1997 Sport running rough after warming up

[Gilpine]

1997 XJ 2DR Sport, 4.0, 214,000 miles, AX-15, Dana 30 front, Chrysler 8.25 rear. Hey guys, been having some rough idle and RPM surging problems with the XJ the last few days and hoping someone has some ideas...

The Jeep runs great upon initial startup and idles around 1,000 RPMs, but after warming up for 5ish minutes it starts to run really rough and RPMs start to drop to about 600-700, then spike to about 1,000 and then drop again. This continues as long as the engine is running. If I try driving it once fully warmed up it feels like I'm pulling a boat anchor and I can't go faster than about 25-30 MPH. If I drive it before fully warming up it runs fine and seems normal. I think this has something to do with the differences between running in open loop vs. closed loop?

The Jeep is not throwing a check engine light; spark plugs, wires, distributer, and camshaft position sensor all replaced within last 10,000 miles; the instrument cluster only reads codes 12 and 55 when I do the ignition code trick; I cleaned the IAC and the throttle body, but it didn't help; and my scanner also shows no CEL.... My scanner does show live readings though, and shows the O2 sensors both fluctuating between .1 and 1 volts while it is running well, and then shows O2 sensor 2 flatlining at .1 volts when it starts to idle rough, and shows sensor 1 staying at about .8 volts. I figure it's likely the upstream O2 sensor causing the issue since the downstream only checks to see if the cat is working if I'm correct...

The scanner can not scan codes for the O2 sensors apparently until I have "completed a drive cycle", but that will be hard to do while it's running this rough. I'll let her cool down and drive around the block a few times later and see if it pulls any codes. Trying to not throw parts at this issue that do not fix the problem as I'm a poor college student.

 

[Unclewolverine]

#1 check when having drivability issues on an xj is always the the tps. Check with an analog meter. I've never heard of 02 sensors causing such bad issues. Bad ones will just typically run rich and have poor mileage. Another suspect would be clogged cat though.

 

[75SV1]

I did have an O2 sensor in a Ford Tempo cuase issues. I would be doing say 65 on the highway. Then I'd have to have the petal to the metal to barely do 55. Then it would be OK after a bit. Not fun.
There is the actuator in the throttle body. Or is it the IAC valve. I think I had some issues with one of them. Rough Idle and then varied RPM. Also, I'd look for a vacuum leak.

 

[Tim_MN]

Any engine sensor that is not genuine Jeep is suspect. O2 sensors should be NTK and not Bosch or parts store generic. Also it would logical to suspect chafed or melted wire insulation, and corroded, bent, or pushed back wire pin plugs for any sensor or wire harness.

Most auto parts stores sell cheap crappy Chinese made parts, some that even come with a "Lifetime Warranty". These parts are poorly manufactured and/or made from inferior materials. They are often out of specification, or even failed, right out of the box. The ones that are not faulty many times will have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep engine sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts.

Cheap parts are cheap for a reason.

A good O2 sensor should produce an oscillating waveform at idle that makes voltage transitions from near minimum (0.1 v) to near maximum (0.9v). Making the fuel mixture artificially rich by feeding propane into the intake manifold should cause the sensor to respond almost immediately (within 100 milliseconds) and go to maximum (0.9v) output. Creating a lean mixture by opening a vacuum line should cause the sensor’s output to drop to its minimum (0.1v) value. If the sensor doesn’t flip-flop back and forth quickly enough, it may indicate a need for replacement.

 

[clean96XJ]

Is it running rich?
Symptoms occur slowly or all at once? After new parts? anymore info?

Similar issue... no cel. ran rough. horrible mileage and horrible performance. MAP sensor was dirty. ~$10-15 for MAP sensor spray. (using infamous k&n air filter.)

another idea...
***need someone to confirm
but.. can you disconnect the 02 sensors and see how she runs..
If it runs ok...might be the sensor(s)
(else maybe im wrong. just stays in closed loop and nothing is verified??)

oh yea.. clogged cat. might be something to check. -solution: 1 gallon lacquer thinner per 10 gallons of fuel. at 214k and probably original cat might be worth doing anyway. (funny coincidence. bought some this afternoon.) Per Scotty Kilmer

 

[Unclewolverine]

Or if you live in a non inspection state, try it without the cat.

