Another 01 with a bad idle/stall NO CEL

All of my searching has left me clueless still, here what ive got...

01 XJ with new Jasper 4.0 with roughly 5k miles on it since install 4 years ago. Jeep dosent get driven much but im hoping to change that and I want to get it it good mechanical condition.

As the title says it has a rough idle which feels like a slight miss. Hard to tell but it looks like if held at constant throttle it has the same miss

If I blip the throttle at idle it has a very slight bog then normal rise, when the rpms drop they drop below idle rpm and will sometimes stall. Ive never had it stall under normal driving conditions just in park throttle blips.

Ive cleaned the throttle body, clean IAC, new IAC, and new map sensor

I have a cell phone type scanner, just need to know what im looking for and maybe some ideas

Did you use Mopar sensors? I would also suspect the wiring harness.

not Mopar but not bad quality

I might try the temp sensor at the thermostat housing. I've heard from two different sources that it can cause running issues. On one of my XJs, I did have rough running issues. I did change that, but I think the main cause was the spark plugs had to large of a gap. Also, O2 sensor.

No trying, diagnostics first.

Use genuine Jeep engine sensors, and NTK O2 sensors or you will be chasing ghosts. 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.



The TPS, IAC and their wire harness/wire plug should be primary suspects.



The TPS is sensitive to heat, moisture, and vibration, leading to the failure of some units. The sensor is a sealed unit and cannot be repaired only replaced. A TPS may fail gradually leading to a number of symptoms which can include one or more of the following:

- Poor idle control: The TPS is used by the ECU to determine if the throttle is closed and the car should be using the Idle Air Control Valve exclusively for idle control. A fault TPS sensor can confuse the ECU causing the idle to be erratic or "hunting".

- High Idle Speed: The TPS may report faulty values causing the engine idle speed to be increased above normal. This is normally found in conjunction with a slow engine return to idle speed symptom.

NOTE: The throttle position sensor is also DIRECTLY involved with transmission shifting characteristics. The TPS function should be verified early in the troubleshooting process, when a transmission issue is suspected.

TPS TEST

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 4.8 volts at WOT wide open throttle.

Perform the test procedure again and wiggle and/or tap on the TPS while you watch the meter. If you notice any flat spots or abrupt changes in the meter readings, replace the TPS.

will test the TPS to rule that out, any way to rule out IAC?

Perform a compression test on all cylinders. Write down the results and get back to us. Many parts stores will let you rent or borrow a compression tester.

Non genuine Jeep parts are always suspect. I do not know of an IAC test, but I have had one original IAC, and two junkyard $3 specials fail on me. I have a box full of good used Jeep engine sensors from the junkyard.

So if the XJ runs rough before it reaches operating temp does that rule out o2 sensors?

Also if I take the vacuum line off the PCV (rearward vent on valve cover) the idle will rise then drop back to normal range and it seems to run better with that off? I dont know if that helpful for anything just something I noticed

Check and replace vacuum lines and elbows. It is possible to have a pinhole leak in them that seals up as the rubber softens from heat. Also check and replace the breather/pcv grommets and valves. TB removal and cleaning, etc.

3.5-4mm ID silicone hose cut to fit works for replacing the vac elbows if you cant find them (my local chinese big box parts house didnt have the right size dorman elbows in stock). Silicone hose is cheap by the foot and more resistant to elements.

Dont forget the elbow on the underside of the MAP sensor. That will most likely require an actual elbow instead of vac hose (its tight in there)

Easiest way to check vac lines is with an *unlit* propane torch. While idling, hit them all with propane. If idle goes up, theres your problem. Works for intake/exhaust manifold leak testing too.


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Also, fuel pressure and leak down test...and check ignition components for corrosion, spark plug gap, etc....all the basics. From there id consider looking at sensors, checking resistance and voltage with a meter.

Barring all that, make sure battery is in good shape, ground straps are clean and tight, etc


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Patriot_XJ said:

So if the XJ runs rough before it reaches operating temp does that rule out o2 sensors?

Click to expand...

Only if you test the O2 sensors, check their fuses, or have proof on your OBD-II scan tool with live data.

You mentioned wanting to replace the temp sensor. On a 2001, the computer uses that for the dash display too, so it that's reading correct, it's probably fine.

The CTS is probably the easiest sensor to test on the HOs since it's right up front and easily accessible. Unplug it and measure it's resistance cold and hot and see if they're reasonable according to the table below. They values don't need to be perfect, and a bad sensor will often show shorted or open circuit. Also pay attention to the wiring near it as sometimes it can be pulled tight and damaged.

For HO engines only.
COOLANT TEMP SENSOR & MANIFOLD AIR TEMP SENSOR RESISTANCE TABLE
Temperature *F(*C) Minimum Ohms Maximum Ohms
-40 (-40) .............. 291,490 .......... 381,710
-4 (-20) ............... 85,850 ........... 108,390
14 (-10) ............... 49,250 ............ 61,430
32 (0) ................. 29,330 ............ 35,990
50 (10) ................ 17,990 ............ 21,810
68 (20) ................ 11,370 ............ 13,610
77 (25) ................. 9120 ............. 10,880
86 (30) ................. 7370 ............... 8750
104 (40) ................ 4900 ............... 5750
122 (50) ................ 3330 ............... 3880
140 (60) ................ 2310 ............... 2670
158 (70) ................ 1630 ............... 1870
176 (80) ................ 1170 ............... 1340
194 (90) ................. 860 ................ 970
212 (100) ................ 640 ................ 720
230 (110) ................ 480 ................ 540
248 (120) ................ 370 ................ 410

For older RENIX non-HO engines.
CTS and MAT sensor
*F (*C) Ohms
212 (100) .................................. 185
160 (71) ................................... 450
100 (38) .................................. 1600
70 (21) ................................... 3400
40 (4) .................................... 7500
20 (-7) ................................. 13,500
0 (-18) ................................. 25,000
-40 (-40) .............................. 100,700

just fixed

1996 XJ tough start, rough idle/stalling and horrible mpg all with no CEL

running rich (original symptom - bad gas mileage)

cleaned the MAP sensor.
~$11 only use MAF sensor cleaner (quick evaporation is critical)
also sprayed the IAC (replaced recently -3 years ago)
cleaned throttle body (wiped it down)

now starts/runs flawlessly

cheap and easy to do

(wow looking back, my post wasnt super helpful...)

just realized you replaced the MAP sensor.
unplug it and then try to see if symptoms persist.

And noticed you mentioned the sound of a "miss" was matched with engine speed.
Does it go away when engine is warm? (injector issue?)

Also..
check to see if all the cylinders are firing. water on individual exhaust manifold.

might try an injector cleaner and make sure plug wires are secure

How old is the gasoline?

Any exhaust leaks?

FYI-
If the MAP sensor is dirty (gets stuck in low vaccum mode), engine will run rich.
Cold start, had to open throttle to get her to start and run. But once warm she ran ok but not smooth always a little hesitation and horrible mpg. (86 miles on 3/4 tank)
(now she starts beautifully with only turn of the key, no throttle use at all)

If the jeep has a vaccum leak, it will run lean.

(check MPG, spark plugs and tail pipe emissions to see if its running rich or lean)

When my jeep's IAC failed..
Jeep would not start at all. And check engine light was on.