1. #1
    olrac's Avatar
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    Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    Hello all,

    Just joined the forum and I hop you can help. I bought a 97 Jeep Cherokee 2.5l petrol. The jeep has always been rough on idle and very reluctant to pull away cleanly, with some backfiring. Usually I need to depress the clutch pedal and rev like mad to get through this. Once into 3rd gear and further there are no more problems.

    Just after buying the beastie, the cylinder head gasket went and made a big mess. I replaced the original engine which included all the sensors and the inlet manifold has been renewed with new gasket as well. I cannot find any vacuum leaks. The throttle body has been cleand.
    The Lambda sensor has been replaced, the CAT, a new custom SS exhaust, new radiator.

    The jeep has a 2 inch lift and has new alloys, wheel spacers and big tyres on it. New front and rear suspension, new clutch and all brakes replaced , including lines.

    All the symptoms of low erratic idle were there prior to all this happening.

    Can anyone crack this one?

    Once vehicle is up to speed there are no problems - it is getting there where the problem lies....

    Any help greatly appreciated!

    Regards,

    David

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    Re: Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    No DTC?
    Confucious says: Don't eat the snow where the huskies go!

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    Re: Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    Hello Mikey,

    many thanks for taking an interest and I like the prunes...

    you are speaking to an absolute numpty when it comes to mechanics and their abbreviations -no idea what a DTC is...but taking a guess you mean codes? none are showing and the local mechanic tried it on several units to make sure.
    Thanks,

    David

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    Re: Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    Erratic idle , backfires and lack of power usually indicates a bad timing. I would start verifying those before moving on further.
    Confucious says: Don't eat the snow where the huskies go!

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    Thumbs up Re: Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    Hello,

    We had replaced the plugs, wires and distributor cap but not the shaft, which has some play in it.

    Could this be suspect?

    The car will sit ticking over happily and then, maybe after 10/15 minutes the idle dips but does not stall out. The car will sit and run rough for 2 or 3 minutes and then pick up revs, sometimes it settles down again.

    It is ony in first and second gear that we get problems. If I make a short half hour run, let it sit for about 30mins or so and then restart she is back to her old tricks.

    Will get the timing checked and let you know the result -any more thoughts?

    Many thanks!
    David

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    Re: Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    Here's another thoery in operation as you described and may help aid in finding the culprit. Keep in mind the PCM functions between open loop and closed loop operation.
    As input signals to the PCM change, the PCM adjusts its response to output devices. For
    example, the PCM must calculate a different injector pulse width and ignition timing for idle than
    it does for wide open throttle. There are several different modes of operation that determine how
    the PCM responds to the various input signals.
    There are two types of engine control operation:
    open loop and closed loop.

    In open loop operation, the PCM receives input signals and responds according to preset
    programming. Inputs from the heated oxygen sensors are not monitored.
    In closed loop operation, the PCM monitors the inputs from the heated oxygen sensors. This input
    indicates to the PCM whether or not the calculated injector pulse width results in the ideal air-fuel
    ratio of 14.7 parts air to 1 part fuel. By monitoring the exhaust oxygen content through the oxygen
    sensor, the PCM can fine tune injector pulse width. Fine tuning injector pulse width allows the
    PCM to achieve optimum fuel economy combined with low emissions.
    GENERAL
    INFORMATION
    5
    The engine start-up (crank), engine warm-up, and wide open throttle modes are open loop
    modes. Under most operating conditions, the acceleration, deceleration, and cruise modes, with
    the engine at operating temperature, are closed loop modes.
    Ignition Switch On (Engine Off) Mode
    When the ignition switch activates the fuel injection system, the following actions occur:
    1. The PCM determines atmospheric air pressure from the MAP sensor input to determine
    basic fuel strategy.
    2. The PCM monitors the engine coolant temperature sensor and throttle position sensor
    input. The PCM modifies fuel strategy based on this input.
    When the Ignition key is in the ‘‘on’’ position and the engine is not running (zero rpm), the auto
    shutdown relay and fuel pump relay are not energized. Therefore, voltage is not supplied to the
    fuel pump, ignition coil, and fuel injectors.
    Engine Start-Up Mode
    – This is an open loop mode. The following actions occur when the starter
    motor is engaged:
    1. The auto shutdown and fuel pump relays are energized. If the PCM does not receive the
    camshaft and crankshaft signals within approximately one second, these relays are
    de-energized.
    2. The PCM energizes all fuel injectors until it determines crankshaft position from the
    camshaft and crankshaft signals. The PCM determines crankshaft position within one
    engine revolution. After the crankshaft position has been determined, the PCM
    energizes the fuel injectors in sequences. The PCM adjusts the injector pulse width and
    synchronizes the fuel injectors by controlling the fuel injectors’ ground paths.
    Once the auto shutdown and fuel pump relays have been energized, the PCM determines the fuel
    injector pulse width based on the following:
    – engine coolant temperature
    – manifold absolute pressure
    – intake air temperature
    – engine revolutions
    – throttle position
    The PCM determines the spark advance based on the following:
    – engine coolant temperature
    – crankshaft position
    – camshaft position
    – intake air temperature
    – manifold absolute pressure
    – throttle position

    Engine Warm-Up Mode
    – This is an open loop mode. The PCM adjusts injector pulse width and
    controls injector synchronization by controlling the fuel injectors’ ground paths. The PCM adjusts
    ignition timing and engine idle speed. The PCM adjusts the idle speed by controlling the idle air
    control motor.

    Cruise or Idle Mode
    – When the engine is at normal operating temperature, this is a closed loop
    mode. During certain idle conditions, the PCM may enter into a variable idle speed strategy. At this
    time, the PCM adjusts engine speed based on the following inputs:
    – throttle position
    – battery voltage

    – engine coolant temperature
    Confucious says: Don't eat the snow where the huskies go!

