Diagnosis help please!

Hi Gang,

We are on the road with limited time and very limited computer access.
I apologies in advance for my very limited pre post searching, thanks for your tolerance.

Here is the deal: 1999 XJ AX15 127,XXX miles. very well maintained, very good condition. Daily driver, frequent long distance high way driving.

Over the past year or so I've noticed, very infrequently (maybe 5-8 times total for both) two different symptoms.

1) A hard or "false" start. This would happen hot or cold. I turn the key, it seems to have started, but upon releasing the key it dies. Evey one of the 3-4 times this has happened, it started on the second try. I should add that it's starting is normally very quick and crisp, starting with very little effort.

2) Erratic idle and associated lack of throttle response. IIRC, this has only happened on a hot restart (i.e. coming out of a 7/11 to grab a quick snack.) This condition subsided, more or less after reving the engine in neutral. when this happened many months ago, I used a scan tool borrowed from an auto parts store to check codes and it did not throw any.

Yesterday, after going out for breakfast, we left the XJ parked and went for a hike for a few hours. Upon returning, it exhibited the "false start" described above. It did start normally on the second try. I did notice a gasoline smell just before pulling away. We returned to our friends house and parked. After dinner, I decided the we all needed some Ben & Jerry's so I hopped in the XJ and you guessed it, it wouldn't start. Dual AGM batteries plenty of cranking power. I tried 3-4 times waiting 10 minutes between attempts with no luck. I then checked for spark at both the coil and at a plug and also listened for the fuel pump. Pump sounded normal and I did have spark. It was late, so I gave up for the night and we used another car to get the ice cream... This morning, I went out to start checking in the daylight and I thought I'd try it again. It did the false start thing again and almost caught with a normal very short cranking cycle. I was initially thinking crank position sensor, but after searching, it seems that if I have fuel pump activation and spark that this would rule out the CPS. I will check fuel pressure if I can get my hands on a test gauge.
What are the other things that would cause this condition?
What to test and how?
I would add that I'm a experienced mechanic but this is my first Jeep and I've not worked on them before.

Please help me figure this out! We need to be back on the road on Thursday.

Many Thanks...

Have you tried the NSS test? Start it in Neutral when it happens? I'm guessing its not the NSS though. The other thing I would check is the CPS.

As you suggest, it could be fuel pressure related. Hard to test without a gauge but do you have some vice grips that you can use to pinch off the return line to eliminate the fuel regulator as the cause?

I had a hard start problem with a '90 XJ and found that it took a long time to build fuel pressure. The issue turned out to be that the small hose in the tank between the fuel pump and the output port had come loose so the pump was working overtime trying to push gas into the rail. It would eventually start but since the connection was barely holding on, I couldn't get it above 20lbs. I hesitated to spend the $$ on a fuel pressure gauge and shotgunned it and started swapping parts (changed the fuel regulator and fuel filter because they were relatively easy). After finally caving in and buying the gauge I found the real source and was able to fix it pretty quickly.

HTH
Todd

You are describing two different symptoms that are unrelated, and have nothing to do with the NSS.

The slow start is your check valve.

The no-start is most likely a failing CPS.

Test your way to a confirmed diagnosis.

Check Valve

When your 1997 to 2001 Cherokee won’t start on the first try, the first thing to suspect is a failed check valve on the fuel pump. The fuel pump check valve is supposed to hold pressure in the fuel line while the engine is off. The check valve is located on the fuel pump and located in the gas tank. A failed check valve allows the fuel to drain back to the tank. This results in the following symptoms:

SYMPTOMS

• Longer than normal cranking times
• Never starts on first try, almost always starts on the second try
• Rough idle for a few moments of idling
• Rough running for first ½ block of driving
• Little or no fuel pressure at the fuel rail test valve similar to this -

1- Key in on position; fuel pump primes for 2 seconds then stops. 0 PSI
2- Cranking over the engine. 2-3 PSI
3- Engine starts and idles for 20 seconds. < 5 PSI
4- Engine idles and fuel pressure creeps up to 45-50 PSI.

POSSIBLE CAUSES
• Failed check valve on the fuel pump in the gas tank.
• Leaky fuel injector(s) may be the true cause of your problem, but more likely they are simply contributing to the check valve issue.

Testing of the fuel injectors should show if any are faulty and are allowing fuel to drain into the cylinder. Either problem can allow heat soak to vaporize the remaining fuel in the fuel rail and you may be dealing with a bit of vapor lock as well, especially in warmer weather.

