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Idle too high on TBI motor, won't pass smog

Discussion in 'The Garage' started by Eric M., Mar 17, 2006.

  1. Eric M.

    Eric M. 1/2 ton status

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    I have an 87 454 w/ TBI. The idle is too fast and I can't pass smog. There doesn't seem to be any adjustment for idle. Any idea how to get it down or why it's so high in the first place?

    Thanks,

    Eric
     
  2. onlychevy6

    onlychevy6 1/2 ton status

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    could not tell you why idle is high. but there is an adjustment. on my 88 1ton truck 454 TBI on the front of the unit on the right hand lower side there is a rubber grommet looking thing that you pop out and it take a star bit to adjust.
     
  3. tomseviltwin

    tomseviltwin 1/2 ton status

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    I'm guessing there is a vacume leak into the TBI.
     
  4. gonger

    gonger Registered Member

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    I have this same problem on my 350 tbi right now, theres a plug soldered over the idle adjuster, it pops out with a screw driver, the adjustment didnt help mine though I backed it all the way off and Idle is still to high, so now I figure I must have a timing problem or vacuum leak.
     
  5. HarryH3

    HarryH3 1 ton status Author

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    Most likely a vacuum leak somewhere. Time to break out the carb cleaner and start looking for the leak.
     
  6. jekquistk5

    jekquistk5 Weld nekid Premium Member GMOTM Winner

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    wanna trade my 350 tbi has a very low idle... I broke out the carb cleaner didn't find anything. I'm planning on getting new injectors see if that fixes it
     
  7. ryoken

    ryoken Puppy Fabricator Premium Member GMOTM Winner

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    loose throttle shaft and vacuum leaks can cause high idle.

    you can't just turn the screw for idle. you need to set minimum air. ..

    Take a paper clip and jump the ALDL in the 'A' and 'B' pins.

    Turn on the ignition, but don't start the engine. Wait 30 seconds. Now, go remove the connector from the IAC. pull paper clip out of ALDL.

    Start engine. You are now going to adjust "minimum air". There is a Torx screw on the side of the throttle body. This is what needs to be turned to adjust minimum air, or more commonly known as "idle speed". It comes from the factory with a protective metal cap over it. If the cap is still there, use a small punch to knock it out. Set the idle speed to 450 rpm, rotating the Torx screw clockwise to raise rpm, and counter-clockwise to lower rpm. Once the idle rpm is set, turn off the engine.

    Re-connect the connector onto the IAC. Start engine. Idle speed is now once again governed by the ECM, but your idle should be smooth and steady, approximately 600 rpm in Drive.

    If you set an SES light by having the IAC disconnected, then after shutting down the engine disconnect the negative battery terminal. Wait 5 minutes. This will clear the ECM of all trouble codes. Re-connect the battery and drive the car for 20 minutes to allow the ECM to relearn your driving style.
     
  8. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    Just like the man says, but if you've recently accuired this high idle you need to look into why. Maybe air leaks or sensors, constant a/c command? Whatever it is needs to be sorted first.
     
  9. ryoken

    ryoken Puppy Fabricator Premium Member GMOTM Winner

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    i somehow think we've been here before... :wink1:
     
  10. HarryH3

    HarryH3 1 ton status Author

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    Injectors are pretty expensive parts to just throw in and see if it fixes anything... :eek1:

    Low idle sounds like the IAC motor is dead. But do some more troubleshooting before you just start throwing piles of money at it.
     
  11. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    High idle strikes me as a coolant temp sensor issue. Best option is to borrow / buy an ALDL cable so you can hook your laptop up to it to see exactly what the ECM is seeing, give you a far better idea of what the problem may be than just tossing random bits and peices at the system.

    Also, when you set minimum air, you want to set it to where ever your engine runs best at it's lowest idle speed. That is not 450 RPM for all engines. If its 500 or 600 rpm, then set it there. You may also need to get your chip changed to change the target idle RPM if that is the case.
     
  12. 1-ton

    1-ton 1/2 ton status Premium Member

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    A lot of good info in this thread.
     
  13. southernspeed

    southernspeed 1/2 ton status GMOTM Winner

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    Yep, but is deja-vu, beer or old age?:D
     
  14. opfor2

    opfor2 1/2 ton status

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    I just went through this with my 90, I replaced the TBI with one from Turbo City and needed to set the idle. I have the OEM service manual and in order to set it correctly you need a OBD-1 code reader so can read the IAC counts and eng idle
     
  15. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Yeah, that is the truely way to do it, but not everyone has a scanner, so the paperclip method is often people's best bet, lol
     
  16. jekquistk5

    jekquistk5 Weld nekid Premium Member GMOTM Winner

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    I will start Troubleshooting next week sometime, I plan on having it fixed by next friday. It runs fine at like Higher rpms but it has trouble at the lower rpms
     
  17. Eric M.

    Eric M. 1/2 ton status

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    Thanks guys.

    What is the process for setting min. air using a scanner? I have one, but there isn't any method listed in the directions for that kind of proceedure.

    Eric M.
     
  18. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Just let the truck get up to full operating temp, then adjust the switch up front till the scanner reports approx 20 iac counts at an idle.

    If your engine is basically stock, the stock target rpm should also be fine, so just set it till its got the iac counts at about 20 or so, and that the rpm is something like 650 - 700 in park.
     
  19. Creek

    Creek Registered Member

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    How fast is it idling? Does it fail emissions because the idle speed is out of range or because the hydrocarbons are high? Is the idle steady or does it surge? Does it run okay above idle?

    The ecu goes into idle mode when the vehicle speed is zero and the throttle position is below a certain percentage of opening (generally between 2 and 2.3%). The idle speed is controlled by the IAC which is basically a vacuum leak around the throttle plates. It tries to maintain the idle speed within ± 25 rpm of the value in the Target Idle RPM vs. Coolant Temperature table (I don't have a binary for a California 7.0L so I don't know what those values are. A 49 state 5.7L would be 575 rpm at normal operating temps.).

    If you have a vacuum leak or the idle air control is bad, the idle speed should be fairly steady and the hydrocarbons should be normal. If the throttle position sensor is bad the idle will fluctuate by two or three hundred rpm and the hydrocarbons will be high. If the water temperature or inlet air temperature sensor (if you have one) is bad, the idle should be fairly steady and the hydrocarbons will be high.

    David
     
  20. Eric M.

    Eric M. 1/2 ton status

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    Took me forever to actually look into this, but, here's what I found.

    Thermostat broken, put new one in (180 not 195 as what was stock). Temp. went up and idle went down, but, not low enough to pass smog.

    Broke out the carb. cleaner and sprayed the TBI unit. There are leaks at the sides where the throttle shaft goes through the TBI body.

    I used my scan tool and the proceedure detailed above to adjust the min. air idle. It did get the idle down, but, as I drive I can see the computer is pushing the idle back up. I'm hoping it will stay low enough (below 1100) to pass smog this morning.

    I am also getting an intermittent bad oxy sensor code. The sensor is new. I'm wondering if the vacuum leaks caused by the loose throttle shafts could be causing this?

    What is the best fix for the loose shafts? Is there a bushing kit to rebuild the TBI, do I get a new body and swap it out, are there rebuilt units that are any good? Any recommendations as to where to shop and where not to for the parts I need?

    Thanks,

    Eric M.
     

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