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    Re: Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    Almost sure: BAD Timing: Had the same problem with my freelaner: Exhaust cam was one tooth off: I had HG changed before and seems that the mechanic was in a hurry...For more search google for "
    Timing belt off a tooth - symptoms (kinda long)

    (It's my first post so I cant enter the link!)

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    Re: Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    Cam sensor one tooth off when they installed it. Happens on the 4.0L all the time. That or a bad coil/bad plug (someone may have dropped it before installing it)

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    Re: Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    Hello All, thanks for the updates and the news is that the old engine blew up last week. Internal strife, big bangs etc and I have bought a replacement engine. It shoud be going in this week and I will keep you informed of progress-many thanks!
    David

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    Re: Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    did it have an orange filter on it? Enquiring minds need a lobotomy!

  11. #11
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    Re: Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    No there was not an orange filter on it -is that signifcant?
    Engine now in but waiting for new starter motor to see/hear what it is like.......more soon.

    David

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    Re: Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    I was wondering if it was a victim of a certain brand filter collapsing internally. Take a can opener to that one and see what it looks like if you have time. Sort of a post mortem as it were.

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    Re: Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    Hello all-new engine fitted and "CHECK ENGINE" light is on. I did the three times ignition turn and get engine fault codes 21 and 24. Anyone any idea what these relate to?
    Many thanks,

    David

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    Re: Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    MIL 21 - OXYGEN SENSOR (O2S) MONITOR

    Effective control of exhaust emissions is achieved by an oxygen feedback system. The most important element of the feedback system is the O2S. The O2S is located in the exhaust path. Once it reaches operating temperature 300° to 350°C (572° to 662°F), the sensor generates a voltage that is inversely proportional to the amount of oxygen in the exhaust. The information obtained by the sensor is used to calculate the fuel injector pulse width. This maintains a 14.7 to 1 Air Fuel (A/F) ratio. At this mixture ratio, the catalyst works best to remove hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx) from the exhaust.
    The O2S is also the main sensing element for the Catalyst and Fuel Monitors.
    The O2S can fail in any or all of the following manners:
    • slow response rate
    • reduced output voltage
    • dynamic shift
    • shorted or open circuits
    Response rate is the time required for the sensor to switch from lean to rich once it is exposed to a richer than optimum A/F mixture or vice versa. As the sensor starts malfunctioning, it could take longer to detect the changes in the oxygen content of the exhaust gas.
    The output voltage of the O2S ranges from 0 to 1 volt. A good sensor can easily generate any output voltage in this range as it is exposed to different concentrations of oxygen. To detect a shift in the A/F mixture (lean or rich), the output voltage has to change beyond a threshold value. A malfunctioning sensor could have difficulty changing beyond the threshold value.
    MIL 21 - OXYGEN SENSOR HEATER MONITOR

    If there is an oxygen sensor (O2S) shorted to voltage DTC, as well as a O2S heater DTC, the O2S fault MUST be repaired first. Before checking the O2S fault, verify that the heater circuit is operating correctly.
    Effective control of exhaust emissions is achieved by an oxygen feedback system. The most important element of the feedback system is the O2S. The O2S is located in the exhaust path. Once it reaches operating temperature 300° to 350°C (572 ° to 662°F), the sensor generates a voltage that is inversely proportional to the amount of oxygen in the exhaust. The information obtained by the sensor is used to calculate the fuel injector pulse width. This maintains a 14.7 to 1 Air Fuel (A/F) ratio. At this mixture ratio, the catalyst works best to remove hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxide (NOx) from the exhaust.
    The voltage readings taken from the O2S sensor are very temperature sensitive. The readings are not accurate below 300°C. Heating of the O2S sensor is done to allow the engine controller to shift to closed loop control as soon as possible. The heating element used to heat the O2S sensor must be tested to ensure that it is heating the sensor properly.
    The O2S sensor circuit is monitored for a drop in voltage. The sensor output is used to test the heater by isolating the effect of the heater element on the O2S sensor output voltage from the other effects.
    24**P0122 Throttle Position Sensor Voltage LowThrottle position sensor input below the minimum acceptable voltag * Check Engine Lamp (MIL) will not illuminate if this Diagnostic Trouble Code was recorded. Cycle Ignition key as described in manual and observe code flashed by Check Engine lamp.
    ** Check Engine Lamp (MIL) will illuminate during engine operation if this Diagnostic Trouble Code was recorded.
    *** Generator Lamp illuminated
    Confucious says: Don't eat the snow where the huskies go!

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    Re: Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    Ok -got the bit on the 21 reading for o2 sensor. what does the 24 code mean as well?

    The engine got diagnosed yesterday and the mechanic cleared the 21 code. The other code is relating to "Throttl Position Sensor". He took this off and gave it a clean but it made no difference -any ideas on this one?

    Many thanks,
    David

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    Re: Jeep Cherokee 1997 Erratic idle and jumpy accelaration

    Quote Originally Posted by olrac View Post
    Ok -got the bit on the 21 reading for o2 sensor. what does the 24 code mean as well?

    The engine got diagnosed yesterday and the mechanic cleared the 21 code. The other code is relating to "Throttl Position Sensor". He took this off and gave it a clean but it made no difference -any ideas on this one?

    Many thanks,
    David
    The TPS is not serviceable, it must be be replaced if the transition of resistance is not smooth. Also make sure there's no corrosion in the terminals and connectors, check the wirings as well. EDIT: Also, a bad Map sensor could give you a bad TPS dtc. The symptoms for an intermittent erratic idle fits on that description.
    Confucious says: Don't eat the snow where the huskies go!

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