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Thread: TBI 350 Target Idle Speed

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    TBI 350 Target Idle Speed

    Hi guys, im in desperate need of help with my 93 silverado that has a 5.7L TBI motor, I have a high idle issue ive been chasing for a couple weeks now. I have done a full rebuild on the TB along with new injectors, new intake to TB gasket, New coolant temp sensor (scanner verified that its reading correctly) New idle air control value (yes i went through the calibration process) Adjusted TPS sensor to .59V. timing is set at 0 and there are no vacuum leaks but the truck still idles at 1k. So i decided to purchase an OBD1 scanner that shows live data i hooked up the scanner and i can see that the idle req is set at 1000 rpm so im starting to think that my computer is demanding a 1000 rpm idle and no mater what i do once its fully warmed up it idles at 1000 rpm in neutral (i have a 5 speed). So with all that being said i did research and found the BIN and XDF files for my truck and downloaded them and im able to view everything in tunerpro RT. My question to you guys is what perimeters does the computer use to determine idle speed? is it possible for the idle command to be set at 1k in the reman computer? my BCC code is AZDR which is the correct one for my truck and i opened up my computer to verify that is has the correct chip and it does. the previous owner had the computer changed with a reman one but i dont think that matters as the chip inside is still an AZDR. when i view the target idle rpm vs coolant temp in tunerpro for the AZDR bin and $9A XDF file i downloaded i see that when the coolant temp is 195 my idle should be between 650-700 but its not so thats why i want to know what perimeters are used to determine idle speed because obviously its not just the coolant temp.
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    Throttle is open (.73V). Throttle 53%. What is the IAC value?

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    Quote Originally Posted by MO LS Noobie View Post
    Throttle is open (.73V). Throttle 53%. What is the IAC value?
    Suggests the manual throttle setting may not be set right. Maronielb have you done the manual throttle set procedure?

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    That picture was taken before i adjusted the TPS to .59V IAC counts fluctuate between 5-10

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    What is the manual throttle set procedure? is that the one where you remove the cap and adjust the idle screw on the TB? if so no i havent and im not willing to mess with that, its set from the factory and is not suppose to be messed with.

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    Quote Originally Posted by maronielb View Post
    What is the manual throttle set procedure? is that the one where you remove the cap and adjust the idle screw on the TB? if so no i havent and im not willing to mess with that, its set from the factory and is not suppose to be messed with.
    Ok, with all due respect, I beg to differ with you. 99% of the mechs that deal with TBI understand that at some point you have to set/adjust the manual setting. Especially if it has been rebuilt. Anyway, here is the procedure. Oh and what did your IAC value read?

    https://www.gmcmi.com/wp-content/upl...alibration.pdf

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    Quote Originally Posted by maronielb View Post
    That picture was taken before i adjusted the TPS to .59V IAC counts fluctuate between 5-10
    If your TPS and IAC are as stated, could be vacuum leak, especially check brake booster.

    Desired idle is 650, that is what it is shooting for. You are getting more air somewhere. What are your block learn and integrator fuel trims?
    Last edited by MO LS Noobie; 02-28-2024 at 03:41 PM.

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    after making sure there are no leaks and timing is correct....any tbi i usually adjust the throttle plates so the IAC counts are in like 10-20 counts at full operating temp in neutral
    its always worked for me
    tbi is the only system where hot idle air should be all through the plates

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    how would i check the brake booster? and yes i agree 650 sounds like the perfect idle speed. i have no idea what a block learn it or integrator fuel trims. i dont think my scanner shows those. i will post a poc tommorow with everything that my scanner shows.

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    if all else fails i might just end up adjusting it manually even though i really dont want to.

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    Pinch off hose going to Booster, this works for any regular, soft, rubber hose. There should be no change in idle speed, or fuel trims. Also test any vacuum lines to carbon canister, HVAC system, or anything else vacuum operated.

    Block learn/integrator are the terms GM used for fuel trims, long/short term before OBD2. 128 equals 100%, or 0 trim. Below 128 is taking away fuel, above is adding fuel. Anything within 10% is considered normal.

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    Ok i will try that tommorow, here are the pics from
    All the data my scanner shows me, it does not so block learn or anything. Also i had adjusted myTPS to .59 but now it’s showing .65, does that mean the sensor is bad? Truck started idling at like 1300-1400 rpm today fully warmed up, I’m about to turn this thing to the ground

    IMG_8342.jpgIMG_8343.jpg
    Last edited by maronielb; 03-03-2024 at 10:43 AM.

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    The thing that really stands out to me is the IAC is at 0%. First thing I would do is block off the air supply to the IAC, this is the kidney shaped hole next to the throttle blade directly above the IAC valve. Is there any air at all going through that valve? With 0% I there should not be any air at all. If there is air going through that valve then either the valve is bad, wiring, PCM, or the computer just hasn't learned the home position. See post number 6 by TriumphR3 above.

    If there is NOT any air going through the IAC valve then you have too much air getting in some place, the most common place is the mounting gasket for the TBI unit, but you already replaced that. I use propane to sniff out vacuum leaks on the engine itself, typically a missing vacuum cap, compromised vacuum line, the PCV valve, or intake manifold leaks at the gasket. You can use carburetor cleaner to do the same thing, but it tends to puddle and is a severe fire hazard. Not being able to see the fuel trims makes things harder but you should be able to hear a vacuum leak because the engine will smooth out and speed up.

    Do you have access to another scanner? There are a couple of other things that your scanner is showing but that IAC percentage is the first place I would look.

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    So today i re did the IAC reset produce and reset the tps voltage to .59 again which brought my idle back down to about 1k. Then I checked all vacuum lines and pinched the line from the brake booster, and pcv valve and there was no change in idle. The IAC counts are now around 4-10. The thing is the scanner is showing a 1k idle requirement so no matter what i do the computer will still idle the truck at 1k. The whole reason for my original post was to find out how the computer determines the idle requirement so i could dig into that rather then just trying to chase my tail on this high idle

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