Erratic and fast idling Lantra 1.6

Discussion in 'Hyundai Elantra / Lantra' started by Ed Gasket, Mar 18, 2007.

  1. Ed Gasket

    Ed Gasket Guest

    Here are the symptoms:
    When the clutch is depressed, revs surge to 1500 rpm and then drop
    back to around 1000 to 1200 rpm.
    With the engine hot and idling the revs vary between 1000 and 1200
    rpm over a 3 second period i.e. in 3 secs, revs go from 1000 to 1200
    then in the next 3 seconds they fall back to 1000 again.
    I have checked the following :-
    1) there is slack in the throttle cable
    2) The throttle - stop screw just touches the plate; if I turn it out
    any further then it is the throttle flap that stops the throttle
    mechanism rather than this screw. According to my information, this is
    now adjusted as it should be.
    3) Resistance measured on the TPS (throttle position sensor), 990 ohms
    to 3000 ohms in smooth progression. The manual says the range should
    be 700 ohms to 3000 ohms but I dont know if this slight difference is
    the problem. I'd have thought the only problem you'd get with a TPS is
    it not making contact at all due to wear; hard to see how resistance
    at the low end could increase unless its not going back far enough.
    I do not have an idle switch on this car.

    Has anyone had these symptoms and found out what the answer is?
     
    Ed Gasket, Mar 18, 2007
    #1
  2. Ed Gasket

    hyundaitech Guest

    1000 rpm is still too fast when warm. I'd suspect either a defective TPS
    (too high a reading when throttle closed) or excessive air entering the
    engine.

    One of the failure modes of the TPS is that the contacts inside become
    dirty, causing excessive resistance. 1k Ohms vs. 700 Ohms is an error of
    about 40%, so I'd primarily suspect the TPS. Attach a voltmeter between
    the signal wire and ground. Then grab the connector and push it back and
    forth. If there's any significant change in TPS reading, replace the
    TPS.

    I don't recall the specific reading to be expected from the TPS at closed
    throttle. On the Mitsu engines, it was to be set at about 500mV, but your
    TPS is nonadjustable. Unfortunately, it's probably necessary to have a
    scan tool that will read percentage of throttle opening to diagnose this
    properly.
     
    hyundaitech, Mar 18, 2007
    #2
  3. Ed Gasket

    Ed Gasket Guest

    Thanks Hyundaitech. I will spend some more time on the TPS; I guess if
    the total resistance is 3K and I'm getting nearly 1K at idle then the
    voltage is going to be over a volt. Difficult to measure though with
    the connector on as you can't then get to the contacts but maybe I can
    backprobe or use thin wire. Its now idling at around 1200 rpm and
    fluctuating slightly since I set the throttle-stop to actually make
    contact with the throttle plate so something's not right !
     
    Ed Gasket, Mar 19, 2007
    #3
  4. Ed Gasket

    hyundaitech Guest

    Let's pause for a minute. You adjusted the throttle stop screw? I thought
    you checked it and the throttle was closing fully (against the screw).

    Put the throttle stop screw back where it was and then adjust your
    throttle cables so they allow the throttle plate to fully close. The
    reason that you're is high and fluctuating is probably that the throttle
    is held too far open (by the stop screw at this point).
     
    hyundaitech, Mar 20, 2007
    #4
  5. Ed Gasket

    Ed Gasket Guest

    Hi, At first, the throttle was not contacting with the throttle stop
    screw and the throttle would close and stick a little in the bore.
    Then I adjusted the throttle stop screw so that now the throttle just
    contacts with it. There is plenty of slack in the cable. The throttle
    cannot be held open very much as the throttle only just contacts the
    throttle stop screw. If I unwind the screw anymore then the throttle
    would not touch it and becomes 'sticky' in the bore as the throttle
    flap closes fully (but even then the revs are high and fluctuating).
    I will try adding a resistor between the signal wire and ground on
    the throttle position sensor and see if that lowers the revs. If so I
    probably need a new TPS. If not maybe its the idle actuator.
     
    Ed Gasket, Mar 21, 2007
    #5
  6. Ed Gasket

    hyundaitech Guest

    No, don't put a resistor inline. First, clean your throttle body to make
    sure the carbon buildup isn't preventing the throttle plate from closing
    properly.

    Figuring out why the throttle mechanism doesn't touch the stop screw is
    the first and primary order of business. The stop screw should never be
    adjusted once it has been set from the factory. If no one moved the screw
    prior to the throttle not hitting the screw, then there's a problem that's
    preventing the throttle from fully closing.
     
    hyundaitech, Mar 22, 2007
    #6
  7. Ed Gasket

    Ed Gasket Guest

    I would imagine the previous owner unscrewed the throttle stop in an
    attempt to bring the idle revs down. Anyway I have it sorted now.
    After the unpromising resistance readings from the TPS, I managed to
    read off the voltage from the TPS. I had to use thin wires poked into
    the plug and then push the plug and wires back on as it was not
    possible to 'back probe' the connector. It read 0.48v at idle and over
    4v at full throttle so everything fine there (must be something more
    than just a potentiometer inside as my resistance readings and
    potential divider theory would have indicated a voltage of 1.5v or
    so).
    I had read elsewhere that some people had fixed this problem by
    resetting the ECU (ECM or PCM; anyway the car's computer !). So rather
    than start ripping things apart, I tried this. I took out the ECM fuse
    (labelled Burg Alarm) and left it for 30 mins (I think less time would
    be OK). Put it back and hey presto, runs like a dream. No more revving
    above 2000 rpm on cold start (cold start revs now around 1200), a nice
    warm idle at around 800 and no fluctuations ! I wonder how much
    Hyundai would have charged to resolve that one?
     
    Ed Gasket, Mar 24, 2007
    #7
  8. Ed Gasket

    Mike Marlow Guest

    Congratulations. Nice when a plan comes together, isn't it?

    Nothing. They do this kind of work for free... when you can get them to do
    it. The Hyundai dealers on the other hand, probably would have charged a
    couple bucks.
     
    Mike Marlow, Mar 24, 2007
    #8
  9. Ed Gasket

    Ed Gasket Guest

    Wow Mike, you must live in a utopian society. I can't think I'd get
    much change out of £60 ($120) minimum here in the UK if I visited the
    Hyundai dealership and booked the car in for them to investigate
    idling problems. Never known anything free or a couple of bucks; only
    big bucks ! Seriously labour here is £100 ($200) per hour and minimum
    charge is 30 mins and on top of that add VAT at 17.5%. Anyone else in
    the UK care to comment?
     
    Ed Gasket, Mar 25, 2007
    #9
  10. Ed Gasket

    Mike Marlow Guest

    Wow Mike, you must live in a utopian society. I can't think I'd get
    much change out of £60 ($120) minimum here in the UK if I visited the
    Hyundai dealership and booked the car in for them to investigate
    idling problems. Never known anything free or a couple of bucks; only
    big bucks ! Seriously labour here is £100 ($200) per hour and minimum
    charge is 30 mins and on top of that add VAT at 17.5%. Anyone else in
    the UK care to comment?



    I was just trying to be funny Ed, but talk about Wow - you guys are paying
    way too much in labor. Clearly the Artful Dodger is alive and well in the
    UK, huh?
     
    Mike Marlow, Mar 25, 2007
    #10
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