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TBI failed smog: Idles @ 1400 RPM, HELP!

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Blazinaire, Jun 8, 2002.

  1. Blazinaire

    Blazinaire 1/2 ton status

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    Well I went to have my truck smogged today and my emissions were clean as a whistle....But the idle would not drop below +/- 1400rpm, they will only allow 900(?) rpm. I have noticed that since I have owned my truck that the idle has been high, and seems to have gotten higher. Is there any way to adjust the idle on these things? What are some things that would cause this to happen, there aren't any vaccuum leaks anywhere. I have untill the 26th to pass and need to get this fixed ASAP. Any help is greatly appreciated!
     
  2. BobK

    BobK 1/2 ton status

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    TPI and throttle body injection use what's called a IAC valve(idle air control).It helps maintain and controll your idle.The computer controlls it.It opens and closes a small valve in the throttle body allowing more air in(increase in idle)or decreaseing the amount of air at idle(decreases idle).That could be faulty.check the conection to the IAC makes sure it's on.I replaced a faulty one on my TPI and it was almost 50 bucks!
    Good luck.
     
  3. Twiz

    Twiz 1/2 ton status

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    Sounds like it sucked the TBI to manifold gasket (pretty common).
    To check, spray some carb-spray around the base of the TBI, when you hit the spot (vacume leak) idle speed should change. It's really easy to replace the gasket, the fuel-lines can be a bugger tho' and keep an eye out for the fuel line O-rings when you pull 'em off.

    This would also be a good time to check for throttle shaft play because the TBI will be off the truck any-way.
    To check for throttle shaft wear, spray some carb-spay around the TBI casting, where the shaft enters - again, idle speed should change. Also, grab the shaft and wiggle it, if there is any play other than rotationaly, the TBI casting is worn and will need to be rebushed. I would bring the TBI to a dealer, or crab shop to have the bushings installed.
     
  4. Blazinaire

    Blazinaire 1/2 ton status

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    I already checked all of that stuff and it is fine... Still searching around /forums/images/icons/tongue.gif . BobK: is the IAC the brass(?) fitting on the passenger side of the throttle body towards the back of the engine compartment? I will try to clean it with some carb cleaner to see if that will help any. Are there any other possibilities?
     
  5. Twiz

    Twiz 1/2 ton status

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    Sure it's not the TBI gasket?

    Any vacume leak below the throttal blades will have the same effect too. I'd take a close look, and check all your hoses and fittings. Better to check the quick/simple fixes before you get in to detailed diagnostics.
     
  6. 90K5

    90K5 1/2 ton status

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    It sound sto me like the IAC...yes, it is the brass fitting on the back of the TBI has a four plug harness into it. Mine cost $30 I think. Really smoothed out the idle, even if it isn't the problem, its worth it.
     
  7. Thunder

    Thunder 3/4 ton status

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    Before you replace the IAC. Take a look at the left front side of the throttle body next to the linkage. You will see a hole with a metal plug in it. behind the plug is a adjustable throttle stop screw. If the plug has been removed maby the previous owner messed around with the idle.
    You also might want to check for trouble codes. A bad IAC will usually throw a code 35.
     
  8. Blazinaire

    Blazinaire 1/2 ton status

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    Okay, I did some tinkering with the IAC and got my idle down where it is supposed to be..... Until I plugged the wires back in /forums/images/icons/mad.gif ! It took a couple seconds, but it went back up to around 1400 rpm. The base gasket is good for sure. Is there a motor inside of the IAC that makes the "plunger" screw in and out? That would explain the idle picking back up after I plugged it back in. Does the IAC have any affect on high rpm's, or just the idle? Please help, must smog by the 26th! /forums/images/icons/ooo.gif
     
  9. BobK

    BobK 1/2 ton status

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    The IAC has a step motor in it and it should effect idle only.SIgnals from the ECM will move the pintal in or out and that (when threaded into the throttle body) will allow more or less air past the throttle plates.If the idle increases after you hook up the IAC wire harness then you know the IAC works but the ECM is making the IAC idle the eng. higher than it should.
    WHY?
    The ECM is getting info. from the various sensors and is interpreting(spelling) those signals and for whatever reason is increasing the eng. rpm.
    Is it a MAF or speed density system?Possibly the MAP sensor?
    Any codes set?
    Sorry,just throwing out some ideas.It has to be one of the sesors feeding the wrong info. to the ECM since you had the idle down when the IAC wasn't hooked up. /forums/images/icons/confused.gif
     
  10. Blazinaire

    Blazinaire 1/2 ton status

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    Well what would happen if I adjusted the IAC so that the idle was where it needed to be, and I don't plug the wires back in? The idle would stay where I put it for sure that way. BUT, I have been told by a few mechanics that this will change the amount of air going in at high rpms and change my emissions (but they aren't exactly TBI experts). My goal right now is just to pass smog.
     
  11. BobK

    BobK 1/2 ton status

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    Well if the IAC is adjusted without the harness connected,at off idle rpm, the amount of air let into the eng. would be the amount the throttle blades are open plus the amount the IAC lets in.But your test is at idle only right?
    My guess is that if you can get the rpm down to normal by bypassing the IAC you may get away with it.THIS IS A GUESS THOUGH!ECM will still be dumping too much gass into the system with the IAC disconnteced.
    Idle mixture is mainly controlled by the TPS(throttle position sensor) + o2 sensor.One reason the ECM may be increasing the rpm is that the o2 sensors is saying there is too much unburnt fuel in the exhaust.The ECM inreturn lets in more air(via IAC)to help burn that unburnt fuel but then that increases rpm.If the TPS is out of adjustment or defective it can give thr ECM the wrong readings.Poss. making the ECM think the throttle blades are more open than they are thus dumping more fuel into the system.

    Have you checked the voltage output at the TPS.If you have a voltmeter is fairly simple.Might want to check the TPS.

    If it's not within spec's that my be the problem.If it's inadvertantly bumped or hit it can be thrown out of adjustment(if the adjustment screws were not tightened enough).I think you backprobe terminals "A"+"B" on the TPS harness.If the voltage reading you get(eng. at norm. temp.,ignition on but eng NOT running)is 0.525volts +or- 0.075 volts the TPS is o.k.,if not,adjust untill it is within those readings.Then check and see if the idle returns to normal.
    The code that is set in the ECM for any TPS prob. is code 21 or 22.
    Good luck man.
     

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