'01 XJ rough idle problem...

can't find an existing thread with this problem.

'01 XJ, 4.0, automatic

Its a very intermittent problem. I get a low/rough idle on startup. Most often happens from a cold start, but once or twice from already warm. applying/releasing brakes and shifting between park, neut, and drive has no effect. Turning it off and restarting, sometimes works, sometimes doesn't. the only thing that does work is giving it gas, which at first drops idle to nearly stalling, then with a sudden "pop" the rpm jumps and then when i let off the gas it goes to a normal idle. any ideas?

*Cleaning the throttle body and idle air control is NEVER a bad idea for idle issues, as they get carboned up

*But your symptom could be related to the throttle position sensor, which I would suggest testing. Here is more on it.
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The throttle position sensor is connected to the throttle shaft on the throttle body. It sends throttle valve angle information to the PCM. The PCM uses this information to determine how much fuel the engine needs. The TPS is really just a simple potentiometer with one end connected to 5 volts from the PCM and the other to ground. A third wire is connected to the PCM. As you move the accelerator pedal with your foot, the output of the TPS changes. At a closed throttle position, the output of the TPS is low, about a half a volt. As the throttle valve opens, the output increases so that, at wide open throttle, the output voltage should be above 3.9 volts. Testing can be performed with an electrical meter. Analog meter is best. You are looking for a smooth sweep of voltage throughout the entire throttle band. While slowly opening and closing the throttle, take note to the movement of the voltmeter needle. There should be a direct relationship between the needle motion to the motion of the throttle. If at anytime the needle moves abruptly or inconsistently with the movement of the throttle, the TPS is bad

You should have 5 volts going into the TPS. At idle, TPS output voltage must be greater than 200 millivolts. At wide open throttle (WOT), TPS output voltage must be less than 4.8 volts.. The best is 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. One of the other wires should show .26V (or so). The other wire will be the ground and should show no voltage. Move the throttle and look for smooth meter response up to the 4.49 at WOT.

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.

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: -

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

 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.
 Slow engine return to idle: A failing TPS can report the minimum throttle position values incorrectly which can stop the engine entering idle mode when the throttle is closed. Normally when the throttle is closed the engine fuel injectors will be deactivated until a defined engine RPM speed is reached and the engine brought smoothly to idle speed. When failing a TPS will not report the throttle closed and fueling will continue causing the engine to return to idle very slowly.
 Engine Hesitation on Throttle Application: The TPS is also used by the ECU to determine if the driver has applied the throttle quicker than the Manifold Air Pressure sensor can read. The fueling is adjusted acordingly to cope with the sudden increase in air volume, however a faulty sensor can cause the ECU to ignore this data and the engine will "hesitate" when applying the throttle. In extreme cases with the engine at idle, a sudden application of full throttle can stall the engine.
 Engine Misfire: A fault TPS can report values outside the deined acceptable range causing the ECU to incorrectly fuel the engine. This is noticable as a slight misfire and can trigger the misfire detection software and/or Malfunction Indicator Lamp (MIL) light on the dashboard. Extreme cases can cause excessing misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.

I'm with birchlakeXJ; clean the TB and IAC port, the IAC pintle (gently, it can break) and test your TPS.

alright, I'll try that. unless testing the TPS tells me anything, I'll just have to wait and see if it does it again. (pain in the rear to test a solution to a problem that happens whenever it feels like)

sbrewster91 said:

alright, I'll try that. unless testing the TPS tells me anything, I'll just have to wait and see if it does it again. (pain in the rear to test a solution to a problem that happens whenever it feels like)

Click to expand...

Intermittent problems can be difficult, but you can start with cleaning the IAC and throttle body. Don't need symptoms for that!

i have a problem similar to yours on my 99 4.0

but mine does it mostly on hot soaks it will start and idle fine and i will be waiting to pull out of a driveway and soon as i get out of the drive and on the road it will spit spudder and pop for about 10 seconds ish then come out and drive fine seems to do it under light accel. Stomping gas pedal somtimes helps. i have replaced tps/ cleaned throttle body/ iac no change hardly ever does it on cold start and almost never in summer. no codes or lights and only does it once per ingition cycle
wierd!!

