9 times out of 10 a PO401 Code is fixed by . . . . .

Discussion in '80-Series Tech' started by aktundra, Jul 23, 2009.

  1. aktundra

    aktundra

    Messages:
    560
    My SC'ed 1995 threw a PO401 code about 6 months ago. Cleared it with the scan gauge and forgot about it.

    It threw the code again and I am getting ready for a 7 hour trip this weekend. when I researched the "cure" I found several threads outlining the process to diagnose the various components on the EGR system and dial in on the faulty component.

    I work so slowly, I just do not have the time to do this properly. If I were to just slap in a part and cross my fingers, is there any consensus as to where the fault lies, say 90% of the time, with a PO401 code?
  2. super90

    super90

    Messages:
    901
    Location:
    TEXAS !!!
    Nope.

    And I wouldn't really recommend that approach anyway.

    You have to figure out which component within the system is causing your issue. I would first make sure that port going from the EGR valve through to the top of the intake is clear. That is a known culprit that doesn't require parts to fix. From there, you just have to follow the troubleshooting to find your problem(s). On a rare occassion it's multiple things. Top offenders include that port, the VSV (under the intake), and the vacuum modulator. Good vacuum lines are helpful too. Less frequently folks have mentioned the temp sensor, or on rare occassion the EGR valve itself.

    Others will chime in and keep me honest, but that's what I have directly experienced and heard/noticed in posts and discussions with other owners.

    Keep the Cruiser Faith,

    R.
  3. SeanAndHis80

    SeanAndHis80

    Messages:
    561
    Location:
    Chester, NH
    Why do you need to fix it so fast anyway? Clear the code again and wait for the right time to fix - that is what I did for three years. To answer your question I think VSV edges out the diaphragm which edges out the rubber tubing, which edges out a clogged throttle body passage, which edges out the EGR valve, which edges out the EGR temp sensor. Nothing definitive and a lilely waste of $$$ if you throw parts at it.
  4. Ebag333

    Ebag333

    Messages:
    8,163
    Location:
    Medford, OR
    The FSM has lots of tests you can try to see if parts pass/fail. Some parts may only fail intermittently and may pass the FSM test, only to fail in specific circumstances (the VSV is common for that).

    Do the tests and see what passes and what fails....or disable EGR totally. ;)
  5. hzavarce

    hzavarce

    Messages:
    298
    Location:
    Frisco, TX
    Leaks is also common. I started getting the code after my SC install and I realized it was because when re-installing the EGR I did not tightened the bolts enough. It was a very, but veryyy small leak, but enough to throw the code.

    my 2 cents...
  6. toyteg08

    toyteg08

    Messages:
    154
    Location:
    CA
    When I first got my LC, it threw the code the next day. I took the EGR valve off and the VSV, and thouroughly cleaned everything, and made sure it held pressure. Put it back together, and it didn't show up again.

    My tranny died, it sat until I had time to replace it, I put a different tranny in, started driving it again, and the code came back.

    To finish off the rest of my baselining, I changed the spark plugs and wires, and sanded off the corrosion on the cap and rotor. Haven't had the code come back since.

    Maybe that would do it for you. Never thought a plug and wire change would have affected it.

    Jon
  7. Grench

    Grench SILVER Star

    Messages:
    2,664
    Location:
    Omaha, NE
    I had the P0401 going into last year and seem to have fixed it.

    I changed the exhaust to 3", added a Supercharger, replaced the EGR valve and the modulator valve (3x... long story), replaced the radiator, all belts & hoses, water pump, new alternator brushes, new starter motor, added a pusher fan, replaced the plugs with cooler MR2 turbo ones, replaced the plug wires, distributor cap, dizzy, fuel filter, oil filter, head gasket, fan clutch, fan from a V6, both knock sensors, both O2 sensors, ceramic coated the exhaust collectors, dropped the transmission pan to clean the filter screen, added a snorkel, replaced the EFI computer and re-wrapped the wiring harness by the EGR.

    Somewhere in there the P0401 stopped triggering. I think that would solve it in at least 9/10 cases.

    Last year was expensive.
    Last edited: Jul 23, 2009
  8. SBFJ60FIG

    SBFJ60FIG

    Messages:
    1,015
    Location:
    Chicago!
    The diagnosis is really quite simple. Make a few minutes to read through some of the postings that are well-done. I forget whose it is that uses a vacuum gauge to confirm VSV operation but that is super easy to do.

    I had a hard fault in mine (just "fixed" it a few weeks ago actually) that is, clear the code, and within 3 miles the light would pop on again. All I did was:
    • Take off the VSV, tested it and confirmed that it functioned properly (easy to access when the throttle body is taken off, which is pretty easy to remove as well)
    • Checked all vacuum lines and replaced some that seemed loose/might not be sealing.
    • Checked the EGR vacuum modulator.
    • Checked EGR valve diaphragm

    Installed the vacuum gauge and drove it around and BAM...no more MIL. So in my case, what I "fixed" is unclear. But it really is a pretty easy thing to repair. You sound a little intimidated, don't be...read the threads and fix it when you have the time.

