RPMs not dropping normally when in neutral - JeepForum.com
 
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post #1 of 7 Old 05-10-2012, 12:54 PM Thread Starter
ttrevino
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1998 XJ Cherokee 
 
Join Date: Mar 2012
Location: Chappell Hill
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RPMs not dropping normally when in neutral

I just noticed yesterday while I was driving around, my rpm's were staying above 1000 while in neutral, or park. Or at least it would take about 5-8 seconds for it to drop below 1000, back down to the 'normal' 750-800 range. I witnessed it also jump up to close to 2000 when I would switch into neutral (no foot on gas), from drive, then drop down to 1000, still higher than what I think it should be in neutral or park. I don't really notice this when the truck is just sitting in park.

Any ideas why it might be doing this? Is the rpm computer controlled? I was going to take off the breather and spray some carbon remover into the throat of the throttle body, just to see if that helps? Thanks, T

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post #2 of 7 Old 05-10-2012, 01:00 PM
CJ7-Tim
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Clean the Idle Air Controller. If that doesn't help, test the Throttle Position Sensor.


IAC

Cleaning the Idle Air Controller (IAC) :

Purchase sensor safe Throttle Body cleaner spray.


1. Remove the Throttle body from the intake manifold.
2. Remove the IAC with a TORX 15 driver (2 bolts)
3. Gently wiggle out the IAC from the throttle body
4. Clean the IAC with Throttle Body Cleaner (not carburetor cleaner). Use cleaner, a rag, and a toothbrush. Be gentle; don’t twist or pull on the pintle as it is fragile and can be damaged easily
5. Also clean where the IAC seats in the throttle body with the same throttle body cleaner
6. While you have the Throttle Body off, give it a good cleaning also.
7. Reinstall IAC, the Throttle body, and check idle quality.

.

.
“When the people find that they can vote themselves money, that will herald the end of the Republic.”

- Benjamin Franklin
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post #3 of 7 Old 05-10-2012, 01:10 PM Thread Starter
ttrevino
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That's awesome Tim, thanks for the quick and detailed response! This is what I had to do with my '06 Expedition also, and it helped, I just wasn't sure (since I haven't had time to look) if it was the same thing. If this cleaning fails, how do you test the throttle position sensor?
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post #4 of 7 Old 05-10-2012, 01:15 PM
CJ7-Tim
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test the throttle position sensor

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.

.
“When the people find that they can vote themselves money, that will herald the end of the Republic.”

- Benjamin Franklin
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post #5 of 7 Old 05-10-2012, 01:45 PM Thread Starter
ttrevino
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Again, thanks again! Awesome information!
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post #6 of 7 Old 05-10-2012, 01:47 PM Thread Starter
ttrevino
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And btw, it doesn't do any of this, so hopefully just cleaning the idle air controller will resolve the issue. I need a easy fix since I'm finding more and more things that need addressing...
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post #7 of 7 Old 05-18-2012, 01:51 PM Thread Starter
ttrevino
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I figured out what the problem was and thought I'd post the results for others.
I cleaned the throttle body with TB cleaner, first wiping down everything, then scraping out the intake with a hard plastic tool, to get all the carbonized stuff off from the inside. Then vacuumed out the inside so there wouldn't be any loose debris around. While spraying the body, I sprayed the idle air control passage inlet, as I didn't have a T20 torx to take off the IAC. This 'might' have caused some loose junk to clog it up, but I'm not sure. The next day, I bought the T20 and removed the IAC, cleaned it and reinstalled. When I would start the Jeep, it would still stay revved around 2K, and not drop, which made putting it into gear horrible. I figured the IAC was actually bad, so I ordered one from TNA Auto for $17 and free shipping! Great site, only offers a few things, but waaaaay better than spending $90 locally, and the part arrived in 2 days! Did I mention free shipping??
https://www.tnaautoparts.com/
Anyway, I put it on, and the idle dropped to 1K, so I thought the problem was fixed. I drove it to get some gas and realized it was still doing the same thing. At least I wasn't out $90, only $17, so not that big of a deal. I drove it to work today and a friend looked at it with me, and he noticed there was a small nut screwed onto the top of the threaded stub, just under the butterfly linkage, kind of like an idle adjustment stop? We removed it, and it started idling like it should! AWESOME!
We think what happened was the previous owner must have had a problem with low idle conditions (maybe bad IAC?), and added the nut to increase idle (and gas) all the time, rather than fixing the problem. When I cleaned the throttle body and replaced the IAC, it started working normally, but was still idled up from the extra nut.
At least the SOB is running normally again and hopefully I won't run into this problem again! Lord knows I have plenty of other areas to address... Thanks for all the help, T
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