I had the same problem on my 01 classic and found a TSB saying that the injectors get hot and the fuel vaporizes inside the injector and locks it up when the engine is shut off really hot. I got some ceramic exhaust wrap and wrapped the injectors and it took care of the issue. Sorry I can't recall what the TSB number was or which injectors it specified to wrap off the top of my head

found it...

NUMBER: 18-031-03
SUBJECT:
4.0L Rough Engine Idle After Restart Following A Hot Soak
OVERVIEW:
This bulletin involves the installation of a fuel injector insulator sleeve.
MODELS:
2000-2004 (TJ) Wrangler (Domestic and International Markets)
1999-2004 (WJ) Grand Cherokee (Domestic and International Markets)
2001-2004 (WG) Grand Cherokee (International Market)
2000-2001 (XJ) Jeep Cherokee (Domestic and International Markets)
NOTE: This bulletin applies to vehicles equipped with a 4.0L engine (sales code = ERH)
SYMPTOM/CONDITION:
Customers may describe a 20 to 30 second rough idle following the restart of a heat soaked
engine. This condition may be most noticeable when the engine is restarted following a
prior 10 to 20 minute heat soak in hot ambient conditions of approximately 32 C (90ºF)
or higher. This condition may be consistent with short city stop-and-go driving trips and
can be aggravated by the use of fuel with a high ethanol content. Depending upon
various conditions a MIL may occur due to DTC P0303 - Cylinder #3 Misfire.
This condition may be caused by heat from the exhaust manifold that following engine
shut down migrates to the area around injector #3 and causes fuel vapor to form
within the injector. This in turn may cause a momentary misfire of cylinder #3 until
the fuel vapor is cleared of injector #3. The insulator sleeve lowers the injector #3
temperature to a point below which the fuel will not normally vaporize.

now that i think about it, that probably the best bet mrmuelle. problem happened in the summer/fall(which down here is still a hot season), normally on the return drive from work/school. even though it was a "cold" start (engine off for several hours), I'm thinking maybe the outside temp may have been hot enough to keep the fuel suspended after the exhaust vaporized it to begin with. Then "low" overnight temps would let the fuel re-condense before startup in the morning. guess its off to the parts store tomorrow...

You should be on the right track. Although the source of the problem is heat, you can have the cold start issue because the fuel in injector 3 has boiled off when you hot parked it yesterday, now you are cold starting on 5 injectors.

LOTS of manufacturers, when faced with similar problems, added a timer to the auxiliary fan circuit so it would run after then engine was shut down to cool then engine compartment. Chrysler came up with an injector cozy. Don't be surprised if the cozy doesn't work.

Finally got around to doing something about it yesterday. dealership wanted $10 for the injector koozie, adv. auto parts wanted $12 for a 15ft roll of heat tape. Now my whole fuel rail is wrapped. will post results once the weather gets warm enough to tell.

There is also a TSB regarding cold start issues.. requires a trip to the dealer for PCm reprogram.

sbrewster91 said:

Finally got around to doing something about it yesterday. dealership wanted $10 for the injector koozie, adv. auto parts wanted $12 for a 15ft roll of heat tape. Now my whole fuel rail is wrapped. will post results once the weather gets warm enough to tell.

Click to expand...

Yeah, if your issue was/is heat soak I have ZERO confidence you will have defeated it.

The TSB injector "cozy" was nothing more than a bad joke.

I was experiencing a similar symptom OP for almost 2-3 months at very random time. I changed the upstream O2 sensor & the rough idle has gone away.

joe_peters said:

The TSB injector "cozy" was nothing more than a bad joke.

Click to expand...

Amen to that. Ditto the 'debris shield' installed to the top of the manifold. It's good for maybe 3-5 years, will become substantially less-effective the more your valve cover gasket leaks, and never really did much to solve the problem anyway.

The only effective workaround I've found for the heat soak problem is to start the XJ with the A/C off and let it idle out (don't be tempted to work the accelerator to speed up the process) until the roughness stops before driving it. Doesn't seem to matter if it's 20degF or 120degF out; this is the one thing that substantially cuts the likelihood of a misfire and attendant CEL.

omg reading this post just made me realize that my XXXXing fix at aamco was irrelevant, i know they scammed me out of a transmission but this post let me know the actual problem of course i wish i would have found it 2 years and 3500 bucks ago