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EGR MIL P0401, Code 71, engine check light, ad nauseam (long)

Discussion in '80-Series Tech' started by ParadiseCruiser, Jul 30, 2003.

  1. ParadiseCruiser

    ParadiseCruiser

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    Short version.

    If you don’t have the time to read this post, it was the vacuum modulator.

    Long version.

    Background.
    1997 1FZ-FE, 80K miles, MIL = P0401, EGR gas underflow detected.

    History.
    Have suffered the little idiot light for about 6 months.

    Observations.
    The most common fault reported among Cruiser owners - and their mechanics - is failure of the vacuum switching valve (VSV for EGR), occasionally carbon build up in the EGR pipe, intake ports, or elsewhere (though most commonly on higher milage rigs), and even less often, the EGR temperature sensor, or possibly (but rarely) the vacuum modulator. All of these preclude the more obvious disconnected or cracked and leaking vacuum line(s).

    Troubleshooting.
    Working from the easiest, to the most obvious...

    1. The vacuum lines are all connected, not brittle or cracked, nor plugged.

    2. The EGR temperature sensor measures within tolerance on an ohm meter.
    Digression. In my haste to solve this problem last December, I misread the FSM as to the allowable tolerance, and happily announced my solution. Even an over exuberant Gumby went dashing out “into the cold Chicago night” to measure the resistance on his temperature sensor, only to find that some fool out West had misread the book and had sent him out there for nothing. In his carpet slippers, no doubt. ( I still don’t feel particularly bad about it though, as it was snowing here at the time...)

    3. Replaced the VSV. As soon as the valve came out, I realized this was not it because the resistance of the coil was within the published tolerance. The new one went on anyway, because this is the most common cause of this particular fault, and I did not want to mess with this particular PITA a second time.
    For those who would like to know, the swap was done with intake manifold on - removing the valve bracket from below with a very long extension. It took two of us a little over 2 hrs, with some horsing around... and the MIL returned almost immediately, as expected.

    4. Decided to pass on the EGR valve/pipe because with such low milage, I did not expect carbon build up to be an issue. In fact, when the temperature sensor came out, there was almost no carbon present, confirming this observation. The difficultly in removing the EGR valve/pipe also helped sway the decision to pass on this step.

    5. Pulled the vacuum modulator (simply slips out of its holder after removing the vacuum lines). Removed the filter in the top, blew it out and replaced. Blew through the valve (as described in the FSM), and though extremely subjective, it appeared to be ok.
    This initially appeard to have solved the problem... but after about 200 miles, the MIL returned with another P0401 from the OBD.

    Final Solution.
    Ironically, the solution - in theory anyway - comes from Gumby. Here is an observation he made last month:

    I'm taking an advanced FI course for Fords this week and today we talked about EGR for a bit. Fords have terrible EGR systems too, so I learned a lot. Some problems they are having with low flow include small cracks in the EGR tube, Their version of the VSV, and their pressure monitoring system. They no longer use the temp sensor, but use a MAP type piezio crystal sensor. Their VSV gets plugged on the vent that releives the vacuum to the EGR when the valve shuts. I assume that's what the modulator does on the Toyota system. That would account for high flow, though. I wonder if a leaky modulator would not let the EGR open enough?


    A leaky modulator would not let the EGR open enough?” = EGR underflow = P0401

    1. Again, remove the vacuum modulator.
    2. Blow out all passages really good. What I mean by this, is firmly plug two of the four ports with the tips of your fingers, then blow about 20-25 psi through one of the open ports in a couple short, quick bursts, then from the other side, blow back in the other direction. Proceed to block each of the ports, and force air through the open pair in both directions, making sure you have accounted for every possible combination of blocked/open ports.
    You will be able to hear the internal valve popping open and closed, in some cases letting the air pass, and in others blocking it completely. My suspicion was that the valve was not completely opening (or closing/seating) - perhaps from dislodged carbon, or whatever - and allowing the modulator to “leak” when it should have been sealed.
    3. Reinstall the modulator, and clear the MIL.


    It is now 1000+ miles later - up and down 4K-10K foot grades in the Sierra Nevada, more drive cycles than I can count - and no MIL / engine check light. No more P0401 codes. No more “soft” codes from OBD either, for that matter.

