGR tested with vacuum tool to confirm proper operation
2. Cleaned and tested Vacuum Modulator
3. Dorman VSV replacement
4. Replacement of EVERY vacuum tube
5. Cleaned out the "pass through" from VSV to EGR
6. Pulled and Cleaned out bent ports E, R and P on throttle body

The puzzle I can't solve which appears to be causing the P0401 code is the lack of Vacuum coming into the VSV from the throttle body. There is a very strange passageway which passes through the gasket to get around the butterfly. I've tested for leaks and blockage but can't see any vacuum.

Any ideas???

Thank you

The very strange pasageway is the bypass for the idle air control valve. What you didn't say was that you tested the Dorman VSV. Also, as Izzy says, you may not yet have a clean TB. Try spraying Brakleen through all the ports and see where that gets you.

 

[Imogene]

Thank you. I've ordered a new Throttle body gasket, and a multimeter and will use brake cleaner to get the throttle body clean and then test the VSV.

 

[Imogene]

I figured out my issue. I thought I'd share to maybe help others in a similar situation. The problem was the Vacuum Modulator. Everyone says it doesn't fail often so I cleaned it and tested it moved on but in my case it was "a little bit clogged" and passed my test but did not operate properly. The new Modulator was $17 so I'd recommend just replacing right off the bat instead of attempting to clean as I did.

 

[Box Rocket]

I figured out my issue. I thought I'd share to maybe help others in a similar situation. The problem was the Vacuum Modulator. Everyone says it doesn't fail often so I cleaned it and tested it moved on but in my case it was "a little bit clogged" and passed my test but did not operate properly. The new Modulator was $17 so I'd recommend just replacing right off the bat instead of attempting to clean as I did.

Where did you find the modulator for $17?

 

[b16]

my modulator seems to get clogged every year. clean out all ports with TB cleaner and it is good to go again.

 

[jpoole]

I had the modulator clogging issue a few times a year for multiple years after cleaning and replacing most of the system. I would blow the clog out (from P to Q ports) with a quick squirt of electrical parts cleaner and follow that with running the engine to be sure to pull the solvent out and avoid modulator damage. If you put a clean paper towel on the outbound, Q port you'll see the residue show up on it once it's clear. You can test the passage for a clog by putting a vacuum on the P port to see if it holds (length of vacuum line to your mouth is a quick way to test).

Eventually I added an inline fuel filter between the charcoal canister and the vac system under the modulator as I suspected bits of charcoal were leaving the canister and causing the modulator clog but this didn't fix the problem.

In early 2022 I installed a tiny inline filter between the EGR valve and the vacuum modulator and haven't had a PO401 since. I'm not sure where the little bits of crud were coming from but the filter seems to have worked.

Below is similar to the filter I used. It's too big for the vac lines but they can be squeezed on. I assume there may be a better option but haven't put more thought into this in the year and a half since the PO401 disappeared

Amazon product ASIN B08P2NJM5H​