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· Registered
7 Posts
Discussion Starter · #1 · (Edited)
Solved: Rough idle, misfiring/buckling >1200 RPM, PCM readings go nuts, NO CEL's

Solved: Need help with tracking down phantom like symptoms that are rendering our '00 TJ 2.5L undriveable.

Turns out I had numerous problems:

- Stuck lifters #3 cyl, convexed face w/ metal edges flared outwards. Replaced
- Bad Camshaft, lifterswore grooved into lobes. Replaced
- Melted wiring harness around valve cover causing numerous sensor signal noises.
Replaced 1/2 of wires between ECU & connectors, all wires insulated w/ heat shrink tubing
- Other parts replaced: Coil, CPS, Evap Purge Valve and Pump, Original CKP reinstalled.

Runs well now. May need new Cat to complete readiness to pass SMOG test.

Only remaining issue: slightly rough idle. At low RPM's (in park or driving), when applying slight throttle, the increases as expected, then drops back down withing a second. TPS monitor shows the slight increase despite RPM drop. No apparent "bald" spots. No CEL's, pending or otherwise. IAC appears normal in response to RPM changes and has no RPM surges or stall conditions. AT seems to lurch at certain RPMs, as if it's changing between gears and reverts back to prior state.

Oddly, when the Evap Valve was set to Open by a local shop, the idle seemed to smooth out. I don't recall exactly, but I believe the RPM drop improved too. Last night, I noticed variations in the vacuum readings. Don't know if this is a fluke/delayed response to my OBD scanner and Torque Pro app.

Not sure how to figure this one without paying to have it scoped. May have to pull TB and clean it thoroughly, not just spray more cleaner in it.

Suggestions would be appreciated!

----------- Original Post ------------

NO CEL's thrown. OBD-II readings indicate different problems on different tests/days. ~98k miles on engine.


- Several ignition components and sensors were replaced including the fuel injectors (Bosh upgrade) and a top end rebuild.
- Numerous other parts and connectors have been checked and/or cleaned.
- Top end rebuilt. Cyl walls still have some crosshatching and no scorch marks or signs of unusual wear.
- CAT seemed OK based upon rear side reading ~100 degrees hotter than front, no rattles, sounds etc.
- Distributor coil resistance in spec, Distributor shaft has minimal play ~1.5-2.5mm.
- Timing chain seems ok - minimal crank rotation at damper bolt. No noises around cover plate.
- Replaced bad 2nd IAT, then IAT spiked indicating wiring problem, went away next day.
- No RPM surging.
- Fuel rail pressure (not running) in spec. No further testing due to rail leaks at the time.
- PCM tested under varying conditions, loads AND when heated with a heat gun. Program updated. NO problems found.
- PCM Black Connector: DID find corrosion on right side roughly, say near pins 1-3, 12-13, 22-24!! Cleaned w/ electrical contact cleaner.
- PCM reset after each new component replaced or as indicated.
- Drained oil looks clean (NO metal observed visually or w/ magnet).
- No loud/odd sounds under engine oil pan area.
- Battery fully charged, Alt charges properly, all ground straps look good.
- No signs of wiring harness or connector problems.
- Evap ECS cleaned, some hoses replaced, hoses secured w/ clamps or twisted wire.
- Generally cold starts fine with very slight misfire until....

Current Observations:

-Upon cold start, the PCM OBD-II readings generally look good at idle. Warm restarts can be a different issue...
Once it warms up a bit (closed loop), increasing the rpm's above 1200-1300 usually causes misfiring, knocking and buckling. On a very few occasions, I'm able to get it up to around 3k rpm but may have to feather the throttle to return to idle w/o stalling.In both situations, upon returning to idle long enough, and throttle feathering if needed, the OBD-II reading often settle down. Other times, I have to restart the engine once or twice.
-Overall, the OBD-II sensors appear to be working properly but show the PCM is trying to compensate for odd inputs.
-The timing usually advances above spec and often the neg ST and/or LT fuel trims and O2 sensors would indicate a high fuel/air mixture. Other times, the high O2 readings indicate the opposite. Given the slow OBD-II sampling rate, I can't extrapolate any deeper.


- A wiring harness short somewhere is at the top of my list. Could be at either end connectors, in between, or somewhere else in the ignition/fuel delivery/sensor circuit. On the other hand, I don't know if it's heat, vibration, rpm or load related.
- The observed and cleaned PCM corrosion (not too bad), still haunts me. I'm leary of trying to open up the black connector given how brittle many plastic parts have become.
- Could a weak PCM harness sensor ground connection (A4) or perhaps any/several of the other close-proximity sensor connector pins?


Has anyone experienced this type of issue or related problem(s) and have any solid advice?
I'd like to avoid ripping open all the harnesses to inspect every inch only to re-wrap everything with a bunch of cloth tape rolls.