1. Connect the gauge at the fuel rail and start the engine. The gage should read 49 psi plus or minus 5 psi.
2. Turn the engine off and immediately clamp the fuel line just ahead of the fuel tank. Watch the pressure gage and see how long it takes to loose pressure.

If the pressure remains at 49 psi for an extended period of time then the problem is in the tank - probably the check valve. If the pressure falls below 49 psi fairly rapidly then the problem is probably a leaky injector.

There are a few solutions to the problem –
1. Turn the key to ON for 5 seconds and let the fuel pump prime. Turn the key to OFF, turn the key back to ON for 5 seconds, start the engine.
2. Crank the engine for 3-5 seconds, pause, and crank the engine again. It will almost always start on the second try.
3. Replace the fuel pump assembly in the gas tank. The check valve is not a separate part and cannot be serviced. This is both expensive and time consuming because you have to remove any hitches and skid plates and then remove the gas tank from the vehicle.

You can use solutions 1 or 2 for as long as you want to, they do not harm anything and the fuel pump will continue to function for many more years.

If you do replace the fuel pump assembly, purchase a high quality OEM style unit. Some cheap aftermarket fuel pump assemblies, like Airtex, don’t hold up and fail completely within 12-18 months.



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.

Typical Symptoms –
- Starter cranks and cranks but engine won't start up and run.
- Fuel gauge and voltage gauge 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.

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 or connectors and replace as needed. Do the same for the grounding wires from the starter to engine block, and from the battery and engine to the Jeep's frame/body. You must remove, scrape, and clean until shiny, the cable/wire ends, and whatever they bolt to. Jeeps do not tolerate low voltage, bad connections, or poor grounds and the ECM/ECU may behave oddly until you remedy this.

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. Check the ASD relay fuse.

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. Don’t get tunnel vision and assume the sensor is bad (unless it tests bad with a meter). Damaged wiring or a dirty connector can inhibit the signal from making it to the computer. Inspect/test/clean/repair wiring or connectors as necessary.

If the CPS/CKP is failed sometimes the OBD-II trouble code reader cannot make a connection to the ECU/computer or cannot read Check Engine Light/MIL codes because the CPS/CKP has failed. Disconnect the code reader, disconnect the CPS/CKP wire connector, and reconnect the code reader. If the code reader establishes contact with the ECU and scans, your CPS/CKP is failed and needs to be replaced.

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

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

1) 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 display correctly and/or the No-Bus is gone, replace the CPS.
2) If the CEL does not come on when the key is turned to the RUN position... then the CPS/CKP is bad.
3) 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.

The 2000 and 2001 will have the CPS in the same location on the bell housing, but the wire connector is near the Transfer Case, not as shown in the diagram below. Simply follow the wire from the sensor to the connector.

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 PROCECURE 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.
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Thanks Guys! great info.

I just got back in. After checking a few things out, I decided to give it a crank. After about 10 seconds, it seemed like it would start so I kept cranking and went to WOT it fired up! I let it run for a few minutes and now it's *fixed* Not! I'm stumped! I'm happy it's functioning but in this unknowing condition, I'm not happy about embarking on a multi state trip...

Last night when I checked it, it had spark when cranking. Am I correct in my assumption that this rules out the crank position sensor? Excluding the harness, could the crank sensor itself be an intermittent fault? I am about to drive to a local shop and see if I can get them to lend me a pressure gauge. If they won't do that, maybe they will do a fuel pressure test for me.

Could it be the fuel filter? Some other intermittent short? I don't friggin' know and I'm not happy about it...

maybe TPS?

blondejoncherokee said:

maybe TPS?

Click to expand...

Why? How would a faulty TPS intermittently affect starting?

If it started at WOT, I'd suspect it was being flooded. WOT during start disables the injectors and is the correct way to start a flooded engine. Now, what's causing the flooded condition is the real culprit, and I'm not able to offer any help!

What you describe about hard to start after sitting is a well known problem called heat soak. Not the end of the world of a symptom of bad things to come. Factory issued a TSB on this that you can find on the web.

Read post #4 in this thread.

All the info you need to include or exclude any number of issues is throughly detailed, including appropriate testing procedures.

Buy or rent at the auto parts store, any necessary test gauges and/or Volts/Ohms Multi-Meter.

Tim_MN said:

Read post #4 in this thread.

All the info you need to include or exclude any number of issues is throughly detailed, including appropriate testing procedures.

