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1998 Jeep Grand Cherokee 4.0. Wont stay running unless I

Customer Question
hold the throttle but as...
1998 Jeep Grand Cherokee 4.0. Wont stay running unless I hold the throttle but as soon as I let off it stumbles and dies. Smells like fuel and when it dies it back fires through the intake.
JA: Has the vehicle turned into a gas hog? And how does it start -- same as usual?
Customer: Starts and dies. It wont idle at all
JA: Are you fixing your Grand Cherokee yourself? What have you tried so far?
Customer: Yes I am. Replaced cam and crank sensor. TPS, IAC, ignition coil, plugs, wires, cap, rotor. All new ground cables and cleaned all positive cables.
JA: Anything else you want the mechanic to know before I connect you?
Customer: No
Submitted: 1 year ago.Category: Jeep
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Answered in 1 minute by:
9/4/2016
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago
Ron
Ron, ASE Certified Technician
Category: Jeep
Satisfied Customers: 36,855
Experience: 34 year working in the automotive field as a Tech.
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Are you able to scan the system and if so are there any fault codes setting?

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Customer reply replied 1 year ago
No fault codes. Hooked up with personal scan tool. 2015 Snap-on Ethos. Had P0320 at first which is why I replaced the crank sensor but that didn't fix the problem. But now no fault codes.
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

The backfiring as well as the stumbling still indicates you have a valve timing concern. Are you able to monitor the CKP signal while the problem is occurring?

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Customer reply replied 1 year ago
I can monitor the CKP signal when the problem occurs
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

Can you do that and see if the signal is consistent as well as looking at the HO2S sensors signal to make sure there switching normal?

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Customer reply replied 1 year ago
Signal stays consistent and 02 sensor 1 stays around 1 volt and 02 sensor 2 stays around 2 volts but go up and down as the vehicle runs.
Customer reply replied 1 year ago
Ok one question as far as the crank signal goes. it goes up roughly 6 to the cams 1. is that normal for this vehicle?
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

The HO2S sensors working properly should not exceed 1.0 volts. The Powertrain Control Module (PCM) on this engine allows the O2 sensor signal to determine 33% of the fuel control. If the O2 sensor voltage remains too high as the PCM moves to closed loop, the controller will take away fuel. This can be seen on a scan tool, by watching the long term adaptive memory parameter.

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Customer reply replied 1 year ago
Ok so what would cause H02 sensor 2 to have such high voltage? Possible short internal to sensor causing less resistance and therefore spiking voltage?
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

The common cause is a failing HO2S sensor, if your sure the HO2S is okay then the other cause would be the PCM (computer).

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Customer reply replied 1 year ago
Ok so I went and re-ran the vehicle until the fuel system went into closed loop and saw H02 sensor 1 voltage going from .04V (almost nothing) to .80V and H02 sensor 2 voltage going from .80V to .84V. and I have 48 PSI fuel at rail according to fuel pressure gauge.
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

Are you referring to HO2S 2 as the downstream sensor?

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Customer reply replied 1 year ago
sorry.
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

Okay if that is the case your readings look fine. I suspect your dealing with a bad PCM if you are not setting any fault codes but with out it setting any fault codes it would be hard to be 100% sure.

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Customer reply replied 1 year ago
Yeah I don't want to condemn a PCM if I'm not 100% sure. Every now and again I get the P0320 code back but that's after several starts and dies. It will run if I have my foot on the throttle but dies shortly after releasing. Stumbles and runs rough like it has a miss between 1 and 2 thousand rpms. If I give it full throttle it sounds like it isn't getting enough fuel. and if I have a jump pack on it it runs a lot better but still dies in the lower rpms. A new PCM is over 300 dollars and I've spent a lot in parts as it is lately. I just want to be darn near positive that's what it is before I buy one.
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

The code P0320 is in most cases caused by a failing CKP but since you have replaced it I would suspect you may have a circuit concern from the CKP to the PCM, that could also cause this type of backfire. How dose the CKP signal look when the problem is occurring?

