99 cherokee 4.0l misfire, no start
 

Ok. My daughter's jeep was misfiring at speed 45+mph. It was throwing a p0420 code, o2 sensor, PO cut cat off. No biggie. Off the rip, I performed a tuneup (cap rotor plug wires and copper core plugs). It ran fine for a couple weeks. Again a misfire like before, and it would not start. Only crank but never fire up. Of course the CEL came back on. This time 3 codes, p0320, p0340 and p0420. Researching different forums, most culprits pointed to a bad crankshaft position sensor. I replaced that last night. It fired right up. Ran great. Let it get to normal operating temperature and restarted no problem a few more times. Reset CEL. SO I think we're good to go now. She drives it this morning, no problems. Then this afternoon running around town it started missing again, CEL on again. When she stopped at a friends house, it would not restart. Again it only cranks , no fireup. After I get there from work still no start only cranking. I unplugged /plugged Crank sensor. No change. So I unplug/plug the camshaft sensor connector. It fires right up. Now taking jeep back home. Every so often it has a misfire, very profound misfire too. First time I've experienced it in jeep since problem arose. Only going on what my daughter was telling me before this. I unfortunately have not read codes since getting it home. But am betting on a camshaft sensor and O2 sensor codes. Any ideas next? I've replaced the crank sensor thus far (last night). Any help greatly appreciated! Need to get this fixed ASAP

Make sure you only install OEM Mopar sensors. Other brands are hit or miss.

Op,

I don't know where you live but US Federal law requires a Catalytic Converter. See https://www.cherokeeforum.com/f2/cat...er-laws-24337/

Here's some basic diagnostics for P0320 and P0340:

P0320-NO CRANK REFERENCE SIGNAL AT PCM

Possible Causes:

INTERMITTENT CONDITION
5 VOLT SUPPLY CIRCUIT SHORTED TO GROUND
5 VOLT SUPPLY CIRCUIT OPEN
5 VOLT SUPPLY CIRCUIT SHORTED TO VOLTAGE
CKP SENSOR SIGNAL CIRCUIT SHORTED GROUND
CKP SENSOR SIGNAL CIRCUIT OPEN
CKP SENSOR SIGNAL CIRCUIT SHORTED TO VOLTAGE
CKP SENSOR SIGNAL SHORTED TO 5 VOLT SUPPLY CIRCUIT
SENSOR GROUND CIRCUIT OPEN
PCM - 5 VOLT SUPPLY
PCM - CKP SENSOR SIGNAL
CRANKSHAFT POSITION SENSOR
TEST ACTION APPLICABILITY
P0340-NO CAM SIGNAL AT PCM

Possible Causes:

INTERMITTENT CONDITION
5 VOLT SUPPLY CIRCUIT SHORTED TO GROUND
5 VOLT SUPPLY CIRCUIT OPEN
5 VOLT SUPPLY CIRCUIT SHORTED TO VOLTAGE
CMP SENSOR SIGNAL CIRCUIT SHORTED GROUND
CMP SENSOR SIGNAL CIRCUIT OPEN
CMP SENSOR SIGNAL CIRCUIT SHORTED TO VOLTAGE
CMP SENSOR SIGNAL SHORTED TO 5 VOLT SUPPLY CIRCUIT
SENSOR GROUND CIRCUIT OPEN
PCM - 5 VOLT SUPPLY
PCM - CMP SENSOR SIGNAL
CAMSHAFT POSITION SENSOR

I think the first thing you should do is examine all the wiring associated with the Crankshaft Position Sensor and Camshaft Position Sensor from the sensors to the PCM. Look for chafing through the wire looms down to the wires. Check each connector for corrosion in the pins and pin cavities.

If you have a digital Voltmeter you can do some checks.

Split the connector from the Crank Sensor,

Turn ignition key to RUN/ON,

Touch the pin cavity with the Orange wire in the body harness connector half with the meter (+) probe and the battery negative post with the meter (-) probe. You should read 5 +/- .5 Volts (this is the 5 Volt supply voltage from the PCM). If you don't see this wiggle the harness to see if it changes.

Next, repeat the process, only this time touching the pin cavity with the Brown/Yellow tracer wire with the (+) probe. You should see around .012-.015 Volts (this is the ground circuit for the Sensor).

Repeat this test on the Camshaft Position Sensor connector. You should see the same voltages.

If there are no wire chafing seen and the voltage checks are good, I would suspect the Camshaft Position Sensor is defective.

Thanks for the help. I'll check them when I get home.