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XJ Riddle. Help!

DangerCell

NAXJA Forum User
Location
Cape Cod
Hi! Hope someone who knows XJ's makes short work of this riddle:

It is a 98 XJ auto, 4.0, 4x4, 132K miles, and it is in nice shape. Drove home from work 32 miles after fill up, through some stop and go traffic for 15 min, stopped for iced coffee....started it up and drove the remaining 1 mile to home. Did some yard work for an hour then hopped in jeep to go to dinner...NO START.
Did a quick diagnostic:
Pulled intake hose off airbox and put clean rag over it and hose clamped it on, soaked it with fuel and it starts right up. Can keep it running perpetually if ya keep fuel coming with a squirt bottle...
Then checked couple other things and end result as far as I can tell is:
1. Fuel pump pumping
2. Fuel rail pressurized
3. But considering diagnostic "test" NO injectors injecting.

So bought a new PCM and installed it. No change.

Where do I go from here???
 
Could be the common culprit CPS crank position sensor.

But I don't think it would run with your gas test. If it is CPS then maybe it's keeping the injectors from firing. Then the gas test would make sense.

I thought the CPS interfered with timing the spark. I'll have to look it up.


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So from what you are saying, the injectors are not firing. Each injector has a separate wire back to the ECM that drives it. For it to not run at all, all 6 wires would have to be bad....not likely. All 6 injectors share a common line. I am a Renix guy, not a HO guy so I don't know if that common wire goes to 12v or to ground, but it seems probable that the common wire to all the injectors is not going where it is supposed to.

The ECU is reading the CPS correctly because you are getting spark and at the right timing, otherwise it wouldn't start. So you can rule out the CPS. If the ECU knows when to spark, it should know when to fire the injectors. Since you got a new ECU, the odds of it having the exact same issue as the old one are pretty long, so you should be able to rule out the ECU. All 6 injectors failing at the same time is statistically impossible so you can rule out the injectors.

Again, not being HO fluent, and knowing how it handles injector drive, I would pull and check every fuse first. Does the HO have a separate power fuse only for the injectors?

It is time to do a thorough visual inspection. I would look for broken wires or where wires have contacted the exhaust manifold and gotten burned. I would look at the ECU connectors for bad pins.
 
Could be the common culprit CPS crank position sensor.

But I don't think it would run with your gas test. If it is CPS then maybe it's keeping the injectors from firing. Then the gas test would make sense.

I thought the CPS interfered with timing the spark. I'll have to look it up.


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If CPS unplugged it will NOT run with my wet sock trick...so it is not that...
 
THE most common cause of "starter cranks the engine but the engine will not run" is the Crankshaft Position Sensor (CPS).

Once you have confirmed the engine has spark, and the fuel pump provides fuel pressure, the first thing to test is the CPS.

CPS Testing

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. 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.
- Fuel pump should run and prime for 3-5 seconds.
- No spark at the spark plugs.

When the ignition key is first turned ON, 12 volts travels from the ignition switch to the ASD relay. The PCM provides ground to the ASD relay to energize it. The ASD relay sends the 12 volts to the primary side of the coil. If after a few seconds no signal is detected from the faulty CPS, the PCM opens the ASD relay and 12 volts power to the coil and fuel pump is removed.

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 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.
-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 lose communication.

If you buy a new CPS, get a genuine Jeep CPS. Most auto parts stores sell cheap crappy Chinese "Lifetime Warranty" parts that are poorly manufactured from inferior materials and are often out of specification, or even failed, right out of the box. The ones that aren’t faulty often have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts. Buy good quality repair parts and genuine Jeep sensors for best results.

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.
Crankshaft Position Sensor Connector (CPS/CKP)
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standard.jpg

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standard.jpg


CPS Testing
TESTING PROCEDURE 1991 –2001 4.0L H.O. engines
1. Near the rear of fuel rail, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C or 2 and 3 (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 .5vac, 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.
.
standard.jpg
 
I would get a noid light and plug it in place of one of the injectors, just to be sure they are not being sent a signal. See if you can find one you can borrow, or spring the $35 or so to get a set of them.

Could it be that they are all firing, but have gotten plugged by something, like say a deteriorating fuel pressure regulator?

Could it be that you got a batch of bad fuel?

If they truly are not firing, and each one has its own connection to the computer, is it the case that both wires for each injector have their own, independent connection, or is there a common ground they all share?
 
2. Fuel rail pressurized

So bought a new PCM and installed it. No change.

Did you "squirt test" the fuel rail Schrader valve...or put a pressure gauge on it?

AFAIK failure of the PCM/ECU is a fairly rare/unlikely first step...but that's water under the bridge now.
 
Ah. A common positive.

That sounds like the place to start.
 
IIRC, The injectors share +12v from the ASD relay and are driven by ground from the PCM/ECU. I'd check all the fuses in the pdc


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On the 1996 XJ, the injectors do not share the ground wire or/and positive wire. The however all depend on the same ASD (Automatic Shut Down) relay. If that relay fails none of the injectors will fire. I think everything else will work.

Well, according the wire diagrams, the ADS supplies the positive voltage all 6 injectors and the negative is supplied by the PCM on an as needed basis. The ASD relay ground is triggered by the oxygen sensor.
 
Common sense, numerous Naxja no-start threads, and prior experience suggest that testing the CPS should one of the first things done with a no-start.

The testing is simple, conclusive, and quick.
 
I want to see a fuel pressure gauge hooked up and showing 31 psi at least, when the key is turned on.

It can only be a fuel issue if it starts when primed externally.
 
Common sense, numerous Naxja no-start threads, and prior experience suggest that testing the CPS should one of the first things done with a no-start.

The testing is simple, conclusive, and quick.
I agree that the CPS is the most common problem, but logic and troubleshooting has conclusively ruled that out. Without changing anything other than providing fuel, it runs.
 
I want to see a fuel pressure gauge hooked up and showing 31 psi at least, when the key is turned on.

A 1998 XJ should produce 49-psi, plus or minus 5-psi, at the test port, with the key on.
 
Did you "squirt test" the fuel rail Schrader valve...or put a pressure gauge on it?

AFAIK failure of the PCM/ECU is a fairly rare/unlikely first step...but that's water under the bridge now.

Squirt test only...and it turns out it does not always even squirt although pump is running...

So I am thinking it is pump or clogged fuel filter...is there even a fuel "filter" anyplace on a 98???

Crank position and Cam position sensors seem to function given my "backyard beer bottle test"...it runs fine if you provide fuel through clean rag over intake with those plugged in...unplug em and no run...
 
Squirt test only...and it turns out it does not always even squirt although pump is running...

So I am thinking it is pump or clogged fuel filter...is there even a fuel "filter" anyplace on a 98???

Crank position and Cam position sensors seem to function given my "backyard beer bottle test"...it runs fine if you provide fuel through clean rag over intake with those plugged in...unplug em and no run...


The fuel filter on a 98 is in the gas tank attached to the fuel pump assembly. I think at this point you should pull the fuel rail with the injectors attached, point them out of your hood, then crank the motor to see if they squirt fuel. Just make sure it's pointed in a direction you can clean the fuel easily and you don't start a fire.

This will tell you three things,
- if the fuel is actually coming out of the injectors
- which injectors are firing if any
- if the injectors are firing in the correct order

You should also hook up a fuel pressure tester to the fuel rail and make sure you are getting 49psi.

At this point it sounds like it is a fuel delivery issue but from my experience, these 4.0s can have symptoms that damn near tell you it is one thing, but it turns out to be something completely different. I would double check that you are for sure getting spark and maybe a compression test for the hell of it.
 
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