RealtreeAP

I recently bought a 99 Cherokee Classic with the 4.0 motor and 4 speed automatic transmission. For some reason my Cherokee will randomly do two things. At times it will idle erratically, it will act as if its going to stall but never does. the tac will just bounce between 0 and 2000 rpm's after awhile it will straighten out and idle perfect. Second issue my Cherokee will sputter and jolt at lower speeds (10-40 mph) with constant pressure on the gas, if you floor it, it will take off with no problems. It only seems to do it at lower speeds with slight pressure to the gas pedal (has never done it on the interstate or at speeds over 50 mph). At times it will run awesome like nothing is wrong and then it will randomly act up again. There is no check engine light on and I have recently gave it a good tune up (spark plugs and wires, air filter) and I added fuel injector cleaner to a few tanks of gas with no success of fixing the problem. Any ideas what could be causing this?

tjwalker

I'd recommend you proceed in this order.

1. Finish your tuneup with a distributor cap and a rotor. The importance of these things cannot be overstated.

2. Clean your throttle body and idle air control. They get carboned up. More on that below.

3. Test your "throttle position sensor" More on that below too.

4. Test your fuel pressure. For a 99, you should have 49 psi, plus or minus 5 psi. Must be done with a gauge, which can be rented from many parts stores.
-------------------------------------------------------------

The Idle Air Control (IAC) is mounted on the back of the throttle body. (front for 87-90) The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

“Gently” wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Reinstall IAC and check idle quality
----------------------------------------------------------------------
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.

xj4banger

check ur upstream o2 which is the first one after the manifold, unplug it which should throw a code an see if it goes away

waltermitty

You may want to check to see if it's thrown any codes as well. Just for the "what ifs?".

xj4banger

wouldnt be a bad idea

Slimfiredl7

On the back rigth corner of the valve cover there is an engine to chassis ground wire. It goes from the rear firewall to the corner of the back of the engine block. Like I said look to the right rear of the valve cover and you will find it. Its a wide ribbon ground wire. Undo the nut holding it and clean it really good. Clean all the contact surfaces. Good chance this is it. My 2000 started doing the same thing, running idling like it wanted to stall but fine at speed. It also made my oil gauge go nuts. This ground is pretty major and can cause misfires and all sorts of problems. Another thing, when you changes your plugs, did you make sure you changed the boots? If so, did you make sure the springs were snug in place? I just changed mine last night, took it for a test and it was misfiring and running like crap. This morning I found 2 of the springs on the block, they fell out when placing the coil pack. Put them in right and now running like a champ. These springs and the boots HAVE to fit exact and perfect over the plugs or it will misfire and run like crap. Also, The OEM plugs are copper. Dont use the fancy platinum or split fire or any other gimmicky plugs. Simple copper 2$ champions are perfect. OEM gap is .035, this is what I gap at. Hope my novel I wrote ya helps!!!

RealtreeAP

Alright so I replaced the Distributor cap and rotors, the IAC valve, and the TPS sensor, and I also checked for vacuum leaks with no luck of curing the problem. It still continues to idle weird at times and will "lope" or run erratically at speeds under 50 mph...what should i try doing next?

sik_xj

Im having the same problem, in addition my Jeep takes a few cranks to start. I have literally replaced hundreds in parts that I thought was the problem and spent hours trying to figure it out. I however didnt check the ground, but I have a slight exhaust leak and I think that my manifold is cracked, and I have talked to some people and they said that could be part of the issue, so if the ground looks good then im going to have to tear into my manifold.
Im going to keep an eye on this thread.

ZINGGG

How about spark plugs and wires?

ZINGGG

Exhaust leaks at the manifold can definitely cause a sporadic idle. When I replaced my manifold, I immediately saw a lower, MUCH smoother idle

Radi

Check your fuel pressure running, and see if it holds pressure with the key off.
I wonder if you have a leaking or sticky injector.

RealtreeAP

Now that you mention it I actually have an exhaust leak as well! Could this also cause the mysterious jolting at lower speeds as well as the erratic idle?

salad

At least part of it, most certainly. An exhaust leak disturbs the gasses flowing by the O2 sensors which the ECU uses to determine how much fuel to dump in. With changing pressure (temperature, wind) the exhaust mix the computer sees changes and consequentially you can jump around between a lean/rich fuel/air ratio.

RealtreeAP

Ok so most recently I replaced my blown head gasket and ended up welding my manifold which was cracked. Although my jeep runs noticeably smoother now, I still get the mysterious rough idle and jolting at lower speeds. Starting to get very frustrated !!!!!

jatoxico

x2
Had what sounds like similar prob as you. Replaced the upstream even though I did not throw a code, problem solved.