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Ron
Ron, ASE Certified Technician
Category: Jeep
Satisfied Customers: 33460
Experience:  34 year working in the automotive field as a Tech.
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1998 Jeep Grand Cherokee 4.0. Wont stay running unless I

Customer Question

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: 3 months ago.
Category: Jeep
Expert:  Ron replied 3 months ago.

Hello, my name is ***** ***** I am a professional here at Justanswer. Thank you for your question and I will assist you with your concern.

Are you able to scan the system and if so are there any fault codes setting?

Customer: replied 3 months 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.
Expert:  Ron replied 3 months 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?

Customer: replied 3 months ago.
I can monitor the CKP signal when the problem occurs
Expert:  Ron replied 3 months 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?

Customer: replied 3 months 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: replied 3 months 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?
Expert:  Ron replied 3 months 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.

Customer: replied 3 months 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?
Expert:  Ron replied 3 months 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).

Customer: replied 3 months 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.
Expert:  Ron replied 3 months ago.

Are you referring to HO2S 2 as the downstream sensor?

Customer: replied 3 months ago.
sorry.
Expert:  Ron replied 3 months 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.

Customer: replied 3 months 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.
Expert:  Ron replied 3 months 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?

Customer: replied 3 months 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.
Expert:  Ron replied 3 months 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

Customer: replied 3 months ago.
Is there anyway you can send me a wiring diagram? I'm having a hard time tracing the wires through all the loom.
Expert:  Ron replied 3 months ago.

Which circuit are you needing?

Customer: replied 3 months ago.
Preferably the entire engine harness but the CKP harness and everything it goes to/through.
Customer: replied 3 months 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?
Expert:  Ron replied 2 months ago.

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

Customer: replied 2 months 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?
Expert:  Ron replied 2 months ago.

Did you ever run the test steps I sent you .

Expert:  Ron replied 2 months ago.

This sure sounds like a wiring issue to me.

Customer: replied 2 months 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?
Expert:  Ron replied 2 months 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

Customer: replied 2 months ago.
I replaced the ends for all 3 wires going there and ran a cable from battery negative to that stud.
Expert:  Ron replied 2 months ago.

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

Customer: replied 2 months ago.
Well they have a solid ground. No wiring issue for those. Would injectors stuck open cause this?
Expert:  Ron replied 2 months ago.

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

Customer: replied 2 months 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: replied 2 months ago.
Could the engine be out of time perhaps?
Expert:  Ron replied 2 months 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?

Customer: replied 2 months ago.
I have 50 psi at start up and when I hold throttle to keep it running I have about 50 as well.
Expert:  Ron replied 2 months 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.

Customer: replied 1 month ago.
Would a failed ignition switch or neutral safety switch cause this kind of problem?
Expert:  Ron replied 1 month 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.

Customer: replied 1 month 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: replied 1 month ago.
You don't happen to have a schematic for that do you?
Expert:  Ron replied 1 month ago.

Which circuit are you needing?

Customer: replied 1 month ago.
Anything for the ignition switch to firewall/bulkhead.