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Unread 07-12-2013, 11:17 AM   #1
Glavus
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1999 XJ Cherokee 
 
Join Date: Dec 2011
Location: Murray, UT
Posts: 216
Idle speed..

Okay, I've been experiencing some 'things' ..sporadically.

- High idle, sometimes (1,200 rpm and when it does that it only goes back to normal if I restart the engine)

- Random downshifts, sometimes (just from releasing the gas pedal to coast)

- Stops accelerating, sometimes (only for a few seconds)

- Idles at 750 rpm, 3rd line on rpm's (not sure if this is abnormal)

I cleaned the IAC pintle and throttle plate/bore even though they've always been pretty clean prior to all this.

Doesn't seem to be a vacuum leak either, symptoms are completely random.

No codes, and all tune up parts have about 11,000 miles on them.

Overall, my main question is if 750 rpm is a normal idle? I only took note of it after I just cleaned the IAC.

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Unread 07-12-2013, 12:37 PM   #2
CJ7-Tim
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2000 XJ Cherokee 
 
Join Date: Apr 2002
Location: out in the garage - Minneapolis, Minnesota
Posts: 22,080
Quote:
Originally Posted by Glavus View Post
Okay, I've been experiencing some 'things' ..sporadically.

- High idle, sometimes (1,200 rpm and when it does that it only goes back to normal if I restart the engine)

- Random downshifts, sometimes (just from releasing the gas pedal to coast)

- Stops accelerating, sometimes (only for a few seconds)

- Idles at 750 rpm, 3rd line on rpm's (not sure if this is abnormal)


Overall, my main question is if 750 rpm is a normal idle? I only took note of it after I just cleaned the IAC.
600-750 rpm is the normal idle speed.

The other symptoms suggest that your Throttle Position Sensor might be faulty and should be tested.

TPS TESTING

You may have one or more of these Physical Symptoms:

1) The engine loses power and is stalling.
2) The engine will idle, but may die as soon as you press the gas pedal. When driving, it seems as if all power is gone.
3) Sometimes it feels as if the transmission is failed or isn't shifting properly, if at all. If you quickly jump on the gas you might be able to get the transmission to shift, but it won’t shift properly by itself. Shifting manually, the transmission goes through all the gears.

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

TPS TEST

You should have 5 volts going into the Throttle Position Sensor (TPS). It is best 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. At idle, TPS output voltage should be greater than .26 volts but less than .95 volts. Move the throttle and look for smooth meter response up to the 4.49 at WOT. The other wire will be the ground and should show no voltage.

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:

-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 accordingly 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 faulty TPS can report values outside the acceptable range causing the ECU to incorrectly fuel the engine. This is noticeable 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 excessive misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.
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