Hi Guys, I have a couple small issues with my '98 Cherokee Limited "Amanda Lucille", maybe some of you might have some knowledge. She runs like a champ aside from these two issues... Her specs are... Stock 4.0 Liter, Automatic, 2WD, 191,000 miles, no 'check engine' lights on.


# 1 - She has a tendency to idle high. Not all the time, just frequently - and somewhat randomly. 900 rpm seems to be about normal idle speed. But she'll do anywhere from 1000 to as high as 1500 rpm on her own. Punching the gas doesn't alleviate this. Last time she ran high, I noted that her motor spun particularly fast when disengaged. i.e. Park and Neutral.

# 2 - This one is rare due to her use. But we took her on an 800 mile trip last year (fresh tires, oil, brakes, tie rod, front alignment, catalytic converter, etc.) and after a long stretch of driving, and at sustained high speeds, (70-75+ mph) she would begin to make clunking noises as though the trans were confused and shifting up and down harshly - very unnerving at night 400 miles from home! Don't see how the tranny could have been shifting though, because her speed was being held steady while it was happening. She really did not like anything past 70. After a rest stop, she would be okay again... Amanda hasn't been on any extended highway trips since that time, so it hasn't happened again. But she will hit the long distance highway again at some point.


Perhaps these are known issues? I'd like to have a clue either way. Anyone who has anything to add, whether it's their 10 bucks, or their 2 cents is appreciated.

 

Is your check engine on? You should check the throttle body gasket when mine was bad it idled high. Worth trying since its only like $2, then after that you may be looming at a iac

 

No check engine lights. But I'll take the gasket under advisement.

 

Looking* and the iac is located on the throttle body and controls air flow at idle

 

High idle issues tend to occur because of a dirty IAC (disconnect it and scrub it with an old tooth brush with some carb cleaner), a dirty throttle body (take it off and soak and scrub it with throttle body cleaner), or a vacuum leak. So as suggested, throw on a new gasket, perfect time to clean the TB once taken off to put on the new gasket

 

X2 on sounding like a dirty IAC. Clean it up and be careful not to twist it when doing so. Also check maybe if your cables are too tight on the throttle body there should be just a tiny mount out play in the them so the valve in the tb is fully closed.

 

I agree that cleaning of both the idle air control and throttle body is a good idea.

But beyond that, both of these symptoms could be related to a faulty "throttle position sensor", as the TPS is involved in both idle and transmission performance.

I'd certainly test it. Here is how.
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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.

 

Don't mean to try and steal the thread, just avoid making a new thread when we have the same symptoms for the idle.

Got a question on this tps test, I just tried this out and one wire is a smidge over 5v and the other is around 4.3... just sitting there. Should I just replace it? It was fine before I put a 60mm throttle body in.

Also curious if you've made any progress on this lucille?

 

Amanda Lucille is running fine, still idles high sometimes. But I'm going to get acquainted under the hood soon here when I get the time.

Thank you all for your help everyone. Especially tjwalker for that exhaustive rundown, it's very helpful! I'm happy to know that the issue is likely auxiliary and easily remedied.