    It was a “leaky” vacuum modulator. Thanks Gumby. That will learn us to bone up on theory... and make damn sure your students know I said so. ;)

    Cheers,

    R -
     
  2. Gumby

    Gumby Supamod Staff Member s-Moderator

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    Thanks for the kind words. Even a blind squirrel can find his nuts once in a while. :D

    Thanks as well for doing all the work and spending all the money to find out what's going on with this stupid code. Those of us who are lazy and cheap just keep coming up with theories and hope someone else will actually figure it out. ;)

    I will attack the modulator first thing tomorrow. I have to go get emissions tested this month. I'm sure I'd pass, (mostly because they can't load test an 80 on their two wheel dyno :D) except I'd have to reset the light before I went in. They would fail me right away with the light on.


    I'd send you a :beer: except Woody hasn't given us back our smilies yet.
     
  3. Junk

    Junk

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    [quote author=Gumby link=board=2;threadid=4022;start=0#msg29605 date=1059614539]
    I'd send you a :beer: except Woody hasn't given us back our smilies yet.
    [/quote]
    [​IMG][​IMG][​IMG]

    Gumby - I had the P0401 for about a year. Replaced the VSV for EGR and it be gone lickitysplit.
     
  4. Gumby

    Gumby Supamod Staff Member s-Moderator

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    That's what I'm afraid mine is. I left one bolt out and loosened the other one when I had the intake off. It was giving me really strange readings when I checked the resistance. I'm hoping it's a dirty modulator cause I can fix that cheap. I'm having a hard time spending money on this light when the truck probably works better with low flow to the EGR and I have a sick 40 and a newborn T-100 to tend to.

    That reminds me I have to change my sig...
     
  5. landtank

    landtank SILVER Star

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    I've said this before, there is no magic bullet, you need to take a logical approach to troubleshooting these types of problems. Tearing into the truck poking, prodding, cleaning and replacing randomly is just going to waste alot of time and money and can get confusing quite quickly.

    The first thing is to validate the error. This is done by confirming the temp sensor is good. By first cleaning it and then placing it in an oil bath, heating and measuring 3 values at 3 different temps. This is to verify the extremes at cold and hot and also the slope of the sensor with the mid point value.

    Next is to check the operating voltage of the circuit. Bad voltage will produce error accross the temp sensor. Read the voltage accross the connector at the temp sensor.

    After this and resolving any issue if you encoutered any reset the error and take it for a run. If the light stays out, guess what...YOUR STILL NOT DONE! There could be several small problems that add up to getting the code and since you resolved one of them or slightly improved the temp sensing ability you could be only marginally on the ok side not in the sweet spot of the exceptable range.

    So now you need to check how well the EGR system is working. Since you have confirmed that the temp circuit is up to snuff you can use it to evaluate the systems flow. Take that same drive on the highway that the FSM describes but this time pull over and check the resistance accross the temp sensor. If everything is fine you should be in the middle of the range. If you're not then nows the time to start cleaning and checking the controls taking that run in between to evaluate your progress.

    Remember, it's not what is actually happening that the ECM is concerned with. It's what it "THINKS" is happening that matters. Once you're able to produce the proper resistance accross the temp senser at the proper time the error will disappear.

    Incidentally this doesn't come from personal experience with these codes but 20 years of troubleshooting electro-mechanical systems. Beleive it or not this type of circuitry has been around longer than I've been fixing them. Not in cars but most everywhere else.
     
  6. Beowulf

    Beowulf

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    Nice post Ron!
    -B-
     
  7. Eduardo96FZJ80

    Eduardo96FZJ80

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    R-
    I was one of the lucky one's also.
    I R/R my modulator and have had no MIL since :D
    about 5k miles ago
     
  8. landtoy80

    landtoy80

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    The CE light came on and off again. I bet the modulator needs cleaned again (3rd time). I will clean the temp sensor too (2nd time)
    On the 93/94, where is the sensor that sends the signal to the computer that there is a EGR problem code71. I don't know how many EGR signals the newer 80's get but we only get one.
    kurt

    Just for the heck of it I will try the modulator from the 87FJ60. The part # is 25870-61011 The fzj80 #25870-66010. The 60 is bigger.
    kurt
     
  9. Junk

    Junk

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    I think that's 80% hogwash. Everyone I know of, with the P0401 code, eventually fixes it by replacing the VSV for EGR. I replaced mine on blind faith and have no errors and no issues.

    See above :flipoff2: :D

    Seriously, I don't know why folks make such a big deal out of this. The little switch is like what $~75 (is that right Cbiskitdan?) and it's not that bad to replace. Shees, if we winced every time we had to replace a $75 part, we'd sound like Wench. :flipoff2: :D
     
  10. Gumby

    Gumby Supamod Staff Member s-Moderator

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    I have to agree. I don't advocate parts replacement to find a problem, but I do know that mechanics make money by knowing what common problems are and what the fix is. If I could get consistant readings out of my VSV, I would at least have an idea if it needs replacing. For $75, I can look at this light for a while longer. It doesn't hurt anything, in fact, it probably adds a little bit of power.
     
