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stumbling, stalling, no start

souske

NAXJA Forum User
1996, 4.0, AX-15, 231

Had some trouble getting re-started on the trail over the weekend, got it running again with a shot of ether, didn't turn it off again the rest of the run, no problems. Then started bucking/stumbling on the way home, but would run good above 3k, gradually getting worse. Ended up needing a tow home.
Already tested so far:
New fuel filter, pump
Replaced TPS
Cleaned IAC

Compression check today: all at 130, except #6@135
All plugs were fuel fouled.

So what's next?
 
If I remember right, somewhere in 96 Jeep switched to the newer style fuel delivery system which continued through 01. Do you know which you have? If you have an earlier style with the separate regulator, that part could still be in play. If you have the earlier style, pressure should be 31 psi. If you have the newer style, pressure should be 49 psi. Map sensor also is involved with fuel/air mixture. Assuming that there is no check engine light??
 
Fuel pressure should be 49 psi, plus or minus 5 psi for that fuel system. You mention above 40, it shouldn't be below 44 psi.

You have lots of juicy codes to concentrate on, and addressing them is what must be done.

P0505: Not in my book
P0122 and P0123 are throttle position sensor codes
P0300 is random misfire
P0301, P0302 and P0303 are all single cylinder misfires for different cylinders
P1391 is intermittent loss of either cam sensor or crank sensor signal

The P1391 is usually linked to a flaky crank sensor more than cam sensor, but don't get tunnel vision.

But I chase them one at a time starting with the lower number codes, then clear codes, and see what codes returns. Proven strategy.

Start with the codes relating to the TPS.

Did you spray clean the connector for the TPS with electrical contact cleaner? Or did you clear the codes after replacing the TPS? If you didn't clear codes, they may have been there from before you replaced it. Also be sure to trace down all wiring for that throttle position sensor if it keeps coding for it. You really need to trust the OBD system on this one; it isn't perfect, but it is pretty darn good at pointing you to the correct circuit that is faulty.
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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.
 
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With all that mess of codes, I would look at the battery first, monitor the system voltage (Battery, Alternator, voltage regulator which is the PCM), and check the system grounds, look for a damaged spot in the wiring harness, or arcing HV to a nearby batch of sensor wires, driving the PCM computer nuts, and causing miss fires and random sensor faults.

You also have a CPS sensor code, which is know for causing miss fires just before it dies and go to a no start, no spark condition. CPS, CRankshaft Position Sensor, it is on the transmission case, and picks up its signal from the flywheel. It is magnetic, and can get too dirty from clutch plate dust attracted to the magnet, and loose signal strength over time!!! The magnets eventually go bad on their own.

But I would look for a common cause, like a HV arc to the computer sensor wiring harness wires!!! Or a dying battery or alternator running the voltages all over the place.

The chances of a bad CPS, TPS, fuel pump and IAC (Code P0505 is bad IAC....) all at once are very unlikely, highly improbable, even for a JEEP!!! :eek: LOL
 
New CPS did the trick

Getting spark when I checked for it threw me off, guess that's the trouble with an intermittent failure
 
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