I'm not sure how to carefully clean the inside of the black PCM harness connector pin holes near the corrosion area noted above.
So, I was thinking of checking continuity and resistance by inserting test pin probes into several of the PCM connector holes and at the component end connectors. Would that suffice given the possibility of heat, vibration and/or voltage (sensor reading) spikes?


· Retired Staff
2004 Jeep Wrangler
76,755 Posts
Running good when cold but it starts running badly once it starts to warm up immediately makes me think bad upstream O2 sensor. The engine computer ignores the O2 sensors when the engine is cold, then starts using the O2 data... good or bad... once the engine starts warming up. A bad O2 sensor will not always generate a CEL.

· Registered
7 Posts
Discussion Starter · #3 ·
Thanks. I thought about that but problem isn't consistent. I can let it idle up to an hour w/o any O2 upstream unusual readings.
It seems to be reacting rather than causing the observed knocking/misfires, timing advance etc rather than causing air/fuel trims and related cascade.
Since the O2's are new, any suggestions on testing them, like disconnecting or swapping?

· Retired Staff
2004 Jeep Wrangler
76,755 Posts
Since the O2's are new, any suggestions on testing them, like disconnecting or swapping?
If they're new NTKs or NGKs, I'd be happy with that. But if they're Bosch O2 sensors, they're notorious for not working properly in our Jeep engines. And it's only the upstream O2 sensor that could cause this by reporting erroneous air/fuel ratios (incorrect O2 levels), the downstream O2 sensor on the cat is passive and only monitors the catalytic converter for proper operation.

Also try disconnecting (unplugging) the upstream sensor to see if the engine runs better. If it does, that pretty well confirms it is bad.

· Registered
7 Posts
Discussion Starter · #5 · (Edited)
Disconnected front O2 (both are new Denso's). Very difficult to start. after feathering idle for a bit, it could be revved well past 3k rpm w/o the same severity issues and ran quite rich as expected but better overall.

Upon reconnecting the front O2, everything went nuts as usual. Could barely keep it running. Possibly be due in part to clearing PCM memory earlier(?).

Here's the kicker... after reconnecting the front O2, the IAT temp readings jumped ~ 50 degrees above earlier OBD-II readings.
The initial temp readings (O2 disconnected) generally agree with IR gun redings.

That leads me back to wiring/connection concerns. Any thoughts in that regard?

· Registered
7 Posts
Discussion Starter · #7 ·
Thanks. FYI - I made an edit to the previous comment.

I'll look around to see if I still have the original O2 to reinstall for a cross check.
If not or no difference, I'll get a NGK or NTK to eliminate that part from the equation.

In the mean time, I'm still curious about the odd IAT temp (OBS-II) reading jump after reconnecting the O2 and restarting the engine.

Doesn't that seem at all odd in any way or do you just suspect some kind of short or unusual current load is effecting the other sensors like the IAT in particular?

Thanks for now... Time to look for the old and/or new O2 sensor. I'll post a follow-up as soon as possible.


· Premium Member
7,158 Posts
The trouble starting and having to feather the throttle could be indicative of a weak battery which wouldn't help things so you should consider getting the battery properly load tested just to be sure.

Since the O2 sensors seem to affect running so badly I'd replace them with new NTK sensors myself (NGK is the spark plug division).

The change in IAT readings is interesting. The O2 sensors share a common sensor ground connection to the PDC. The voltage from the O2 sensors must be affecting that somehow and upsetting the computer. I think all of the engine sensors are connected to the same sensor ground so it could be throwing out quite a few of the inputs to the PCM.

One thing that you might consider checking is the continuity between the different engine sensors and also the PCM on the brown/yellow wire. If you don't have good continuity between all the sensors and PCM there could be a bad splice in the wiring, or maybe some corrosion at pin 4 of the PCM connector where the brown/yellow wire is at the PCM.

· Registered
7 Posts
Discussion Starter · #9 ·
Thanks Johnny Jeep.

Ok, removed C2 from the PCM. Disconnected positive battery cable and all C2 sensors BUT the downstream O2 and Oil pressure. All the A4 ground to sensors connectors read about 0.6-0.7 ohms as did the sensor signal wires back to the C2 connector. While at it, I checked the other sensor wires against other C2 pins for any shorts or odd readings until my DVM batteries died.

That said I have a few questions:

PCM - C2 A4 sensor ground read 10.65K ohms against the chassis ground. Shouldn't that be isolated (0 ohms)?
Tomorrow, I'll remove the Oil and rear O2 sensor connectors to see if that makes any sensor ground difference.

PSPS - The black lead (not A12) read 0.6 ohms against C2 A31 and A32 (Ground) pins. Is this a ground circuit as I couldn't trace it in my diagrams.

COIL - The OR/BG connector lead traced back to several C2 pins: 6.7 ohms to A31 and A32 (ground), 17.2 ohms to A2 (ign switch) AND 8.5 ohms to A6 (P/N Position switch). I couldn't re-verify these readings since the DVM pooped out. Typical. The only diagram I could find showed it going to the other fuse box. Clearly, this doesn't sound normal and I fear the worst. PLEASE tell me otherwise!

