nitrousjunky

Last week my 2001 XJ with 190,000 miles just started idling very low or stalling out after it gets warm. Pulling up to a redlight, it will idle down to 200-250 rpms and/or stall out. It idles fine when first cranked cold, even if I have it warming up for 5 minutes or so in the mornings. I'm getting by currently by just giving it enough gas when stopped to hold the rpms around 1000. Otherwise it drives pretty normal and I don't notice any other issues. Tail pipe shows signs that it might be running a tad rich.

Check engine light hasn't come on yet. I have replaced the crank position sensor a couple times in the last year-year and half. Could it be that going out again or anyone have an idea if it could be another sensor? I don't currently own an OBD scan tool, probably should though. Have been borrowing one from a friend when needed. Did also replace the IAC valve about a year ago too.

I haven't pulled plugs to check them yet either, but I did replace coils, wires, and plugs about 15,000-20,000 miles ago.

Any ideas before I start blindly replacing sensors or just take it to my trusted mechanic (who is always swamped with work)? Or recommendations on which sensors you would start with?

jordan96xj

A mixture problem that comes on after warm-up is most often associated with an O2 sensor that is no longer providing accurate values to the computer. O2 sensors are good for about 110k miles, so factor that into your troubleshooting.

If the low idle can be entirely corrected by depressing the gas pedal a little, that is also a sign of the IAC not properly managing the air flow. Unfortunately, this can also be the result of a CPS that is reporting strange RPM numbers to the computer (would suspect IAC first). One that is working when it is cold, but gets worse as it warms up.

If I was in your position, my first goal would be to get an OBD2 scanner on it so that I could see the RPM values and Long Term Fuel Trim values. If the RPM values appeared reasonable, I would not suspect the CPS involved. I would be looking for higher than normal Long Term Fuel Trim values (numbers greater than about 8% after the vehicle is warmed up), if I observed those higher numbers, O2 sensors would be next on my list to test. I would use the scanner to show me what the voltage on the O2 sensor was doing, I would be looking for a full range of numbers from .1 - .9 over time if one appeared to be stuck at a low number or high number all the time I would replace that O2 sensor.

nitrousjunky

Thanks, that's exactly the kind of info I was looking for. I was suspecting possibly the O2 sensors or the IAC possibly again. I picked up the IAC from the same source as the first CPS I changed out, that cps lasted about a month.

Just purchased an OBD2 scanner and have that on the way. Between this XJ and my 96 4 runner, I'll get good use out of it.

Are parts house (Bosch, etc..) O2 sensors decent for the XJs ? It may be the original O2 sensors in it.

ericjw1991

I used Bosch's o2 sensors, they are OE i think, and replacement Bosch's are fine.

PatHenry

NTK is the recommended brand for o2 sensors. They're comparably priced and available at retail parts stores.

jburg318

Second NTK. They are the OE manufacturer. Can be had through Rock auto for about $35 a piece. Only the upstream O2S controls fuel mixture/ pulse width.

I highly recommend BlueDriver. I have diagnosed a bad upstream O2S, downstream O2S, MAP sensor, IAC, and found a bad coil wire using the STFT and LTFT real-time monitoring and available real-time data.

If the fuel trims are crazy rich (highly negative) it could be causing the stall - a good upstream O2S will cause the engine to go into closed loop in about 30 secs. When the engine first starts, it operates in open loop, default programming and is typically around -7 to -5% on the LTFT and 0% on the STFT.

If the heater is bad on the O2S, it can't properly measure O2 in your exhaust. In this case, your O2S voltage should be low on the sensor, reporting a lot of fuel in the exhaust gas and therefore trying to further make the FT negative, backing off the pulse width of the injectors.

I would also check the reading on your coolant temperature sensor, and intake air temp, as this can screw-up how the vehicle is maintaining the pulse width and how the ECU is entering into closed loop.

tjwalker

Absolutely go with NTK oxygen sensors; they are the original equipment mfg. for the oxygen sensors on your 2001.

I really do like most Bosch products and while Bosch oxygen sensors work fine for the early Renix XJ 87-90 as they were OEM at that time, from 91-01 it's been NTK.

Not that expensive and widely available. Go NTK.

nitrousjunky

Thanks guys!! I had researched a bit and saw a few others saying NTK on the O2s as well.

We'll see what the real time data shows when the OBD2 gets here.

ericjw1991

NTK from factory? Wow! Mine must have been replaced in the past then lol. A lot of things on mine were factory, and incredibly worn out and never replaced. Can't go wrong either way, NTK or Bosch.

nitrousjunky

Alright update time.

Finally got my OBD2 scanner, long story (horrible ebay seller). In the mean time I picked up a Mopar CPS and swapped that out. Definitely wasn't the CPS.

Just sat in the Jeep and let it idle for about 25 minutes while monitoring. Here's what I saw in that period letting it idle-
Long Term Fuel Trim - Bank 1 was 3.1-3.9% and Bank 2 was staying around -1.6%
All 4 O2 sensors were fluctuating between .1-.8
Coolant temp got up to 205 degrees and intake temp got up to 145 degrees.

Idled fine the entire 25 minutes, I shifted the Jeep into Drive and it stalled out.

Only had one pending code, a misfire on cylinder 1.

I may monitor it again on the drive home this evening, but I'm thinking the next step is ordering the Mopar Idle Air Control Motor. Unless someone suggest otherwise.

nitrousjunky

jordan96xj

Those initial idle numbers you posted definitely were reasonable. I can't quite make sense of your follow up numbers. But the B1S1 and B2S1 being at short term 1.6% is reasonable.

I re-read your initial post. So, if I understand correctly, the problem does not present itself until you shift into gear (any gear other than Park or Neutral I would assume). An engine that goes very low rpm and/or stalls when slowing down at stop signs, or that stalls when shifting into a gear other than P or N can also be caused by a torque converter lock-up clutch that is not properly disengaging. So that instead of the engine free-wheeling at idle with your foot on the brake (while in gear), the motor is trying to turn the transmission (which isn't moving because you are stopped or slowing down), so the transmission drags the motor down with it until it stalls.

It can be caused by a variety of things. A failing torque converter lockup clutch solenoid (one of the 3 shift solenoids in your transmission). Or a failing brake pedal switch (are your brake lights working properly?). Or a failing speed sensor on the transmission.

Next time you can scan while driving see what the scanner is reporting for speed and engine rpm. See if anything looks unusual. Also it should be able to present throttle position (as a percentage) take not of that value at idle, and while driving. It should be higher while driving, and then reduce back down to the idle value when your foot is off the pedal. On my 96 the idle throttle position value is about 16.9% though let's not assume that the 2001 would be the same.

nitrousjunky

jordan96xj

1. yes a tc lockup problem would probably be counter-acted by quickly shifted back into P or N.

2. Perhaps, but downstream sensors usually just measure cat efficiency and are not involved in the mixture adjustment process. But different years might handle this differently, so I don't want to assume. About clogged cats...perhaps but they would be clogged from the time the vehicle is started. So not sure that adds up either.

nitrousjunky