bigbird

I have a 1990 xj that im getting back on the road. It has been running fine until all of a sudden it sputtered and died at idle. it cranks over fine but no start. it almost sounded like it was out of gas. I changed out the fuel filter and check to see if the pump was working by leaving the output end of the filter unpluged and draining in a bucket. cranked over and a lot of gas coming out so i would assume the pump was working. checked to see if i was getting fuel at rail and i was but was noticed very little pressure out the shradder valve. i dont have a pressure gauge yet to confirm but does not seem like it is getting recommended 35psi. what could cause this and could this be the reason for the no start. ive also been reading about the cps could cause a no start but i want to trouble shoot before putting more money into it. history of the jeep 1990 xj with 150k miles auto
thanks for helping

dukie564

CPS would seem to be the main suspect.


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 should run and prime for 3-5 seconds.

If the CPS/CKP is failed sometimes the OBD-II 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/CKP)


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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.

TESTING PROCEDURE for 1987 – 1990 4.0 L engines

Test # 1 - Get a volt/ohm meter and set it to read 0 - 500 ohms. Unplug the CPS and measure across the CPS connector's A & B leads. Your meter should show a CPS resistance of between 125 - 275 ohms. If the CPS is out of that range by much, replace it.

Test # 2 - You'll need a helper for this one. Set the volt/ohm meter to read 0 - 5 AC volts or the closest AC Volts scale your meter has to this range. Measure across the CPS leads for voltage generated as your helper cranks the engine. (The engine can't fire up without the CPS connected but watch for moving parts just the same!) The meter should show .5 - .8 VAC when cranking. (That's between 1/2 and 1 volt AC.) If it's below .5vac, replace it.


The 2000 and 2001 will have the CPS in the same location on the bell housing, but the wire connector may be on the passenger side, near or on top of the Transfer case, not as shown in the diagram below. Simply follow the wire from the sensor to the connector.

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bigbird

i just came in from working on the jeep. poured a little gas down the throttle body got the jeep to start but would stop after burning the poured gas. could this still be the cps or is it a sign of a bad fuel pump. oh and i pushed in the valve on the fuel rail after the start up and very little pressure. any thoughts are helpful thank you

dukie564

A faulty CPS will disable both the spark and the fuel, so sounds like CPS is ok.

Definitely sounds like a faulty fuel pump at this point. The first thing you should do is check for a blow fuse (underhood fuse box) or a faulty fuel pump relay (swap with another one not used for starting - like the A/C relay).


Your model also has a ballast resistor the you should check. Disconnect the two wires connected to the resistor and jumper them together. I don't think this is your issue though.


This should help too:
http://www.ehow.com/how-does_4811117...fuel-pump.html

Harper

My money is on a fuel pump. i just had this same problem in my '91.

Change the fuel filter first, then check the fuel pressure at the rail again. If it's still low, then change the pump.

I could hear my pump running, but it had no pressure.

A couple of hints if you end up changing the pump:

1. Drop the tank... Yes, I know it can be removed with the tank in the vehicle, but you will likely never get the lock ring back on with the gasket in the proper position. It will leak.

2. DO NOT buy an Airtex pump! They are crap. The little rubber adapters they give you will swell up inside of a week, the pump will rattle loose and short out on the steel mount.

Mycherokee96

Thats a fuel pump.......try banging your tank as best you can while someone tries to start the truck....if it starts and runs .. there you go...fuel pump...P.S...use a rubber mallet if you can...The CKS will allow the engine to run on its last parameters until you shut it off....then it will not start. True, some fail only when hot. Far and few between. My guess would be the fuel pump......