[jeremy8302]

Last week the engine would crank but it would not start. My mechanic put a new fuel pump assembly in the tank, and it was fine for a few days but the same starting problem resurfaced agian. Does anyone have any idea what my problem could be? I don't want to keep throwing money at this.

[CJ7-Tim]

Originally Posted by jeremy8302

Last week the engine would crank but it would not start.

This is common with the XJ Cherokee and is usually the CPS.


The most likely cause of it cranks and cranks but won't start up is the Crankshaft Position Sensor (CPS) located on the transmission bell housing. Often this part is also referred to as the CranKshaft Position Sensor (CKP) CPS/CKP failure is very common. The CPS/CKP can stop working with no warning or symptoms and the engine will not run or the engine may randomly stall for no apparent reason.

Crank Position Sensors can have intermittent “thermal failure”. This means that the CPS/CKP fails when engine gets hot, but works again when it cools back down.


Symptoms –
- Starter cranks and cranks but engine won't start up
- Fuel gauge and voltage gauges may not work or display properly.
- You sometimes will have No Bus on the odometer after 30-60 seconds.
- A failed CPS/CKP may or may not throw a CEL trouble code.
- No spark at the spark plugs.
- Fuel pump will run and prime for 3-5 seconds.

If the CPS/CKP is failed sometimes the OBDII code reader cannot make a connection to the computer or cannot read Check Engine Light/MIL codes because the CPS/CKP has failed.


Diagnostic steps to confirm the CPS is the cause of your no-start


You should be able to verify a bad cps, by unplugging it, and turning the ignition key to on. If the voltage gauge and/or the fuel gauge now displays correctly, replace the CPS.

Unplugging and reconnecting the CPS sensor where it connect to the main harness near the back of the intake manifold usually resets the ECU and if the jeep fires right up after doing this you can bet that the CPS is faulty and needs to be replaced.

Exchange the fuel pump relay and the ASD relay with one of the other similar ones in the PDC to eliminate the relays as the cause of the no-start Confirm that the fuel pump to runs for 3-5 seconds when you turn the ignition key to ON.

Eliminate the NSS as a cause of no start. Wiggle the shift lever at the same time you try to start. Put the transmission in Neutral and do the same. Do the reverse lights come on when the shifter is in Reverse?

Inspect the wires and wire connectors at the O2 sensors on the exhausts pipe. A short circuit from melted insulation or from broken O2 sensor wires can blow a fuse and the ECU/ECM will loose communication.

Begin with basic trouble shooting of the start and charge systems. Remove, clean, and firmly reconnect all the wires and cables to the battery, starter, and alternator. Look for corroded or damaged cables and replace as needed. Do the same for the grounding wires from the battery and engine to the Cherokee's frame/body. Jeeps do not tolerate low voltage or poor grounds and the ECM/ECU will behave oddly until you remedy this.

CPS Testing


Crankshaft Position Sensor Connector (CPS/CPK)





TESTING PROCEDURE 1991 – 2001 4.0L H.O. engines
1. Near the rear of intake manifold, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C (See Image). Ohmmeter should be set to 1K-to-1OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.




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[jeremy8302]

I think my problem is a crank sensor. I have a FSM, but I can not find the sensor on the transmission bell housing. Is this a fix I can do in my drive way?

[doublejz]

check to make sure its not the good ol PCM issue. When you turn the key to on do you hear the fuel pump priming? If your not sure, crawl under the fuel tank and have someone turn the key to the on position and you should hear the pump prime for like 5 secs. Also, does the CEL come on when its in the on position? Most of the time if its the PCM you can turn the key to on, wait like 10 secs, turn it off and repeat like 5 times and you'll eventually hear it prime and the CEL comes on (but turns off once its running like normal).

Theres a right up on here showing how to replace the capacitors in the PCM if this lands up being the issue. It'll cost like $5 vs a new PCM.

[doublejz]

Heres the link to the write-up if you land up needing it

http://www.jeepforum.com/forum/f12/crash-course-brain-surgery-replacing-capacitors-ecu-492597/

[jeremy8302]

Originally Posted by doublejz

When you turn the key to on do you hear the fuel pump priming?

I had a new fuel pump and filter put in. Even after that it will start sometimes and not another time.