Sudden sooty plugs

C

Cherokeedad2

NAXJA Forum User
The 98 4.0 suddenly started running rough and wouldn't idle/ it would die
Like, went to the store, ran fine. Came out, issues
As this seemed to be ignition issue, checked the cap and rotor, fairly burnt, so replaced
No change
Pulled the plugs and they're all black, cleaned and reinstalled, it would idle, but still rough.
Tried a test drive, went 1/2 miles and it died.
Limped it back, no idle again
It's leaving a sooty patch on the driveway

I thought that maybe the cold start enrichener is somehow engaged, but this is happening very fast

Any ideas?
No CEL
 
First thing is to check for active or pending OBD-II Trouble Codes.

Basic rich running should make you suspect the O2 sensors, an exhaust manifold leak, or faulty fuel injectors.

Test the Throttle Position Sensor and the Coolant Temperature Sensor.
 
Last edited:
The problem is, it wasn't showing any codes
So, off to YouTube, found a similar condition in a video that was caused by O2 sensors.
So, ordered NTK O2 sensors, installed, fresh plugs, it runs better but idle,/off idle is still very rough, and the CEL finally came on.
Tested, O2 sensor 2 is the problem
I ordered the O2s thru NAPA, so I doubt they're fake
 
There are several things which can cause the computer to thing the downstream O2 sensor is bad when it actually is not. Do a search for PO138 and PO141 and you will find the list.

Tim is correct in what he has pointed out. A substantial leak at the intake or exhaust can cause this problem. Poor fuel pressure could be another potential cause.

That said, I have a bad NTK O2 sensor sitting in my garage which I recently got from Rock Auto. There could be an entire batch of bad ones out there. Quality isn't what it used to be.
 
Test the Throttle Position Sensor and the Coolant Temperature Sensor.

Test the O2 Sensor fuse. Inspect the O2 sensor wiring for chafed or melted wire insulation and check the wire plugs for corrosion and bent or pushed back wire plug pins.
 
Update
I checked the O2 sensors connections and they are ok
I checked the O2 fuse with my VOM. And it's ok.
Left the battery cable off overnight
Started the jeep, even rougher idle/off idle than before
Took it on the test loop up the hill, runs decent from 2,000 rpm and crap below that. It died coming down the hill. Started back up and letting it cool in the drive.
Haven't checked the TPS yet, had trouble with that last year, got 2 defective ones and finally a decent one
 
Monday update
Since it was idling poorly, but felt much better over 2,000 rpm, I pulled the AIC and cleaned that.
Idle issues decreased by 50%, but still remain
Test drive showed improvement in throttle response, but still troubled.
Cleaned the AIC and it's passage again, no real change.
 
Can we surmise that it is not getting enough air?

Sooty plugs would say the mixture is too rich. That could be a result of either too much fuel or not enough air.

Having made a difference with the IAC (by 50% should be meaningful) that suggests there was a useful difference made in the air side of the mixture. I am thinking a clean IAC is going to allow more air to get in at idle. What else could make a difference in this direction? Do you have a clean air filter? (I have seen some seriously neglected air filters.)

This is just train of thought, trying to follow the clues.
 
Tuesday update

I pulled the plugs and cleaned them. Odd results, most were dirty on one side, two were dirty all around and #4 was extra dirty and I had to hand clean it after 10 minutes in the ultrasonic

I tested the CTS, it was out of range, replaced it, no change.

Currently disconnected the battery to reset the computer
 
TPS is Out of Tolerance.

The TPS is mounted on the throttle body. The TPS is a variable resistor that provides the Powertrain Control Module (PCM) with an input signal (voltage) that represents throttle blade position. The sensor is connected to the throttle blade shaft. As the position of the throttle blade changes, the resistance of the TPS changes. Along with inputs from other sensors, the PCM uses the TPS input to determine current engine operating conditions. In response to engine operating conditions, the PCM will adjust fuel injector pulse width and ignition timing.

The PCM supplies approximately 5 volts to the TPS. The TPS output voltage (input signal to the PCM) represents the throttle blade position. The PCM receives an input signal voltage from the TPS. It is best to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. This will vary in an approximate range of from .25 volts at minimum throttle opening (idle), to 4.5 volts at WOT wide open throttle.
 
See post #9

Tested the TPS and it's within range

Leak tested the TB and it's leaking badly around the shaft. Removed, disassembled, ultrasonic cleaned, going to get new O rings and reassemble and reinstall
 
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