98 xj rough idle no dtc stall

Ok so last night my truck stalled turning a corner. After I started it back up it seemed to only be running on a few cylinders. I thought it was the same issue i had with my old jeep with the PCM having a misfire cond. after heat soak. I parked it overnight night and in the morning started it up and it was running like a bag of a**holes. I threw my code reader on and the only code that came up was a speed sensor A P0500. I replaced the plugs wires cap and rotor 2 months ago for the hell of it had no issues then. I read the 02 data and it didnt seem like they were switching. and one thing i noticed was the fuel system would be in open loop at idle and closed loop off/above idle...wont stay in closed loop at running temps. Possible bad 02's causing all this? Any help would be great. Thank you

Check the wiring to the O2 sensor, see if a driveshaft, mice, stealth mode jeep gremlin,... or something damaged the wiring to the upstream O2 sensor. Check for voltage, 12-14 at the O2 sensor heater wire. Bad relay/fuse on that heater wire to the O2 sensor?

Also check the wiring to the VSS sensor!!!! P0500 code.

Sounds like your fighting two issues. With a bad o2 your only discernible issues should be poor fuel mileage...I wouldn't contribute any stalling to it unless it was stuck WAY rick or WAY lean. What were the voltage readings on them? Should switch above and below 450 mv to maintain 14.7:1. Remember on scan tools though that you are only reading what the computer thinks it is looking at. They can fool you sometimes...

Loss of power to the O2 sensor heater at idle can cause it idle rough, send the idle speed on a hunt up and down, and cause it to stall.

Until it goes into closed loop the computer doesn't look at the o2 for fuel control. If it is truly staying in open loop it is only making calculations based on IAT and MAP to determine fuel. He says it runs badly right on startup and his scan tool says the vehicle is not going into closed loop. If the scan tool is correct then the o2 sensor is not being used...

Sounds like coolant sensor. Double check your coolant temp readings with a non contact thermometer. Just spitballing.

joejeep92 said:

Until it goes into closed loop the computer doesn't look at the o2 for fuel control. If it is truly staying in open loop it is only making calculations based on IAT and MAP to determine fuel. He says it runs badly right on startup and his scan tool says the vehicle is not going into closed loop. If the scan tool is correct then the o2 sensor is not being used...

Click to expand...

OP post #1 says:

I read the 02 data and it didnt seem like they were switching. and one thing i noticed was the fuel system would be in open loop at idle and closed loop off/above idle...wont stay in closed loop at running temps.

Click to expand...

To me that is dead on clue that the O2 sensor is getting too cold at idle, and the PCM is flip flopping in and out of closed loop/open mode and making the engine run rough in the process. Thus the O2 sensor heater is either bad, or not getting voltage. Since he also has a VSS code, I suspect wiring harness damage that caused both problems!

four_shot said:

Sounds like coolant sensor. Double check your coolant temp readings with a non contact thermometer. Just spitballing.

Click to expand...

If it was never going closed loop I would agree!!

O2S heaters fuse location in your PDC:

thank you for the suggestions I was able to use the solus at work today to take a look at things just as i thought the o2s are not switching. the other thing is on the scan tool the upstream was reading 0.89v which would indicate a rich condition. downstream was at 1.03v. I was messing around with the injectors to see which cylinder was missing I unplugged cyl:4,5,6 and it made no difference cyl:1,2 made slight changes. Not quite sure if that curveballs the issue or if Im just over thinking it, (they are all hitting bty, used a noid light) Im headed outside now to take a look at the wiring to the o2s and check for voltage on the heater side.

ok so after i looked at it tonight I found that on the upstream o2s im getting 4.5v but at the fuse im only getting 8.8 mV?? I did some research on alldata and found out that the ASD relay supplies the heated oxygen sensor fuse (23) and the fuel injector fuse (20) with power. I tried four of the same relay in that spot and had no change in readings at fuses. I then jumped a wire from battery + to where the relay would have been sending the power to the fuses and my readings came to be source volts. Im beginning to think PCM only because I believe it supplies the ground for the ASD relay.

Are you testing the right wire? One is supplying 5 V from the ECU/PCM to the O2 sensor, the other is 12-14 volts from the battery to the O2 sensor heater!!! The last 2 wires are grounds (and if the heater wire ground is bad that will cause the problem too)

If you connected 12 volt battery power to the 5 V ECU/PCM to O2 sensor supply wire, I hope you did not damage the computer?

00jeep said:

ok so after i looked at it tonight I found that on the upstream o2s im getting 4.5v but at the fuse im only getting 8.8 mV?? I did some research on alldata and found out that the ASD relay supplies the heated oxygen sensor fuse (23) and the fuel injector fuse (20) with power. I tried four of the same relay in that spot and had no change in readings at fuses. I then jumped a wire from battery + to where the relay would have been sending the power to the fuses and my readings came to be source volts. Im beginning to think PCM only because I believe it supplies the ground for the ASD relay.

Click to expand...

update: with the truck running I got 12v at 1 wire 5 at another and ground at 2. with what i did last night with the jumper i had both fuses and the relay disconnnected so there was no way it could back track to the pcm. that said i decided to go thru the test procedure for the P0500 VSS test. I skipped a bunch of steps which I know your not supposed to but one test lead to replacing the PCM. So what I had to do was. KOEO,disconect PCM, measure the resistance between the sensor ground circuit and ground>is the resistance below 5OMHS>yes-repair the sensor ground circuit shorted to ground::No>Replace the Powertrain Control Module. Well when I did this "shortcut" test I had my DVOM one lead on BATTERY ground and the other on the round for the sensor...I had a reading of O.L.. If I'm not mistaken that means I had more than 5ohms. And yes I did double triple quadruble check to make sure I was acctually touching the terminal on the connector and I asked my dad about a dozen times if he was on the ground side of the battery (he was) If the pcm is the case any ideas where to go to have it rebuilt or if a JY has a PCM for my truck. I dont think i mentioned this before about my truck but its a 2 door 5 speed 4.0L 1998. Not many around where I live...hence why I bought it lol