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Unread 06-08-2013, 09:20 AM   #1
Grabo60
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1998 XJ Cherokee 
 
Join Date: Jun 2013
Location: Palos Hills, Illinois
Posts: 7
XJ Weird Accerleration Problem

Hi everyone, I recently bought my first car, a 1998 Jeep Cherokee Sport. And when I start it and start driving when I get to 30. It accelerates in short bursts by itself. I'd apply more pressure on e pedal but nothing would happen. Any ideas?

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Unread 06-08-2013, 11:18 AM   #2
tjwalker
It's the crank sensor!
 
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1999 XJ Cherokee 
 
Join Date: Nov 2007
Location: Central Minnesota, MN
Posts: 8,446
1. Any check engine light? If so, you must retrieve codes for clues.

2. Verify fuel pressure with a gauge, which can be rented. For a 98, you should have approximately 49 psi at idle.

3. How old is tuneup hardware? Never overlook it. For a new-to-you vehicle, unless you have believable proof that it has been replaced recently, I'd replace plugs, plug wires, distributor cap and rotor. Champion copper plugs gapped to .035 are a good choice for your 98 4.0 engine.

4. If none of the above are contributing, I'd test the "throttle position sensor". Very important engine management sensor for acceleration. More on this below.
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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 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 fault TPS can report values outside the denied 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 excessing misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.
__________________
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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Unread 06-08-2013, 12:03 PM   #3
Grabo60
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1998 XJ Cherokee 
 
Join Date: Jun 2013
Location: Palos Hills, Illinois
Posts: 7
I changed the plugs yesterday, I went to autozone because the check engine light was on. They said it was the O2 thing. Idk if that's been making it fun weird but ill definitely look into what you said, thanks.
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Unread 06-08-2013, 04:49 PM   #4
tjwalker
It's the crank sensor!
 
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1999 XJ Cherokee 
 
Join Date: Nov 2007
Location: Central Minnesota, MN
Posts: 8,446
Quote:
Originally Posted by Grabo60 View Post
I changed the plugs yesterday, I went to autozone because the check engine light was on. They said it was the O2 thing. Idk if that's been making it fun weird but ill definitely look into what you said, thanks.
Always, always chase and resolve a check engine light/code. Period, exclamation point.

This is almost assuredly one of the best piece of advice I have given, and will continue to give.......If you have a check engine light/code and you don't resolve it because it possibly doesn't give you any symptoms, and then down the line you develop up another problem; well then your troubleshooting has become exponentially harder. You'll wonder if the initial code is responsible or is it a new problem?? Or a combination of the two? You can get to chasing your tail real quickly when this occurs.

The fact that you have a check engine light AND symptoms is almost always not a coincidence. You must resolve the code and see if that changes/resolves your symptom. And even if it is not directly related, it has to be resolved anyways! OBD codes are the first thing I retrieve when working on a vehicle.

Post the EXACT code (or codes) number here for comment (not just a description) and we can try to help you interpret them. With OBD codes, the devil is definitely in the details and oxygen sensor codes are among the trickiest to determine root cause.
__________________
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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