P0401 LX 450 Repair Log (1 Viewer)

I've got the dreaded P0401 with the LX450, its been on for about a month now and my MPG has been dropping, so I'm just starting this thread to catalog my experience fixing it and hopefully create a cohesive and streamlined solution/test report once I've gotten everything worked out.

From my searches it looks like this P0401 problem is very common, but there is not a common fix, just alot of common tests. I'm going to make an effort to make this thread the "go to" thread for P0401 issues with pertinent links to the necessary tests, as well as step by step picture aided guides for performing them.

Oh what a little adderall and a dSLR will do for you. Check back here later for updates.
-----Nate

NaterGator said:

From my searches it looks like this P0401 problem is very common, but there is not a common fix, just alot of common tests.

-----Nate

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I would argue, based on what I've read in this forum, that replacing the EGR valve is the most 'common' solution - especially if you have a green top EGR valve.

I think what you are getting at is that there is no 100% effective single repair or replacement part to fix the P0401 and that a P0401 can be the result of many different parts and/or system malfunctions.

I went from a green top EGR to a Blue top about 4 months back and haven't encountered a P0401 since. Before that time, it was a monthy or bi-monthly occurance. So a single repair/part worked for me, but that's not to say it will work for every one, every time.

A first very east step is cleaning the temp sensor.

Ross

450 Cruiser said:

I would argue, based on what I've read in this forum, that replacing the EGR valve is the most 'common' solution - especially if you have a green top EGR valve.

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And your argument would be wrong.

First, the "green" vs "blue" top is referring to the EGR modulator, not the EGR valve. Second, the blue top is just a newer part and nothing more and nothing less. If anything, the most common solution has been the cleaning of all the EGR passages. Next would be the replacement of the VSV for EGR. Third would be the EGR modulator, followed by replacing the EGR valve, then the cleaning and/or replacing of the EGR temp sensor. I cannot recall anyone actually getting a permanent fix from cleaning the EGR valve though there has to be someone that succeeded with this.

-B-

NaterGator said:

I'm just starting this thread to catalog my experience fixing it and hopefully create a cohesive and streamlined solution/test report once I've gotten everything worked out.

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Nate,
Your efforts are applauded though they may be redundant with what Raven is working on in another thread. Please see his thread before you spend a lot of time on this. I'll look up the link if you can't find it.

-B-

Beowulf, I assume you mean this thread:
https://forum.ih8mud.com/showthread.php?t=111750

And indeed, it would seem what I am doing is pretty redundant. Regardless, I'm going to try and make this a meticulous log of what I find and check, and if we have two sort of "logs" for the issue, then oh well, but at least those plagued by this issue will have more resources to consult.

NaterGator said:

... and if we have two sort of "logs" for the issue, then oh well, but at least those plagued by this issue will have more resources to consult.

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I think there is a P0401 section in the FAQ so check that out too. If your work meets the 'Mud standards then we'll nominate it for inclusion in the P0401 FAQ.

The above link is the thread that Raven is working on. Now that I read it again, it seems that his work can be added to your work with the link and you might be able to use some of the testing as you are working through your problem resolution.

Good Luck and keep this thread alive with posts and pictures.



-B-

Well, man I suck! Turns out the first thing I checked fixed my EGR problem (at least for now) so I didn't really do anything or research much. Oh well, maybe it will come back and I'll end up actually having work to do.

Anyways, the problem was my EGT sensor was covered in soot and must have been reading slowly/incorrectly, so I cleaned it with steel wool and finished with a little tooth paste, then rubbing alcohol to get it back to a nice shine.

Here is a pic of my dirty EGT:


Oh well! Glad I got the P0401 issue fixed for now at least.
-----Nate

No bright, shiny "after" pic.??

Err.. let my put my wacom +CS2 to good work

Actually, it was getting dark fast outside and I didn't want to waste time taking an after pic... just a before pic so at least there could be some reference of how bad mine got before the code was thrown.

If I recall correctly, when I encountered the PO401 code a while back. It ended up being the "EGR modulator" valve. A friend did various vacuum test which proved to be the "EGR modulator valve," or the "blue top," as being the problem. $90.00+ later, the problem was fixed.

NaterGator said:

Oh well, maybe it will come back and I'll end up actually having work to do.

Click to expand...

You can bet on it.

-B-

Beowulf said:

You can bet on it.

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Jerk! You jinxed me.

Came back on yesterday. Grr!

I hooked the good-ole Fluke 115 up to the EGT sensor and ran the truck for 30 minutes at various RPMs. The lowest reading I got was 103k ohms, and the FSM specs ~4k ohms once exhaust temps are up to normal (I can't imagine 30 mins isn't enough warmup for the EGT to read nearer than 103k ohms)

Obviously I either do have little/no exhaust return, or the EGT is borked. Since visual check of the EGR modulator looked good, I guess my next options are VSV and the EGT valve itself.

Any tips for getting to the VSV? Looks like its tucked up under then manifold pretty well.

