my jeep keeps dying

I have a 1999 4.0. The first time i was driving down the road and it showed all the signs of running out of gas. I got towed home, than it started right back up. 2 days later it did the same thing as i was pulling into my drive way. I first replaced the fuel pump and the sock filters. i was letting it idle and after 5 min. it died. today i replaced the fuel on top of the fuel tank. again i was letting it idle and after 5 min. it died. i checked the fuel pressure at the fuel rail and it is solid. my next step is to replace the fuel injectors. but after $300.00 and not fixing the problem i'm stumped. any ideas.

The first question to consider is why are you replacing parts randomly before you have done any diagnostic testing ? A logical and step by step inspection and trouble shooting process will, in most cases, discover the actual source of your problem.

Do all the logical things that are simply, and inexpensive.
-Pull and Check all the fuses
-Pull and check all the relays.
-Disconnect, clean with contact cleaner spray, and reconnect all the wire connectors you can get at.
-Test the TPS and CPS.
-Load test the Battery
-Disconnect clean and firmly reattach all the wires to the battery, alternator, and battery. Do the same for the Ground wires at the battery, the firewall and the starter.
-Inspect the wire harness for chafed or melted wires that may be causing an intermittent short or open circuit that could stall the engine.

I paid $800 for a 1997 Cherokee that wouldn't start. After two hours of looking around and poking at it, I found a relay that had corroded contacts and the engine fired right up. Want to buy it ? Its only $2500 and it runs great.

If you are just going to throw more parts at it, I would suspect the CPS is the cause of your stalling. If the engine will turn over, but will not start, suspect the CPS.

A SEARCH of the forum will find threads with details on TPS and CPS testing.

Any check engine light? If so, pull codes and post here for comment. It is very unlikely that fuel injectors are the source of your problem. As Tim mentions, you need to adopt a methodical process of elimination strategy for determining root cause. Don't throw parts at it. It is very expensive and rarely effective. I agree that load testing the battery is job #1 here. Yes, you can have a battery that is strong enough to start the vehicle but not strong enough to keep it running. Happens ALL the time. The engine management system does not like it when available battery voltage is low.

If battery checks out and there are no codes, you need to find out what you're missing WHEN it is symptomatic. Fuel or spark.

Pull a spark plug. Keep it attached to the wire, place the plug electrode near a good engine ground and have a buddy crank the engine. You're looking for a strong, blue, snapping spark. Yellow/orange indicates a weak spark which isn't good.

For fuel, try spraying a small amount of starting fluid into the intake. If the engine starts and runs momentarily after that, you have a FUEL DELIVERY problem, not an ignition (spark) problem. BTW, the only way to accurately test fuel pressure at the fuel rail is with a fuel pressure gauge. Pressing in on the schrader valve tells you very little.

Once you find out what you're missing, the suspect list gets a whole lot shorter. Let us know what you find and we can go from there. Test your way to a solution here....

first thing you should try is pulling the fuel pump relay and swapping it with another one. It could be something simple like that.

Also, if you happen to have another computer you could try plugging it in. It is worth a shot doing that if you have easy access to one.

Thanks. A few things I have since learned. It is not the fuel. i have a fuel pressure gauge that i did hook up to the fuel rail. the reason i replaced the fuel pump and filter is my fuel pressure was not solid. the gauge was bouncing around between 45 and 50. It should be at 49.2 which it now is when i have it running. when i dropped the tank i looked inside and the fuel was clean but the pump and filter were both filled with mud. I do have a computer that i have been hooking up to it, It is a different one than my OBDII computer that i keep under the seat. The first thing i did when my jeep died the first time was to see if it was throwing any codes and no, Not a one. I can start the motor and it will run for a while before it dies. After getting a baseline with the computer I realized that for some reason when the motor heats up the engine stops sparking. I am going to test my battery. That seems like the logic thing to do. Thanks again and i will let you know how it goes. And BTW the next time I have to drop my tank I'm making an access door in the trunk to access the filter and pump. removing the skid plate and disconnecting the exhaust before i can get to the tank is getting old.

mikeam7750 said:

....for some reason when the motor heats up the engine stops sparking. ....

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Crank Position Sensors can have intermittent “thermal failure”. The CPS fails when engine gets hot, but works again when it cools back down

CPS


The most likely cause of it cranks and cranks but won't start up is the Crank Position Sensor (CPS). CPS failure is very common. The CPS can stop working with no warning or symptoms and the engine will not run or the engine may randomly stall for no apparent reason. A failed CPS may or may not throw a CEL trouble code.

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.
-No spark at the spark plugs.
-Fuel pressure is OK at the fuel rail.

If the CPS 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 has failed.

Crank sensors can have intermittent “thermal failure”. This means that the sensor fails when engine gets hot, but works again when it cools back down.

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 the culprit 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?

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





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.

I have checked a lot of things the past few days. What I have found, my water pump is fine but the bottom of the gasket was damaged and it was sucking some air, the connection for my fan was broken so it was not kicking on, my coolant system was full of muck. These things played part in my coolant burning up and causing my thermostat to also fail. When i had my jeep hooked up to the computer it was reading 320 deg. but my gauge was reading 210 deg. I have fixed all those things and today I got it all put back together with a nice flushed coolant system. i even put a new radiator cap on. I started the engine and it died about a min into it. I turned off the ac and it idled for 30 min. with the temp reading just fine. i turned the ac on and it worked just fine. I took it on a test drive with my father-in-law right behind me. I did hot starts and cold starts. It all worked. I went out to get some dinner an hour later and again it died 5 miles down the road. I sat there for 2 minuets and it started right up and I came back home. I don't understand i took it on a longer test drive and it worked just fine. the ac was on for my test drive and it was on when it died. i don't know?

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See the post above on the CPS sensor

lawsoncl said:

See the post above on the CPS sensor

Click to expand...

(Not to pick on lawsoncl, because he is right in concept, but anyway) CPS, CPS, CPS. Sorry, this is wrong according to FSM. Actually CKP, CKP,CKP. According to FSM, CPS is Cam Position Sensor and CKP is Crank Position Sensor. This is probably the only part (CKP) on the XK that should be replaced without thorough diagnostics. "Intermittant not run" is a strong clue.

mikeam7750 said:

I don't understand i took it on a longer test drive and it worked just fine. the ac was on for my test drive and it was on when it died. i don't know?

Click to expand...

A failing crankshaft position sensor may not follow any set pattern for failure.

Replace your crank sensor. With the symptoms you describe, it is by far the most likely suspect.