 

[CJR]

I had a similar problem that started suddenly with my 88XJ years ago and I suspected a cracked intake manifold. After many inspections of the intake manifold my youngest son's "eagle eyes" finally spotted the crack. Apparently, the previous owner had coated the aluminum manifold crack with an aluminum based epoxy and then sanded it to blend perfectly with the aluminum intake manifold. After awhile the numerous heat-cycles(heat-ups/cool-downs), the "aluminum epoxy fix" cracked and the poor idle/performance started. On cool start-ups, it idled perfectly. However after warm-ups, the intake manifold crack opened up and it idles roughly and performed poorly. I replaced the intake manifold and it cured the problem.

Best regards,

CJR

 

[Gilpine]

Update:

Checked the TPS with an analog meter and the readings were good. It's getting about 5v, so it's getting power, and the output voltage when I back-probed it changed smoothly when I opened and closed the throttle. Decided to test the MAP sensor, which is also getting 5v to it, but the output voltage was weird....

When the Jeep is running well while it is cold and idling smoothly, the MAP sensor output read a pretty steady 1.5v, which should be normal. As soon as the Jeep was warmed up and started running like crap, the output voltage started fluctuating between 1v and 3v. I'm thinking this is likely the culprit....

As for symptoms, it does smell like it's running rich, but only after it warms up and starts running like poo. The symptoms are gradual at first as it warms (seems like a slight misfire after a couple of minutes of running), then they escalate quickly as it becomes warmer. The symptoms were not preceded by any new parts (other than a filler neck skid plate for my cut and fold rear). I did have the camshaft position sensor replaced a few months back by a questionable shop because that was where I broke down haha, but it has been running fine since then until about a week ago. Other than that, I can't think of anymore info that would pertain to my issues.

Per clean96XJ's experience, I went and bought some MAP sensor cleaner spray and tried my best to clean out the inside of the thing, but the hole is very small....I managed to make the spray straw more narrow at one end to fit in the hole, but still lose a bunch of cleaner that can't make it in. Anyhow, I cleaned it best I could, reinstalled the sensor, and fired her up. She started running fine, and continued running fine all the way to operating temperature, so I took her for a test drive. She ran great first mile or so, then had one little hiccup where I lost some power for a second, but then it ran great for another mile or so. I get it up to highway speed and can feel a loss of power and throttle surging, so I pull over to check idle and it's idling like poo again....so I limp it home.

Seemed like it improved with the cleaning spray, so I'm going to try cleaning it some more this evening and see if it helps more. If it seems like it does, I suppose it would mean I need a new MAP sensor most likely when I get payed (suckers are $100 at the local shops!). I would get a genuine Jeep one, but can't seem to find them anywhere and don't want to get the crappy $15 ones on Amazon.

I will try removing the O2 sensors and see how she runs once my PB Blaster has worked it's magic as well before I commit to buying a new MAP sensor (They're stuck pretty good currently). I'll check the intake manifold as well just in case it has a crack. I definitely thought of the cat as well because I had that happen to an Isuzu Trooper once with similar symptoms. How would one test the cat? Took it to a mechanic last time that happened. I miss living in a county with no inspections, but sadly I'm in Northern Calirado to finish school and we have emissions testing. Although, I just passed with flying colors and will likely be long gone before my next emissions test is due....

 

[Gilpine]

Another update:

I cleaned the MAP sensor again, but it did not seem to help....So I checked to see if my PB blaster had loosened up my O2 sensors at all, and realized that there was a connection where I could unplug the upstream one without removing the O2 sensor. Unplugged it and now it runs great aside from the CEL being on. Gonna buy a new one on pay day and in the mean time just drive around with the O2 sensor unplugged and the CEL on. Drove great today, did about 20 miles in 98 degree heat with no problems.

Hopefully this helps someone else who has similar issue! Thanks everyone for the advice and tips.