P0401 EGR troubleshooting (1 Viewer)

Joined
Nov 13, 2013
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150
I'm looking for some more hint on my '97 LX. (316k miles)
Almost a year ago and 13k miles, I cleaned out the intake passages, etc. the check engine light didn't come on until last month and of course it has to pass inspection before the end of October.

So, I tested the EGR valve and it wouldn't hold a vaccum, so I replaced it. That didn't solve the problem.
So, I decided to remove the manifold and try to clean it again.

After removing the upper intake manifold, the passages looked good. I ran a wire down the passage and it is clear and also sprayed cleaner through it and it flows freely.
Since it didn't look like clogged passage was the issue I looked at the following:

VSV - when 12v applied it is blocked, with 12v not applied, it's open, that includes getting air through the passage through the intake manifold

Temp Sensor - I'm getting .189 M-ohms at room temp (80ish degrees), if I heat it up, the resistance decreases.

After re-assembly, I'm still throwing the the same code.
(By the way, the idle is too high now, so I need to check for an unmetered air leak also)

Anything else I should look at?
 
Joined
Jan 13, 2011
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Corrales, NM / Boulder City, NV
Did you make sure the vacuum ports on top of the Tbody are clear? I had 2 different plugged vacuum ports plus the port on the intake. Also might check the modulator valve mounted next to the EGR. Do you have the FSM? Theres a step by step diagnosis for this problem.
 
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Nov 13, 2013
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When I cleaned the intake last year, I replaced all of the vacuum hoses. I didn't replace them this time.
 
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Feb 9, 2020
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Houston, TX. USA
I wonder if you could pass inspection by deleting the code with your ultra gauge? I cleaned everything on mine and it actually went away for like 3k miles. Now it's back and I just clear the code 'til it comes back again like every 200-300 miles.
 
Joined
Aug 31, 2010
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Palmyra,PA
Try this, is works for me every time my 401code goes off.

Pull your EGR temp sensor and give it a good cleaning. I use a scotch pad and some brake cleaner to make it shine again.

Clean the probe, install, clear the code, I’m usually good for 8 months.
 
Joined
Nov 13, 2013
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I don't have the FSM, I guess I need to purchase.

I will clean the temp sensor better in the morning.

I can't pass inspection because the code is pending and then sets the code within 20 mils of driving.
I'll check the ports on top of the throttle body in the morning.

Thanks for the ideas. It's got to be fixable....
 
Joined
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Socal
I had almost the exact same issue, p0401. I removed the intake and cleaned everything, replaced the VSV and all hoses, put it back together and Dangit, still there. Cant pass smog and if you clear the code, they won’t smog it because the computer isn’t ready.

i took the intake off and splayed the end of a wire so it looked like a chimney sweep, reamed the heck out of the port on the intake with a ton of carb cleaner. Then I zip tied each vacuum hose to make 100% sure no air was getting by. I put it all back together and it worked.
 
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If you get the egr test kit from hudd expo, it will never come back..but you should do your due diligence with cleaning and replacing vacuum lines..
my understanding is that you need to get it turned off and the test passed, then you can get the test kit.
right now, the computer shows that the EGR test needs to pass and they can catch that at the emissions inspection.]
 
Joined
Nov 13, 2013
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I had almost the exact same issue, p0401. I removed the intake and cleaned everything, replaced the VSV and all hoses, put it back together and Dangit, still there. Cant pass smog and if you clear the code, they won’t smog it because the computer isn’t ready.

i took the intake off and splayed the end of a wire so it looked like a chimney sweep, reamed the heck out of the port on the intake with a ton of carb cleaner. Then I zip tied each vacuum hose to make 100% sure no air was getting by. I put it all back together and it worked.

I'd rather not spend another day taking it apart, but that might be what it takes.
 
Joined
Nov 13, 2013
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150
found a couple of things...
one - the PCV hose was cracked at the valve cover, definitely leaking. I tried to trim the end and re-use but it just split more, so I had a hose that I hose-clamped on. I'll have to order the correct hose.

I also found that the throttle cable was too tight, between those 2, that looks like it has solved by high idle problem. With that big of a vacuum leak, I wonder if that can affect EGR, even though it's not directly connected. I guess I'll have to see on that.

Thanks for the hints.

forgot to say.. I also cleaned the temp sensor again... this time, basically polished it.
 
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Joined
Jan 5, 2017
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If it really goes for 20 miles before the code reappears, plug your OBDII reader into the port in the parking lot and clear the code. That'll get you past the smog Nazis.

Not a long term fix, but you have to be able to drive it (I assume). As to buying the FSM, I have electronic copies of several years. PM me and I'll see if I can help you out.
 
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Nov 13, 2013
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In our county, they will not pass a car that has just had the codes reset, and they can tell.