Any advice would be greatly appreciated.

Also, If anyone has download links for any additional wiring diagrams to help with the above AND/OR something showing the various wiring harness segments with names (for example S101, S126) and locations relative to the chassis locations, that would greatly help me with further wire tracing.

Thanks everyone.

· Premium Member
7,158 Posts
PCM C1 connector has the A pin numbers. Corrected that in the quotes below for clarity...

PCM - C1 A4 sensor ground read 10.65K ohms against the chassis ground. Shouldn't that be isolated (0 ohms)?
Tomorrow, I'll remove the Oil and rear O2 sensor connectors to see if that makes any sensor ground difference.

- I don't know if/how the sensor ground is isolated from the chassis ground but it might be expected to reduce electrical noise from affecting sensor readings.

PSPS - The black lead (not A12) read 0.6 ohms against C1 A31 and A32 (Ground) pins. Is this a ground circuit as I couldn't trace it in my diagrams.

- Yes it is a ground, Z denotes battery/chassis ground. The PSPS and PCM both ground at G105 (right rear of engine block).

COIL - The OR/BG connector lead traced back to several C1 pins: 6.7 ohms to A31 and A32 (ground), 17.2 ohms to A2 (ign switch) AND 8.5 ohms to A6 (P/N Position switch). I couldn't re-verify these readings since the DVM pooped out. Typical. The only diagram I could find showed it going to the other fuse box. Clearly, this doesn't sound normal and I fear the worst. PLEASE tell me otherwise!

- I'm not sure why there would be continuity to the ground wires. The coil gets 12V from the ASD relay in the underhood PDC. This should be a dark green/light green wire according to the FSM. It also supplies 12v to the injectors and a feed to the PCM, so the PCM knows the ASD is activated. PNPS works uses a 12V feed to power back-up lamps and an input to the PCM at A6 so I'd say that's all normal. Seeing a few ohms is to be expected due to the length of wiring and switch contacts between the pins.

How plugging/unplugging the bad O2 sensors screws up the temp reading I don't know, but, all the wiring related to it seems to check out from what you've done. If you haven't done so already, get the battery tested just to be sure and swap those O2 sensors. If it all runs fine after that I'd call it good.

· Registered
7 Posts
Discussion Starter · #11 ·
Bump anyone?

- Replaced O2 as suggested. No difference other than now having a spare O2 sensor on hand!
- Engine still runs rough, bogs down and nearly stalls when trying to increase RPM above 1200.
- Just for the heck of it, downstream O2 sensor was removed, signal and ground leads checked out OK as did the other sensor leads to the PCM conectors.
Then restarted. No fuel trims at all and still ran bad.. Downstream O2 read 1V. Is this normal??
- The odd sensor ground reading (10.65K ohms to ground) went away after I removed the first PCM connector. My bad, I thought it was removed.
- I found some FSM pdf download files.

New Observations after replacing the O2 Sensor:

- Hard to start. had to feather throttle to get it to idle in open loop. after a little warming, had a few misfires.
- NO fuel trims at all. Completely different than before changing upstream O2.
- Over a roughly 7 minute period, I noted the following readings:
* RPM idled between 820 and 926, average around 880 RPM
* Timing advance varied between 8 and 17 degrees, averaged high 12 degrees
*Upstream O2 varied between .06 and .48V, Downstream O2 varied between .08 and .3V. Both averaged roughly .1V excluding a few high end spikes.
* MAP varied between 6.1 and 7.1 PSI, average about 6.7
* Coolant rose from about 80 to 135 degrees
* IAT rose from about 75 to 88 degrees
- Then I tried tried increasing RPM to 1200 when it bogged down... ran the engine at idlefor just 1.5 more minutes.... Exhaust smelled rich.
* RPM idled between 765 and 889, averaged higher at first, then lower range.
* Timing advance varied between 11.5 and 25.5 degrees, average around 14 before jumping at the end.
* Upstream O2 jumped to .88V, spiked to a high of.94V, averaged .92V
* Downstream O2 jumped to .9V, spiked to a high of .94V, averaged .92V
* MAP jumped to 7.1 PSI, read between 6.1 and and a spike to 8.8 PSI with no discernible average. Hovered around low 6 and 7 psi briefly
* Coolant rose from about 138 to 154 degrees
* IAT jumped to 136.4 in ~20 seconds then rose to 154 degrees

SO, I really NEED some further suggestions at this point...

- I'm at a loss why there were NO fuel trims registered on the OBD-II which did vary prior to checking out the O2 sensors and replacing the upstream O2
- Still curious about the Coil's 12V feed wire registering resistance at the C2 connector pins (A2, A31 and - A32)
- It seems like there's a weak/bad sensor or possibly a corroded or grounded wire somewhere that goes bad the minute the RPM is increased.


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