    YOU CAN DO IT!!
  9. goat97

    goat97

    Messages:
    6
    mine threw the p0401 code yesterday.
    well my fix as of today was one of the vac lines had a crack.
    code has not come back, yet.
    mabe its something that simple or not.
  10. edog's 80

    edog's 80

    Messages:
    346
    Location:
    Boulder, CO
    Mine was the VSV for EGR. When they heat up and kick the P0401 code I think this is most common, could be vac lines too though and they are only a few dollars and you will want to replace those anyways while you are in there. The FSM tests do clear it up though, probably worth your time.
  11. MoJ

    MoJ Moderator

    Messages:
    3,254
    No need to do it before your trip. It won't hurt anything. Wait until you have the time. It's safe to say there are members that have been driving around for years without fixing it.

    Top 3 causes imop:
    VSV
    dirty TB
    loose/cracked vacuum hose

  12. Grench

    Grench SILVER Star

    Messages:
    2,664
    Location:
    Omaha, NE
    A caveat to that. If I understand it right, and I may be very wrong on this... The engine management may switch to 'safe mode' with the CEL on. This can result in the engine using more gas. MPG may drop by 1-2.
  13. p8nt

    p8nt

    Messages:
    796
    Location:
    Traverse City, MI
    Understatement of the year :beer:
  14. livinoffroad

    livinoffroad SILVER Star

    Messages:
    532
    Location:
    6005 ft. Smoky Mtns. Idaho
    po401

    Quote from super90: "You have to figure out which component within the system is causing your issue. I would first make sure that port going from the EGR valve through to the top of the intake is clear. That is a known culprit that doesn't require parts to fix. From there, you just have to follow the troubleshooting to find your problem(s). "

    When I got my PO401, I replaced the vacuum modulator valve with the new type ($65)...nope. I replaced the VSV for EGR ($73)...nope. Took apart the EGR valve, cleaned and re-installed...nope. Finally, (and I tell this story just to suggest reversing the order of troubleshooting per super90) I cleaned the little vacuum port that goes into the intake with a piece of stiff wire. I thought it went into the intake, but it actually goes THRU the intake, so use a long piece of wire, and disconnect the hose connected to the other end of the port under/inside the intake (sort of hard to access) before you start cleaning. I cracked the flux off a piece of 7018 welding rod and used it to clean with, because it is stiff and straight. I used a drill bit first, before I realized how long the vacuum port tube was. For me, this was the fix. Have not had a PO401 in the 2 years since. If I had tried this first, I would have fixed it for free. Instead, I have a nice supply of back-up parts should either of them fail (for $138 ):bang:
  15. aktundra

    aktundra

    Messages:
    560
    Thanks for all the input.

    Looks like I can just clear it one more time and really dive into the problem in a few weeks.

    Mod'ing & maint on the Cruisers is an "indoor" activity that I try and get done in the spring before things go compeltely nuts in the summer. I just forgot about this stupid CES code.

    Here in AK the summers are short and frantic. I just did the math, and since the second week of June, I have spent more time in my tent than in my own bedroom! (plus for every day in the field it seems like there is 2 days planing, packing, and clean up afterwards) Getting ready to head down to the Kenai to dip net the Red salmon run next weekend and after that things should take a breather for a few weeks until hunting season !

    Thanks for the suggestions. I will clean and clear the easy things, reset it one more time, and pick up the issue in ernest this winter.

    thanks again for the input
    Last edited: Jul 23, 2009
  16. super90

    super90

    Messages:
    901
    Location:
    TEXAS !!!
    Maybe I am misunderstanding, but is sounds like we are both recommending to start with the same thing; cleaning the port that goes from the EGR across the top of the intake.
    Last edited: Jul 24, 2009
  17. Gumby

    Gumby Supamod Staff Member s-Moderator

    Messages:
    11,107
    Location:
    Knee deep in hookers and gin
    I chased it for a while.

    then I ignored it for three years then sold the truck with the light on.
  18. livinoffroad

    livinoffroad SILVER Star

    Messages:
    532
    Location:
    6005 ft. Smoky Mtns. Idaho
    Yes, absolutely, clean the little vacuum port that mounts to the intake FIRST, and save trouble time and money!
  19. LandCrusher'70

    LandCrusher'70

    Messages:
    1,982
    Location:
    Sandy Eggo, CA
    FYI:

    That 'port' goes through the intake and not into it. There is another vacuum hose attached to the opposite side (underside) of the intake. If you are not careful when clearing that port, you can push that lower hose off or tear right through it.
  20. SeanAndHis80

    SeanAndHis80

    Messages:
    561
    Location:
    Chester, NH
    ...or get a drill bit stuck in it like Joey!

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