2 examples. I bought a Subaru Forester (09) that was throwing a code for the o2 sensors. I couldn't register it until it passed inspection. So I replaced the O2 sensors, reset the codes, drive it for maybe 5 miles, took it in for inspection, failed because the tests were pending. I had to drive it for about 50 miles because before the internal test passed and it would pass inspection.

another example. My wife's Porsche Cayenne was throwing a code for emissions. I did what I thought needed to be done to fix it, drove it for 50 miles or so, test was still pending, so didn't pass. I took it back everyday and each time, the test was pending, to the point where I thought the inspection station didn't know what they were doing. I took the car on a road trip, light never came on, took it back to the inspection station and it passed.

The ultraguage in the LX is still showing p0401 as pending, so I know it won't pass inspection, yet.
Maybe after today.
 
Joined
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Understood. That sucks.

Have you tried pulling the main fuse (in the engine bay fuse box, not the one under the dash), after you reset the code? I think this resets all the ECUs and should clear the "pending" status.
 
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Apr 15, 2018
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Hope, maine
I don't have any help as I still have this code as well but very interested in any results. and I am sure you have read a 100 posts as I have but I have hopes for this one when I tackle mine this winter

 
Joined
Sep 15, 2016
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Chattanooga, TN
You can troubleshoot and address many/most of the possible issues without taking things apart. If you had everything working properly and the components like the EGR Vacuum Modulator, EGR Valve and lines are relatively new then my first assumption is that you've got blockage somewhere in the system. There are a few choke points that tend to block repeatedly until no more loose carbon is in the system. This loose carbon could be residual from when the 80 was driven with failed components that introduced gunk, possibly from the vacuum canister, etc.

A quick way to address what I think are the most common vacuum choke points follows. First with the two ports on the throttle body:
  • Grab a can of relatively mild aerosol solvent/cleaner. Perhaps an electronics cleaner? Brake clean, carb clean, etc. could all work but may be harder on any rubber/etc. so keep that in mind.
  • Start the 80 and let it idle.
  • One at a time pull the rubber hoses off of the two ports that are directly on top of the throttle body. These rubber hoses are slid onto 90 degree copper/brass elbows.
  • One at a time, with the straw on your spray cleaner, put the straw into the brass/copper elbows and squirt a bit of cleaner into the port.
  • If the port is blocked there will be no impact on the engine idle speed. Clean as necessary. If you can't clean the port with just solvent you can gently turn/remove the elbow with pliers and then use a small wire to clear the port in the TB itself.
  • If/once the idle speed changes when the cleaner is sprayed switch the the next port.
Next up the vacuum modulator which I think is the most common clog point:
  • If this is your first time working on the EGR Vaccum Modulator just go ahead and remove it. In the future you can check/do the following with it in place.
  • Get a short length of vacuum hose (6" to 1') and connect it to one of the three small nipples at a time on the VM.
  • Blow into the vacuum hose to confirm that air passes through each of the ports. You can use your fingers on the other two ports that don't have the line on them to confirm that all is air tight and to confirm the airflow/etc.
  • The most likely clog is in the straight in line ports and it's the TB side that clogs. This is port P to Q I believe. I've taken an old VM apart to inspect and the passage inside P is reduced to a small size that likes to clog.
  • If you have a clogged port squirt it with solvent to blow it out. This usually means that you squirt into port P. I enlarged the inside diameter on my VM to make the spray straw fit into it a bit so that this is easier.
  • Flow air through the VM to get the solvent out asap so that it doesn't damage the rubber.
The above have been repeating issues on my 80 and I'm slowly troubleshooting why. I've added an inline, clear fuel filter between the charcoal canister and vacuum system to see if that stops the flow of carbon chunks but will wait 6+ months before deciding on that. I've blown the system out, replaced components, etc. While I've applied other relevant fixes on my 80, the above clogged ports are what I've encountered after months of operating without a po401 so they are my best guess as what you may be experiencing. I've never had to clean my intake, temp sensor, etc. a 2nd time and my VSV valve has only been replaced initially, etc.

Beyond the above, which I've not seen documented online much or at all there is a lot of good information on this system available so I won't recreate it. If the above 10 minute clearing of ports doesn't resolve your issue you can move through broader tests to learn the system and identify your problem(s). Try searching the following in Google and note that a few of the videos show nice troubleshooting processes that are easy/quick and don't require taking much apart beyond pulling vac. lines:

Additionally I found it really helpful when I was learning how the system works to use a vacuum gauge connected at different points in the system. For instance you can remove the VSV from the system (leads to a po402 if done long-term) etc. to simplify testing by blocking the line from the EGR modulator to the intake pass-through. Add your vacuum gauge to the system after the EGR valve and you can watch when it should open. I have done this with the gauge mounted to the wiper arm so that I can watch the gauge in real time as I drive. Very informative.

Also keep in mind that after a fix and reset you have to drive to speed (45mph+ ?) and stop then drive to speed again before the computer will throw the code again. As you troubleshoot keep this in mind to help you get a sense of success or not.

In any case, as long as your components are functional I'm sure that you can troubleshoot and resolve this issue without a lot of disassembly or time invested.
 
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