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Customer reply replied 1 year ago
CKP signal is consistent even when vehicle is dying. I've resistance checked the wires from the pcm to the ckp and everything is .1 ohms or less in the wiring.
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

Let run the test in the link below and see if the results check okay.

https://dl.dropboxusercontent.com/u/104923897/CKP%20testing%20Sep%204.pdf

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Customer reply replied 1 year ago
Is there anyway you can send me a wiring diagram? I'm having a hard time tracing the wires through all the loom.
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

Which circuit are you needing?

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Customer reply replied 1 year ago
Preferably the entire engine harness but the CKP harness and everything it goes to/through.
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago
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Customer reply replied 1 year ago
Did the CKP test you gave me and everything came back ok. Had 5 volts on power supply and signal. Ground wire had 14mV with key on. When watching data on the scan tool the CKP sensor was erratic so I swapped out sensor with an oem one and still had the same thing. As this is all going on my scan tool is losing communication to the ecm on and off when the engine is running but when the engine is off I don't lose communication. Also noticed that on one of the pids for auto shutoff it is reading on. Should it be reading that or reading on? Resistance checked CKP, CMP, injector, and ignition coil harness wires and all were .1 ohms or less. Square wave coming from CKP sensor is all good when checked at the connector. Is there anything else I should be looking for before I condemn the ECM?
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

If the wiring and the signal at the CKP check okay as stated I would suspect the ECM may be at fault.

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Customer reply replied 1 year ago
Replaced with new ECM from jeep. Cost 1000 bucks and it did not solve the problem. Returned and got a 2nd one and still starts and dies So now I'm 1500 in parts and jeep still isn't fixed. What else could it be?
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

Did you ever run the test steps I sent you .

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Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

This sure sounds like a wiring issue to me.

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Customer reply replied 1 year ago
I did the CKP tests multiple times and even ran an overlay harness on the off chance the wiring was bad. replaced pins at both the ckp sensor and into the ecm connector. checked with drag test with test pins and everything had proper drag. I did some research and saw that the ECM grounds through the rear 02 sensor. Still no check engine light at all. Also I replaced the ignition coil and saw that it should have 2 grounds. But only 1 ground wire. Does it ground through to the bracket it is bolted to as well?
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

Im showing ground at pin A 31 and A 32 to ground at ground 117 at the right rear of the engine . The HO2S sensor also use this same ground

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Customer reply replied 1 year ago
I replaced the ends for all 3 wires going there and ran a cable from battery negative to that stud.
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

Those are the 3 grounds Im seeing, two for the PCM and on for the HO2S.

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Customer reply replied 1 year ago
Well they have a solid ground. No wiring issue for those. Would injectors stuck open cause this?
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

If they where sticking open you would notice an over fueling condition and lots of black smoke.

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Customer reply replied 1 year ago
I see. Well I'm must be losing power somewhere that I need power at. I have everything I need for combustion. Any suggestion at all on what to do?
Customer reply replied 1 year ago
Could the engine be out of time perhaps?
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

If it was out of time in most cases it would effect the compression. It would not hurt to at least check the compression to be sure its okay. Have you ever verified you have good fuel pressure as well?

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Customer reply replied 1 year ago
I have 50 psi at start up and when I hold throttle to keep it running I have about 50 as well.
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

Your fuel pressure looks good. There has to be something going on here with the computer control system. Finding it is going to be the challenge.

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Customer reply replied 1 year ago
Would a failed ignition switch or neutral safety switch cause this kind of problem?
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

In most cases if the ignition switch or the neutral switch was failing the engine would not crank . I bad ignition switch could cause issues if in fact you loosing voltage to the ignition.

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Customer reply replied 1 year ago
ok I will hook up meter and back probes and see what I'm getting for voltage out of ignition and see if its dropping out at all.
Customer reply replied 1 year ago
You don't happen to have a schematic for that do you?
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago

Which circuit are you needing?

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Customer reply replied 1 year ago
Anything for the ignition switch to firewall/bulkhead.
Jeep Mechanic: Ron, ASE Certified Technician replied 1 year ago
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