Nate,
I can recall 1 report over the past 5 years where someone said they tracked a P0401 to a bad temp sensor. That report isn't reliable AFIAC because it came from a Cruiser mechanic who repaired a customer's FZJ80. The problem is that the replacement of most any component of the EGR system will result in the light going off and the P0401 being cleared for a short time. 1 week, 2 weeks, sometimes a month, and sometimes 6 months (I think that was my record.)

So we really don't know if that temp sensor was just a 2 week fix and the customer got pi$$ed and took it to a Toyota dealer or if the problem was permanently fixed. Other than that single report, there have been no other reports that I can remember (and my memory get's worse every day ) of anyone that had achieved a permanent fix by cleaning or replacing the EGR temp sensor. Does yours fail the tests in the FSM? If not, then move on to the VSV for EGR.

The VSV for EGR can be removed by accessing the bolt from below using about 3' of extension on a socket. Then you remove the brace on the intake (depending on year) and use tiny flexible hands to feel under the intake to remove the hoses and the valve. Installation is the reverse. There are tests for the VSV. Some have seen intermittent failures. Mine was a solid failure but that was after the P0401 became solid. (Reset, 2-trips, P0401... every time.) Until you reach "solid" failure, yours may be intermittent like Raven is seeing. There is a recent picture of the VSV in the other thread so study how it looks so you can remove the hoses.

Good Luck.

-B-

Beowulf said:

Nate,
I can recall 1 report over the past 5 years where someone said they tracked a P0401 to a bad temp sensor. That report isn't reliable AFIAC because it came from a Cruiser mechanic who repaired a customer's FZJ80. The problem is that the replacement of most any component of the EGR system will result in the light going off and the P0401 being cleared for a short time. 1 week, 2 weeks, sometimes a month, and sometimes 6 months (I think that was my record.)

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tarbe was able to clear his EGR code (P0401 or P0402?) by cleaning his temp sensor with steel wool. He also cleaned out all the vacuum line ports. Hopefully he will see this and post specifics. I do know that he has been EGR code-free for many thousands of miles.

Ross

NaterGator said:

Well, man I suck! Turns out the first thing I checked fixed my EGR problem (at least for now) so I didn't really do anything or research much. Oh well, maybe it will come back and I'll end up actually having work to do.

Anyways, the problem was my EGT sensor was covered in soot and must have been reading slowly/incorrectly, so I cleaned it with steel wool and finished with a little tooth paste, then rubbing alcohol to get it back to a nice shine.

Here is a pic of my dirty EGT:

Oh well! Glad I got the P0401 issue fixed for now at least.
-----Nate

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Where is this thing, near the EGR? I wanna clean mine, i am slow today so i need things to do!

Spydaz said:

Where is this thing, near the EGR? I wanna clean mine, i am slow today so i need things to do!

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On your '97, it is on the DS of the intake close to the master cylinder.

GeoRoss said:

tarbe was able to clear his EGR code (P0401 or P0402?) by cleaning his temp sensor with steel wool. He also cleaned out all the vacuum line ports.

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My money is on the cleaning of the vacuum lines. That has proven to be the best "bang for the buck" in resolving P0401 codes. There have been many that have fixed their problem by cleaning the port that goes through the intake.

While that temp sensor may look "bad", I seriously doubt it's ability to measure temps is affected by the carbon. Carbon, on the other hand, is highly effective at blocking air flow which is what the VSV needs.

My 2c.

-B-

Nator Gator, try this out and tell me what you think, it is as thorough an EGR system check out as I can come up with and should lead you to your fault. If it works for others it will become my standard response for EGR problems.


I recently got the dreaded P0401, I started with all the tests found in two different areas of the FSM and found no solid culprit. Those tests are not good at catching intermittent problems. Maybe if I had access to the Toyota/Lexus hand held tester FSM tests would have been more fruitful. But those hand held testers are outrageously expensive and out of my reach. The FSM tests were still important though as they were the basis of later tests that I came up with.


Some other things to try if the FSM tests do not get you there.

Tooling

Hand vacuum pump, a friend of mine had one of these that I borrowed, it is just a simple hand operated pump with a vacuum gauge on top.

Vacuum gauge, I have a MityVac gauge, scroll down and look for "05511 Vacuum/Pressure Gauge" http://www.mityvac.com/pages/products_ede.asp , I think I got it at Pep boys for something like 25$ It comes with the "T"

6’ or more of vacuum line, the hard lines on the 80 are .155” OD, 3/16 (.189”) vacuum line seals but does not hold on very tight works OK for temporary testing, if you could find some 5/32 vacuum line (.159” IIRC) it should work better.

Female vacuum caps, 5/32 would best.


Modulator

The Vacuum modulator is the brains of the EGR system, it has two vacuum inputs from the throttle body to watch throttle plate position and one pressure input from the exhaust to watch engine load, and from these three inputs it outputs a vacuum signal to the EGR valve.

This vacuum signal is nil at idle and provides more vacuum as the throttle plate opens more, at about mid way it reaches maximum vacuum, as the throttle is opened further past aprox mid point the vacuum sharply decreases and is again nil when the throttle is wide open.

