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Old 05-30-2012, 03:35 PM   #1
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Default idle goes up and down

I just bought a 1996 Lorado 4X4, my engine idle goes up and down sometimes when I'm at a stop or if I'm going down hill without throttling the engine I noticed it going up and down on the tach. the AIC got changed right before I purchased it. note: when the engine is running it very smooth until the idle wants to change.
I hope someone can help me out.
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Old 05-30-2012, 03:44 PM   #2
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search cleaning IAC MOTOR
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Old 05-30-2012, 05:18 PM   #3
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Quote:
Originally Posted by freegdr View Post
search cleaning IAC MOTOR
x2.
The IAC regulates the idle speed, and any alternating symptoms in the dile would put the IAC high on the list.
If its been put in recently, check if its tight enough etc.
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Old 05-30-2012, 06:06 PM   #4
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thanks! I'll try that.
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Old 05-30-2012, 06:28 PM   #5
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My first thought is one of two things.

1. You have a faulty "throttle position sensor". It is a pretty common failure. Here is more on it. Check out the part on idle control.

2. If the TPS doesn't do it, I'd remove the throttle body and do a bench cleaning on that.
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The throttle position sensor is connected to the throttle shaft on the throttle body. It sends throttle valve angle information to the PCM. The PCM uses this information to determine how much fuel the engine needs. The TPS is really just a simple potentiometer with one end connected to 5 volts from the PCM and the other to ground. A third wire is connected to the PCM. As you move the accelerator pedal with your foot, the output of the TPS changes. At a closed throttle position, the output of the TPS is low, about a half a volt. As the throttle valve opens, the output increases so that, at wide open throttle, the output voltage should be above 3.9 volts. Testing can be performed with an electrical meter. Analog meter is best. You are looking for a smooth sweep of voltage throughout the entire throttle band. While slowly opening and closing the throttle, take note to the movement of the voltmeter needle. There should be a direct relationship between the needle motion to the motion of the throttle. If at anytime the needle moves abruptly or inconsistently with the movement of the throttle, the TPS is bad

You should have 5 volts going into the TPS. At idle, TPS output voltage must be greater than 200 millivolts. At wide open throttle (WOT), TPS output voltage must be less than 4.8 volts.. The best is to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. One of the other wires should show .26V (or so). The other wire will be the ground and should show no voltage. Move the throttle and look for smooth meter response up to the 4.49 at WOT.

Perform the test procedure again and wiggle and/or tap on the TPS while you watch the meter. If you notice any flat spots or abrupt changes in the meter readings, replace the TPS.

The TPS is sensitive to heat, moisture and vibration leading to the failure of some units. The sensor is a sealed unit and cannot be repaired only replaced. A TPS may fail gradually leading to a number of symptoms which can include one or more of the following: -

NOTE: The throttle position sensor is also DIRECTLY involved with transmission shifting characteristics! It should be verified early in the troubleshooting process, when a transmission issue is suspected!

Poor idle control: The TPS is used by the ECU to determine if the throttle is closed and the car should be using the Idle Air Control Valve exclusively for idle control. A fault TPS sensor can confuse the ECU causing the idle to be erratic or "hunting".
• High Idle Speed: The TPS may report faulty values causing the engine idle speed to be increased above normal. This is normally found in conjunction with a slow engine return to idle speed symptom.
• Slow engine return to idle: A failing TPS can report the minimum throttle position values incorrectly which can stop the engine entering idle mode when the throttle is closed. Normally when the throttle is closed the engine fuel injectors will be deactivated until a defined engine RPM speed is reached and the engine brought smoothly to idle speed. When failing a TPS will not report the throttle closed and fueling will continue causing the engine to return to idle very slowly.
• Engine Hesitation on Throttle Application: The TPS is also used by the ECU to determine if the driver has applied the throttle quicker than the Manifold Air Pressure sensor can read. The fueling is adjusted acordingly to cope with the sudden increase in air volume, however a faulty sensor can cause the ECU to ignore this data and the engine will "hesitate" when applying the throttle. In extreme cases with the engine at idle, a sudden application of full throttle can stall the engine.
• Engine Misfire: A fault TPS can report values outside the deined acceptable range causing the ECU to incorrectly fuel the engine. This is noticable as a slight misfire and can trigger the misfire detection software and/or Malfunction Indicator Lamp (MIL) light on the dashboard. Extreme cases can cause excessing misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.
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Last edited by tjwalker; 05-31-2012 at 04:35 PM.
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Old 05-31-2012, 12:45 AM   #6
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I had a similar issue. I cleaned the throttle body. Cleaned the idle air motor. Checked my TPS. Checked my MAP. After all that it ended up being my O2 sensor. It wasn't reading right and kept switching my engine between open and closed loop which effected my fuel trims and gave me a weird idle issue
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Old 05-31-2012, 09:49 AM   #7
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thank you for you in put, very helpful. I'll get testing and cleaning ASAP.
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Old 05-31-2012, 04:01 PM   #8
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I went through the entire throttle body and everything was clean,I removed the TPS and put it back. after that the check engine light came on so I checked for a code and it turned out to be the TPS. The Cherokee is running fine now. thanks for the tips. now it's time to change rear axle bearings.
see you soon pesnemesis.
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Old 05-31-2012, 04:38 PM   #9
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Do you have the A/C on while this happens?
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Old 06-01-2012, 08:58 AM   #10
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no, you can't even feel the engine change idle when it was on, but all fixed now.
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Old 06-01-2012, 08:58 AM
 
 
 
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