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TBI - stumped

Discussion in '40- & 55-Series Tech' started by woody, Aug 19, 2003.

  1. woody

    woody Internet Fireman Staff Member

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    1991 350 Chev TBI - idles great, "casual" accelleration is great, mash the pedal and it bogs down. Actually, anything beyond a casual accelleration. Back off the pedal, and it accellerates casually...mash again, and it bogs.

    Ran WinALDL, pulled a code 44, replaced the O2 sensor. no change.
    Replace the TBI basegasket, no change.
    Air cleaner is new.
    Fuel pump is new.
    Fuel filters are new.

    NO other codes....drove it last night about 10pm (my neighbors love me), WinALDL shows nothing. Did not turn on the DataLogger, but can if someone thinks that info will note something. All sensors appear to be sending information in correctly.

    I want to mash :D
     
  2. upnover

    upnover

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    Woody,

    Two quick thoughts for you try. Check your static timing and make sure that your cats are not clogged. Also is it manual and auto? If it is auto make sure that the TV cable is adjusted properly, they are very picky and if it is not adjusted properly the trans could slip and cause a bog and wont allow it to downshift.

    Chris
     
  3. woody

    woody Internet Fireman Staff Member

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    easy answers...no cat, original vehicle did not have one either, and it's a stick shift, so no extra cables to stick.

    Got a lengthy discussion going on the PBB....and a dozen more things to test tonight...thanks for the reply!
     
  4. hammerhead

    hammerhead

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    Woody, my brother had a similar problem and replaced dang near the whole fuel system to no avail. Went to the ignition system and found an electronics problem. I laughed at that point because I suddenly remembered what I was taught at YAMAHA tehnical. "If you think it's carbuetion check the ignition. If you think it's ignition check carburetion".
     
  5. db

    db

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    I have the same set up in my cruiser. I have the dealer maintance manual for a 1991 TBI chevy truck. I had a similar problem and it was water in my fuel. Here is a abrievated list of causes from dealer book. If you need voltages and resistances let me know. Does the problem occure in open loop closed loop or both. A short list of code 44 causes (lean exhaust) are
    1. fuel contamination
    2. low fuel pressure
    3. circuit 413 open. this is the ground for o2 sensor at D6 of ecm. This will cause terminal d7 of ecm to remain at .45 volts. might also set code 13
    4. o2 sensor wire shorted to ground
    5. o2 sensor bad.
    These checks are in the diagnostic section for hessitation,sag,stumble
    1. fuel pressure
    2. TPS
    3. map
    4. water/contaminated fuel
    5. cannister purge system operation
    6. spark plug wires or fouled plugs
    7. open ignition system ground wire from ecm b3 to hei module pin a of 4 pin plug
    8. timing
    9. egr valve
    10 generator output incorrect. repair if less then 9 volts or more then 16.
    I know some of these items have been noted already but I listed them anyway. There is more info in the book i have. if you need more, let me know


    derek
     
  6. fr8train

    fr8train

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    My Blazer had that problem for a little while too. Changed the fuel filter and intake pump screen. Double check that your timing( not supposed to change, but hell just turn the distributer till it goes away.) at the distributor to make sure it didnt walk to retard.
     
  7. woody

    woody Internet Fireman Staff Member

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    we checked the dist last night, it had walked slightly. "ear" adjusted it last night, slight improvement, but still feel there's something else wrong.

    db, do you have info on setting base timing and idle speed? My cheap Haynes doesn't have that, and I failed to save the underhood sticker. Also, what do I need to disconnect to cancel the electronic advance?

    We are leaning TPS right now...fortunately, it runs well enuf on the trails that I'm not too worried about this weekend, but would still like to solve it for the long term...
     
  8. db

    db

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    Idle setting.
    auto trans transmission in drive, 525 rpm's should be 5-30 iac counts closed loop.
    manual transmissisn
    trans in netural 600 rpm's 5-30 iac counts open loop
    one other thing that is important (or so I have been told) you need a park netural switch. If you do not have one ground pin b10 of the ecm before adjusting. This tells computer you are in park or netural. I was told by my friend who works for chevy service(who gave me the manual) It can mess with the IAC. I have on on my b&m shifter.
    timing
    put est in bypass mode by disconnecting the timing connector. It is a tan/blk wire with a pull apart connector. mine is located by egr valve.
    base timing is now set to 0 btc. Thats it.
    if you need more let me know

    derek
     
  9. woody

    woody Internet Fireman Staff Member

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    THANKS db for the TBI info....got the idle reset last night and the timing adjusted. Grounded the neutral pin too. Took 'er for a spin and my laptop finally provided an error code, MAP is bad. That's been my guess since this started, nice to have it confirmed. Picking up the part today, installing while the truck is on the trailer tonight, then headed straight up north...hope it's the final straw! lol
     
  10. hammerhead

    hammerhead

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    Good luck Woody. Hope you got it solved. Have a good romp. I wish I was going wheeling and someone else was changing this NP435. This thing is heavy. Oh yeah, Don't forget pics.
     
  11. toddslater

    toddslater SILVER Star

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    Woody,

    Reading this makes me remember why I like carbs so much. Thanks...well I hope all goes well and you get it straightened out....you still have an old intake with maybe a 650 laying around....just in case. ;)
     
  12. '74 UA FJ

    '74 UA FJ

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    Woody:

    Was is the MAP?

    Thanks for the update. ;)
     
  13. theo

    theo

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    I got this one, fellas. Stand by one!
     
  14. theo

    theo

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    http://www.diy-efi.org/gmecm/component_info/

    I was going to rag on you for digging up a 4 year old post, but thought better of it. So just a link with some useful info. HTH :D
     
  15. '74 UA FJ

    '74 UA FJ

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    I'm not having a problem. I just wanted to see if woody fixed his junk back in '03. Thanks though.
     
  16. woody

    woody Internet Fireman Staff Member

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    took a few years, but ended up changing fuel pumps to one from Summit, specifically for TBI applications...pressure wasn't high enuf at WOT...

    she's a screamer now ;)