Buy or rent at the auto parts store, any necessary test gauges and/or Volts/Ohms Multi-Meter.

Click to expand...

Yep. Thanks.
Just got a FP test gauge. It's right at 48lbs. About how long should the system hold pressure after the pump is turned off?

saburai said:

Yep. Thanks.
Just got a FP test gauge. It's right at 48lbs. About how long should the system hold pressure after the pump is turned off?

Click to expand...

Fuel pressure should not fall below 30 PSI for five minutes, according to the FSM.

Thanks!
It took 20 minutes to get to 30psi.
So now am I safe to assume that I can rule out the fuel pump?

I think that is a safe assumption. I would leave the pressure gauge attached as you continue your troubleshooting.

saburai said:

A hard or "false" start. This would happen hot or cold. I turn the key, it seems to have started, but upon releasing the key it dies. Evey one of the 3-4 times this has happened, it started on the second try. I should add that it's starting is normally very quick and crisp, starting with very little effort.

Click to expand...

This symptom sounds like an open resistor to the fuel pump. Battery voltage is applied directly to the pump during start. When you release the key to the run position, the power goes through a resistor to decrease the noise from the pump. You may have an intermittant conneciton in this resistor circuit. Keep an eye on that gauge as you troubleshoot.
It is common for the CKPS to fail intermittantly. It may be a hot or cold thermal, or just not work because of evil spirits. When one is failing, sometimes the signal level from the sensor will be low. Please find a voltmeter and go back to post #4 and continue after the fuel pump checks at the section labled "CPS". If this is really intermittant, you may be forced to shotgun it and just replace the CKPS.

MoFo said:

IThis symptom sounds like an open resistor to the fuel pump. Battery voltage is applied directly to the pump during start. When you release the key to the run position, the power goes through a resistor to decrease the noise from the pump.

Click to expand...

No Fuel Pump Ballast resistor on 1996 and newer OBD-II Jeeps.

Thanks Guys! As per post #4 I did put a meter across the CPS and it tested fine. However I was lucky enough to have the truck malfunction again! When it did I had the FP gauge hooked up, fuel is fine. I tried going to WOT w/o any effect. Next I unplugged the CPS, cranked it again for a touch and then plugged in the CPS and it kicked right over. I think I'm going out in the AM to buy a sensor...

Ghost said:

Have you tried the NSS test? Start it in Neutral when it happens? I'm guessing its not the NSS though. The other thing I would check is the CPS.

Click to expand...

It's an AX-15, no NSS. No crank at all if the clutch interlock goes.

Update: Replaced CPS this morning and it *seemed* to be the culprit. Drove around all day and it was fine. Got to my Sister's house around 3:30 this afternoon, went to leave 6:45 and it would not start! it has spark and fuel pressure at 48lbs. that takes about 20 minutes to drop to 30lbs. I think I can rule out the pump and injectors. swapped ASD relay too... Borrowed a car and went out for Thai. Got back at 9:00 and gave it a crank, zilch, nada. Did I mention that it has only done this at night? It's likely coincidental, but I thought I'd mention it. What could it be? Are there any other sensors that will cause a no start condition while still giving spark and fuel? Would a scan tool be much help in this situation?

I take it you're not getting any check engine light? I don't know of any fatal condition that would show up on a scan tool that wouldn't set the light.

How about some odd problem like a cracked vacuum hose or a bad connection to the MAP sensor? I'm trying to think of things that could mess you up that don't show in the OBD. Mechanical problems with fuel injectors, such as a clog or one sticking open, won't, but a dribbling injector will usually just cause a rough start and smoke, rather than no start at all. Physical problems with the engine such as a bad manifold gasket leak, or a broken hose, won't. A badly worn out distributor rotor and cap won't unless it misfires unevenly.

You say you have spark, but are you sure it's enough spark? It might pay to look inside the distributor and make sure it isn't something simple like a missing button in the cap. I've had that happen and felt like a dam fool when I finally figured it out!

Hi Matt! Thanks for the quick reply.

You are correct, no check engine light.
I'm reasonably certain that the vacuum harness is in good shape. Do you know of a quick method to do a positive check? It holds fuel pressure very well taking 20 minutes to drop 18# after shut down. Do you still think it could be an injector problem? The thing is the truck runs like a champ! It has relatively fresh primary ignition components and such. I will pop the cap off in the AM and take a look, thanks for the reminder. Would a bad MAP (or connection) keep it from starting? Thanks again for the help!