SL2 sticky throttle problem

Discussion in 'Saturn S-series' started by David T. Johnson, Feb 13, 2009.

  1. The throttle accelerator pedal on my '92 Saturn SL2 seemed to be
    sticking so that when you stopped at a traffic light, the idle rpm might
    be at 2-3K before slowly dropping back down to idle speed after 30 or 40
    seconds. The pedal and cable linkage were fine. The 'Service Engine
    Soon' light did not come on. Googling the problem suggested that the
    the throttle body might be gummed so disassembled and cleaned it but the
    problem was still the same. The throttle body had a little solenoid
    valve and piston attached to it that opened and closed a little bypass
    hole around the throttle body butterfly valve. Turns out that is the
    'air idle control valve' or AICV and it adjusts to control the flow of
    air through a little hole about 1/4" in diameter that provides the air
    to the engine for idling when the throttle butterfly is closed. I
    didn't think it could be bad because there was no fault code or SES
    light but I decided to change it as there wasn't much else that could be
    causing the problem. Et voila! The problem was solved.

    What puzzles me is why there wasn't a fault code for the AICV function.
    My guess is that the Saturn doesn't throw a fault code if the idle rpm
    doesn't drop back down like it's supposed to perhaps because the
    programming is set to maintain MINIMUM idle rpm and only issue a code
    when the idle rpm is below that. At higher rpm, the throttle butterfly
    is supposed to be open and the AICV closed but the computer has no way
    of knowing that the AICV was failing to close. The computer should have
    thrown a fault code when the throttle position was closed and the idle
    speed was too fast but the car is a manual transmission so the rpms stay
    up to correspond to the road speed even when you back off of the gas so
    the computer didn't know the car was 'idling' rather that 'driving.'
    The computers on newer OBD2 vehicles know what the vehicle speed is but
    apparently the OBD1 Saturns don't report the vehicle speed to the
    David T. Johnson, Feb 13, 2009
  2. David T. Johnson

    Private Guest

    David, Thanks for posting this.

    I have a very similar problem as I posted last month. (post copied below)

    Mine is an OBD11 car with similar symptoms as yours. Shortly after my
    posting the SES light came on, so I did a scan which returned a code
    ISO9141, which nobody could identify??? On rescan I got a code PO147. I
    cleared the PCM and SES light did not reappear. It was suggested to me that
    I may have an IAC valve problem similar to yours and my reading and your
    timely post seem to confirm that this is the area needing further inspection
    and service.

    Did you attempt to clean the IAC valve or just replace it?
    Cost of new part?

    The scan code ISO9141 was confusing but goggle returned
    I suspect this is important information when purchasing a code reader.

    My error searches found - P0147=Heated Oxygen Sensor HO2S Heater Circuit
    Malfunction (Bank 1 Sensor 3) M/T Only
    I do not understand what this may indicate? The only O2 sensor I am aware
    of is in the exhaust and it does not have a specific heater circuit. I had
    suspected that this may indicate faulty CTS? but now think it more likely
    that a malfunctioning IAC valve may be causing a false reading from the O2
    sensor. Why would this be 'M/T Only'?

    I will admit that I have been procrastinating the service somewhat as the
    car is still very usable and I would prefer warmer weather for any repair,
    and it is not uncommon for many problems to disappear in the spring.

    I think that this is a good example of how a little research and DIY
    mechanicing can eliminate the reliance on dealer service and save a lot of
    $$ and aggravation and disappointment. It does require a bit of learning
    effort but neither the work or the systems are all that complicated and IMHO
    learning is a big part of life. I am grateful for this NG and all who
    contribute here.
    Happy trails to all.

    Bob, Thanks for this suggestion, I had forgotten that replacing the CTS
    seems to solve most similar problems reported here.
    Private, Feb 13, 2009
  3. I just replaced it. I got one at Car Quest for $50. It took about 30
    minutes to remove the throttle body, replace the valve, and reinstall
    it. Youtube has a good video on removing the throttle body on the
    Saturn S series.
    It's always possible that there is more than one malfunction. The
    overspeed 'sticky throttle' idle, though, is a dead giveaway for the IAC
    valve if the throttle body butterfly valve is clean and working properly
    since the only other way for air to get into the intake is through the
    IAC port (assuming that there aren't any obvious vacuum leaks).
    David T. Johnson, Feb 13, 2009
  4. David T. Johnson

    Private Guest

    IAC & throttle body cleaned. (New valve price 80-105 C$) Problem solved,
    total cost <10$ for carb cleaner and gasket.

    Thanks for help and advice.
    Private, Mar 14, 2009
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