hey guys
i have a 99 cherokee sport with 180k miles. this problem has happened once before but hasn't appeared for about 9 months. i have search but haven't found my exact problem. please help.
after engine warm up it will not maintain a constant rpm. regardless of gear or engine rpm the engine cuts out briefly then resumes. its like flicking a switch on and off. if stopped and at idle it will eventually die, sometimes. the problem will go away as fast as it appears just to return again 10 miles down the road. very erratic and unpredictable.
tune up prior to first time this occurred. approx 1 year so i don't think its a "basics" problem. had to replace my battery and that's when the problem appeared again. vehicle was without battery for 4 hours so i think its a"learning" problem. but its not learning the correct info.
first time i fixed problem i sprayed and cleaned all connections including ground connections and sprayed the throttle body to remove the build-up. tried this fix again and i'm having no luck this time.
starts and runs great cold. starts ok when problem is present.
here's the kicker - when its doing its on / off routine i can put my foot into it and it runs like a race car until i hit the speed limit (of course). so i don't think its a fuel delivery problem.
i've disconnected the battery and shorted the battery wires to clear and restart the "learning" process again.
i've cleaned the air idle control motor thoroughly .
i've disconnected the water and air temperature sensor to see if any change might occur. none.
i've changed the crankshaft position sensor even though i can't see the reasoning behind it. but it seems to be a very common part to replace with many stated problems associated with it. i'll try anything.
have NOT replaced O2 sensor yet due to PB spray has not loosened sensor and maybe never will. going to be a real project. i can tell. but next on the list unless someone has any suggestions.
talk to me.

 

What do you mean you sprayed and cleaned all connections including ground connections? What connections? What ground connections? How did you clean them? What did you spray them with?

Did you remove the wire ring terminals from all the ground points and clean them? Did you remove and clean the B+ wires ring terminals from the starter, alternator, and PDC Bus and clean them. Battery posts?

It is essential that all grounds are free from corrosion and securely attached.

Never use Dielectric grease, or any grease, on the ring terminals. Never use PB Blaster or WD40 on the ring terminals. If anything, use a product called NOALOX or a product called OxGard.

Here are photos of the ground points in your engine bay.

G100 and G101 are the most important grounds.

 

It's your TPS.
Has to be.
Okay, it doesn't have to be, but I'd bet money on it.

 

Throttle position sensor came to my mind as well. Here is more on this important sensor. I'd verify it if freshening grounds per CCKen's information doesn't help you.
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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 accordingly 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 denied acceptable range causing the ECU to incorrectly fuel the engine. This is noticeable 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.

 

thanks for the input guys.
just got home. will try some suggestions in the morn and post results asap. beddy by time right now.
2WDUnderachiever - other than gut feel what is your reasoning?

 

id throw my vote for the o2.. but do the free maintenance first. it will help overall no matter what.

 

Because the TPS causes issues like that.

 

update

CCKen - when I first read your reply I asked myself when did I ask for a good *** whoopin? then I realized you have no idea who you are talking to and ask some very valid questions. been in design electronics for 30 years and have some concept of continuity vs dielectric strength so my answer is yes I used the appropriate chemicals on my connections. yes I did all of the grounds you pointed out in your pix. my preference is OxGard. I DO appreciate your thoroughness in answering my dilemma.
turns out the removal of the O2 sensor was not a problem. spraying it down with PB Blaster over the period of a week or so did the trick and it came out fairly easily. I did some driving with NO O2 sensor and the vehicle ran great.

two questions for anybody.
the pre-cat sensor leans the fuel mix out otherwise without one the engine will run rich. this is why the few miles I drove it the jeep ran fine.
true? false?
the post-cat O2 does nothing to modify the engine parameters. it just reports on the condition of the cat itself and turns on the check eng light.
true? false?

anyway. looks like the winner is VTJeep. the new O2 appears to have fixed the problem so far.

I will post again should the problem re-occur and we will go from there.

THANX everyone I do appreciate all inputs.

 

those o2's can be pesky lil bastards sometimes.. I'm pretty sure your correct in assessment of o2's. i know that the upstream can cause cause it to run "dead lean" meaning its basically trying to shut off fuel delivery, so thats what I'm basing it on.. as for downstream, again that sounds correct. you can bypass it, but its 1, illegal to do in the states and 2, can cause issues with CEL..

heres one more cheap fix to try- add an extra ground wire to the o2. literally strip a couple inches of wire, back out the o2 a bit, wrap the wire around sensor and attach to chassis. i used one 18g lead and it seemed to help keep my CEL off, but I've been through endless CEL/o2/ECU issues..