Hi

I've owned my xj for around 3 months now and has always had a problem which appears to be getting worse. It's a 97 4.0 auto.

The whole engine cuts out at around 4000 rpm and then kicks back in at around 3000 rpm and then does the same for as long as I keep my foot down. It also has devoloped a misfire sometimes as well at around 3000 rpm and won't rev past this then all of a sudden it does. It seems to do it more at higher speeds. I also after going off road for a few miles stopping and restarting a few minutes later it misfires and then clears.

I've replaced plugs, leads and rotor arm (dizzy cap looks fine and in good condition)

I'm all the way over in the uk and I really don't want to sell it to buy a discovery like everyone else in this country uses for off reading!! I love the xj and have wanted one since I was a kid but I need to get these issues sorted and no one over here has a clue!!

Please help me.

Thanks in advance

Justin

 

Could be a fuel issue. Why are you in that high of RPM so much?

Does it do this only under load?

 

Check your Crank Position Sensor. Search for a how to.

Have you had the trouble codes? Any CEL (check engine light)?

 

The engine management sensor that might be involved here is the "throttle position sensor".

They can be tested. Here's 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.

 

It does do it under load, mostly on dual carriage ways when over taking etc. it seems to be fine when the engine isnt at full temp. If I put my foot down to overtake the auto box drops down a gear and straight away it misfires and splutters and if i can get through that the at around 4000 - 4500 rpm it just dies and then kicks back into life at around 3000 rpm. its driving me mad.

Justin

 

Ok so I have had all the throttle body off and gave it a good clean, iac off and done the same crank posistion sensor off and done the same. It now runs a bit better but have a really low idle and still a slight misfire. any ideas???

 

Look into the throttle position sensor as tjwalker suggested.

 

Ordered genuine one from jeep. Will be here tomorrow. Ill keep you all posted. Thanks for the help guys.