Sputter of RPMs

So my 01 XJ died today while standing in traffic. At first it dropped RPMs, then picked it up, and immediately dropped again and died. It started right up, so that was not the issue. After that I felt the sputter of RPMs once more, but that was it.

I am also losing fuel pressure as soon as I shut the engine off. Within 10-15 minutes I drop about 6-10 psi. In the mornings it starts up just fine, but once the day is on, it has to turn for about 2 seconds before it fires up.

I thought it might have been that famous valve that sits in the fuel pump that gets bad, but this stumble and dying is new to me. I did read about crank positioning sensor, but I am not sure.

Ideas?

Since you're losing fuel pressure, I'd start by giving the rail and the fuel system a good going-over. A leaky fuel injector, injector O-ring, or really any leak upstream of the check valve in the fuel pump could give the symptoms as a bad check valve.

The CPS usually dies when hot and sometime comes back to life, temporarily, after cooling.
If the fuel pressure is dropping very quickly, after shut down, it is either a leaking injector or a bad check valve in the fuel pump.

I would test the fuel pump's pressure, if in the right range, i would next pull the injector rail, leaving the injectors in the rail, and look for dripping injector, when the fuel lines pressurized

My 2001 XJ had a bad check valve, cured by installing anew Bosch fuel pump.
I noticed a slight stumble occasionally, on steep upgrades, as if it was running out of fuel...evidently it was.
The the performance immediately improved when the pump was replaced and the engine started up much faster.

Thanks for the replies. I did replace the OEM injectors with 4-spray injectors. I might have gotten a faulty one, so I will check that.

I was thinking loss of ignition, as it also likes to give a stumble when driving higher speeds, although not very often. I might have several issues that I need to solve here.

So, update. today on my way to work I was on the highway, and suddenly the engine died while driving 40 miles per hour. I was lucky to be in the right lane so I could move over. Put it in park, fires right up.

I am beginning to think that the fuel pressure issue is a separate problem, while this is more computer or ignition related. Other than this, engine runs great (other than it sounds like a diesel as described in my other thread). It seems to be a moment of a hickup that it happens.

Test the fuel pressure, but the symptoms strongly suggest a failing CPS.


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. Typical CPS lifespan is about 150-200,000 miles.

Crank Position Sensors can have intermittent “thermal failure”. This means that the CPS/CKP fails when the 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 NoBus displayed 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 trouble 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 may 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 these relays as the cause of the no-start. Confirm that the fuel pump runs for 3-5 seconds when you turn the ignition key to ON.
-Eliminate the NS 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 exhaust pipe. A short circuit from melted insulation or from broken O2 sensor wires can blow a fuse and the ECU/ECM will lose communication.

You must also perform 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.

If you buy a new CPS, get a genuine Jeep CPS, or the premium one from NAPA. Cheap crappy “Lifetime Warranty” parts are often out of specification or even failed right out of the box. They usually also have a shorter service life than better quality parts. Buy good quality repair parts and genuine Jeep sensors for best results.



Crankshaft Position Sensor Connector (CPS/CKP)
.





.





CPS Testing


TESTING PROCEDURE 1991 –20014.0L 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 the 10K-or-2OK 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/ohmmeter 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 .5 VAC, replace it.

The 2000 and 2001 will have the CPS in the same location on the bell housing,but the wire connector is 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 wire connector.
.

also if the rpm gauge just drops right to 0 when it dies its a cps issue.

Damn, you guys are spot on with the symptoms. In the morning it starts up fine, but when warmed up it takes a good 2 seconds of cranking before the engine fires up. I also see the rpm gauge floating sometimes to almost zero, and shoots right back up.

My XJ has 190k miles on it, so it might be time to get that part replaced.

Thanks for all the replies and details!