2000 XJ misfire/dying while driving, no codes (4.0L AT, CA emission)

[ihmagellan]

In the 14 months I've owned this XJ, I've driven it over 25k miles. Total mileage is about 241k at this point, but it's a rust-free, well maintained former CA highway department vehicle. It has always had an intermittent issue at highway speeds, usually with the cruise control on, where it feels like the engine cuts out. The cruise cancels but the system stays on and pushing resume brings it back to speed as expected. I initially thought it was a problem with bad gas and Lucas injector cleaner seemed to take care of it for a couple thousand miles. Sometimes it would go for a couple months without happening. I thought it might be an electrical issue causing the torque converter to unlock. Researching that listed the TPS, crankshaft position sensor, and cam position sensor and possible causes. I changed the TPS about 1000 miles ago and the problem seemed to go away until this week.

This time when it happened the cruise control system shut off completely. The engine was cutting out badly. The speedometer would bounce from 55 down to about 30. Once, the needle jumped to about 90. The engine light came on intermittently and the oil light would flicker (idiot lights only). I pulled over and it idled fine with no lights. I'm pretty sure that under load, the engine was shutting off and restarting on its own. I was only about 3 miles from home, so I limped it back with the miss. As I approached the driveway, it died completely. I turned the key and it started back up and idled fine. It still didn't set any fault codes. I changed the crankshaft position sensor, cam position sensor, and coil rail and took it out for a test drive. It was fine for about 6 miles and then about a half mile from home it cut out a couple times.

In almost 30k miles, and with a definite miss for some reason, there has never been a fault code of any kind. Is that a sign that the PCM is going bad? On a side note, a mouse got into the Jeep last week, but I can't find an wiring damage.

I seen that there's a current thread about a similar problem with an '89 XJ, but that would be a Renix system, correct? If so, I'm assuming the root cause is likely to be different from mine.

 

[Tim_MN]

PCM failure is very highly unlikely and should only be considered after a thorough and in-depth diagnostics of the more common and more likely suspects.

The CPS is the primary suspect. Test it. OBD-II sensors that are not Genuine Jeep or NTK are likely cheap Chinese clones and should be tested if they are suspect, and even before they are installed. Often they are out of tolerance, or simply defective right out of the box.

Test the Throttle Postion Sensor (TPS). Visually inspect all the OBD sensor wire harnesses, inch by inch, for melted or chafed wire insulation, failed wire splices, and for corroded, and bent or pushed back wire plug pins. Common damage areas are near hot exhaust parts and next to sharp metal edges of the block and the valve cover. Test wire continuity, signal and ground, between all the OBD sensors, and the PCM.

Don't forget to test the O2 sensor function, and to inspect the O2 sensor wires and fuses. The O2 sensors should be genuine Jeep or NTK, and NOT Bosch or cheap crappy auto parts store brands. How old are the O2 sensors? Are they contaminated by excess fuel, oil, or coolant in the exhaust flow?

Testing the fuel pressure is a good idea.



TPS failure 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 TESTING

The TPS is mounted on the throttle body. The TPS is a variable resistor that provides the Powertrain Control Module (PCM) with an input signal (voltage) that represents throttle blade position. The sensor is connected to the throttle blade shaft. As the position of the throttle blade changes, the resistance of the TPS changes. Along with inputs from other sensors, the PCM uses the TPS input to determine current engine operating conditions. In response to engine operating conditions, the PCM will adjust fuel injector pulse width and ignition timing.

The PCM supplies approximately 5 volts to the TPS. The TPS output voltage (input signal to the PCM) represents the throttle blade position. The PCM receives an input signal voltage from the 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. This will vary in an approximate range of from .25 volts at minimum throttle opening (idle), to 4.5 volts at WOT wide open throttle.

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 fault 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.

Cheap crappy Chinese "Lifetime Warranty" parts are often out of specification or even failed right out of the box. Many times they have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep sensors.

 

[Unclewolverine]

X2 on thermal failure on cps; the spark rail can also cause similar issues.