Alternate tests for the Modulator,
If you blow into the port on the bottom (exhaust pressure port) and air comes out of any of the other ports then the diaphragm is bad, if you can see any carbon in the system then the diaphragm is bad. With the cap and filter off the modulator and the engine running I could feel exhaust pulses coming out of the breather hole in the top of the modulator, later on I cut my old modulator open and found the pin hole leak in the diaphragm,

I replaced my green top modulator with the new blue top, even though it was “bad” and needed to be replaced it was not the source of my P0401. It still passed all the FSM tests and provided the proper vacuum signal to the EGR valve.

The leaking exhaust gasses would have eventually fouled the whole system with carbon witch would have added other problems on top of the existing problems making troubleshooting even more difficult, but I caught it early enough where the fouling was limited to just the modulator and its filter.


EGR Valve

The EGR valve is pretty simple, it has a diaphragm and a spring fighting each other, as the vacuum sent to it increases the diaphragm moves the valve towards open, as the vacuum decreases the spring pushes the valve towards closed. The amount of EGR flow is therefore proportionate to vacuum applied.


Alternate EGR valve tests,
plug one of the two ports with a vacuum cap and apply vacuum to the other with a hand pump, it should hold that vacuum without leaking down, this tests the diaphragm for leaks, if you do this with the engine running it will also stall the engine testing that the valve actually opens and that exhaust gasses do flow. this is an alternate version of the FSM test. If the valve opens and closes and the diaphragm does not leak the valve is good.

Line System

I also checked to make sure all of the vacuum lines were clear including the hidden path from the VSV to the throttle body under the intake by drawing vacuum through them with a hand vacuum pump, They were all wide open. Many have found these plugged with carbon, I also plugged and applied vacuum to different sections to make sure there were no vacuum leaks.

EGR temp sensor

This is the ECU’s only eyes into EGR operation, when EGR flows it gets hot, early FZJ80’s did not have one unless they came from California. If the temp sensor does not get hot when it is suppose to it will throw a P0401, If the temp sensor is hot when the EGR is not suppose to be running (during the early stages of warm up) then it throws a P0402.

I did not bother pulling and cleaning my temp sensor when I was chasing my P0401, the factory seal looked too good to mess up, I did check its resistance per the FSM wile in place and it was in spec, no need to disturb that seal when it is very unlikely to solve the problem. YMMV

VSV valve

The VSV valve's only task is to veto EGR operation, as far as I can tell it is only suppose to do this during the early stages of warm up. it is a simple electric open/close solenoid controlled by the ECU, when it is closed the EGR system is armed and is controlled by the modulator, when it is open the VSV allows atmospheric pressure from the throttle body to flow into the EGR valve defeating the vacuum signal from the modulator.


The VSV is expensive over a weeks worth of groceries, I had no problem replacing it IF it was the culprit but I wanted to be certain before I laid down the $ for a new one, I needed a way to watch the EGR system in action all together wile driving, that is where the FSM calls for the hand held tester, Since I knew that my EGR valve worked and did not leak if I watched the amount of vacuum it was being fed to it I could tell what it was doing and why the ECU was unhappy.

Driving Test

I removed the clutch master plug in the firewall and ran a vacuum line from the engine bay into the cab and hooked it to the vacuum gauge.

Use it one of two ways,

One way to hook it up reads the pressure to the EGR valve with everything intact.

Make a 5” or so vacuum line with a ‘T’ in the middle one end goes on the EGR valve, the other on the nipple on the intake manifold and T in the sense line to the cab, (see pic 1)


When you first start up cold the VSV is open so you will see no vacuum, shortly after the vacuum should start to follow throttle plate position

Idle, no vacuum,
Cruise, lowest pressure, on a slight downhill as low as 12 or 13 inches of mercury on steep down hills the pressure would increase, and uphill it would also increase
Regular acceleration ~5 inches of mercury
Moderate throttle to WOT, no vacuum

If you are not getting any vacuum the Modulator is either not providing any or more likely the VSV is open defeating what is being provided.

To test with is witch the other way is to watch just the modulator alone: remove the short line that runs from the nipple on the intake manifold and goes to the EGR valve, plug the intake nipple and hook the sense line from the cab to the EGR valve, (see pic 2)

Now you can drive around and see the signal that the modulator is sending to the EGR valve without interference from the VSV,

If it works as it is suppose to this way but did not before then most likely your VSV is bad, if you still have no vacuum the modulator is bad or there is a plugged line or a vacuum leak somewhere.

Be aware that because the VSV is out of the picture and unable to kill the EGR during warm up that it will throw a P0402.

Beowulf said:

My money is on the cleaning of the vacuum lines. That has proven to be the best "bang for the buck" in resolving P0401 codes. There have been many that have fixed their problem by cleaning the port that goes through the intake.
-B-

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What's the best way (er, I should say EASIEST way, lol!) to clean this port? Mine is clogged. When I disconnected the v line from the EGR valve and try to blow or suck through the one that goes into the intake plenum, I get nothing--it's plugged good. I tried running a piece of wire through it, but it feels like to goes through the port and into the vacuum hose then stops--I'm sure if I push too hard I'll just puncture a hole in the vacuum line.

How are people cleaning this?

Thanks!

Anyone? I need to clean this port/vacuum line as mine is clogged.....anyone have any advice on how to do this?