  11. landtoy80

    landtoy80

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    [Seriously, I don't know why folks make such a big deal out of this. The little switch is like what $~75 (is that right Cbiskitdan?) and it's not that bad to replace. Shees, if we winced every time we had to replace a $75 part, we'd sound like Wench.]

    I spent the $ for the VSV a month ago, :mad: so replacing it again should not be needed, I hope :eek:
    For some reason the modulator is getting carbon in it. I tried the fj60 and after 40 mi it had some carbon in it and the light came on. When I pulled the 80 modulator it had carbon in it. It is clean and installed again.
    I wounder if there is a way to put a filter in the big vacuum hose to the modulator? I assume the carbon is commingfron the big vacuum hose.
    Would having the motor decarboned via a repair shop help? Or better black tap over the light :D
    kurt
     
  12. Junk

    Junk

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    [quote author=Gumby link=board=2;threadid=4022;start=0#msg30027 date=1059849155]
    For $75, I can look at this light for a while longer. [/quote]

    Fess up Gum. :flipoff2: We know it's not the fla$h, it's just you're a lazy sob. :flipoff2: :D :D :D
     
  13. Gumby

    Gumby Supamod Staff Member s-Moderator

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    That's why I make yo momma do all the work. :D
     
  14. Junk

    Junk

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    [quote author=Gumby link=board=2;threadid=4022;start=0#msg30068 date=1059877536]
    That's why I make yo momma do all the work. :D
    [/quote]
    See, I knew you didn't mind spending $75. :flipoff2:
     
  15. landtank

    landtank SILVER Star

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    Hey,
    I'm just trying to point out the professional way of troubleshooting a circuit like this. Properly executed one can confirm that the circuit is functioning correctly and optimally at a minimal expense of repair. People seem to have these codes come and go even after replacing the "Magic VSV". I personally don't get excited about replacing a 75.00 part and still having the same problem. Call me what you want but I prefer to address a problem once and be done with it.

    By all means stand next to your truck with your dick in your hands and contemplate what 75.00 part should be ordered next.
     
  16. Gumby

    Gumby Supamod Staff Member s-Moderator

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    I have tested each component. I am getting strange readings from the VSV. I can not confirm it is the problem. I will not order the part until I know it is. The biggest issue is that it is an intermittant problem. I can test it all you want, I might never find the actual source. By talking it over with other people who have had similar codes, I hope to establish a pattern of failure to help in diagnosing the problem.
    If I can clean, modify, or adjust something to fix the problem I will do that.

    Besides, it is an interesting diagnostic excersize that some of us find worth talking about.
     
  17. landtank

    landtank SILVER Star

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    Stating that my post was 80% hogwash was a compliment?
     
  18. Junk

    Junk

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    [quote author=landtank link=board=2;threadid=4022;start=0#msg30129 date=1059937378]
    By all means stand next to your truck with your dick in your hands and contemplate what 75.00 part should be ordered next.
    [/quote]
    Chill dude. Take the profanity to chit chat. Yeah, it's even Junk saying that!

    Dude, like I said, everyone I know of that had that code, fixed it with that part. If you don't like those numbers, then ignore my post. In the mean time, I've seen several guys mess around with every component of their EGR trying to find the problem and they end up wasting a ton of time and money.

    Do what ever you want. I don't care. As I said, my problem was fixed. You're the one with the ce light on now. :flipoff2:
     
  19. landtoy80

    landtoy80

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    [Dude, like I said, everyone I know of that had that code, fixed it with that part.]
    Sir Junk Dude, I think you know me? I replaced that VSV and still the light flashes me.

    For my problem, carbon in the modulator. Of the 4 hoses that go to the VSV, which of them would the carbon be comming from? They all are vacuum hoses so they suck :D The big hose below the mod. is my guess? Of corse it could be something eles. The 60 mod. was clean from carbon before I installed it and the light came on after 50 mi. Then there was some carbon in it. I will pull and test the temp sensor.
    kurt
     
  20. Gumby

    Gumby Supamod Staff Member s-Moderator

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    [quote author=landtank link=board=2;threadid=4022;start=0#msg30138 date=1059943892]
    Stating that my post was 80% hogwash was a compliment?
    [/quote]

    Sorry, missed that part. Edited my reply to reflect my mistake.
    Unfortunatey, I read your post as an attack on all of us who are having problems and not going through the detailed diagnostic procedure you described.
    Profanity does tends to bring out reactions.