I included as much detailed information as possible, sorry this is so long. I just wanted to make sure I documented everything I’ve done.
　
1998 Jeep Cherokee Sport 4x4 (NOT Grand Cherokee)
4.0l 6-cyl automatic

Problem: Was working fine, parked it overnight at a friend's house without an issue. Next morning started it up fine, went to pull out on to the road and it stuttered quite hard. Soon as I accelerated the jerking/stuttering went away. It threw a code which I had checked. OBDII said the code was for "Random misfires." Had to drive to work, on the way I had trouble each time I would approach an incline. I took the off-ramp to my work and it died at the light. Put it in neutral, turned it over for ~5 seconds - it cranked great, but didn't even try to start. Immediately turned it over again and it started right up, threw it in Drive and took off. While coasting down hill with foot barely pressing accelerator, it jumped a bit. Parked at my work and:

Changed: Cap, rotor, wires, plugs that I had bought when I had the code tested. Rotor looked in near-new condition. Cap wasn't terrible - was time to change it though. Wires felt fine - no cracks or loose connections. Plugs were nice and dry, no signs of oil, melting, burning - all were nearly the same appearance, a fine dusting of lighter-colored deposits.

Drove home: The drive was perfect - felt back to normal, no hiccups, no stuttering, jerking, or dying. Parked it fine.

20 minutes later: Had to go out, thought the issue was clear. Started up right off the bat, no problems, started coasting down a long hill and it died again (I noticed it died when the braking felt noticeably harder to stop the vehicle.) Same issue again - turned the engine over ~5 seconds, no start. Turned it over again, started up and took off. I decided then to just turn around and head home and not take the risk until I knew the problem was fixed. Still jerking a little, noticeable misfire on way back home.


Things I've noticed/checked for:

1)Idles fine around 700 rpm like it always has
2)I don't feel a loss of power
3)When it is on cruise control it keeps it just about 2000 rpm - as SOON as I begin an incline it starts misfiring, not violently, but quite roughly.
4)Soon as it starts misfiring, if I accelerate the problems go away immediately
5)In weeks past I recall possibly smelling sulfur while going uphill (aka bad cat), but was unable to determine if it was my vehicle or others in front of me since it doesn't happen every time I am going uphill.
6)Never have a problem starting it unless it just died
7)While 2200-2500 rpm, if I quickly release the accelerator the rpm gauge drops very fast down to normal idle rpm - I only say this because this was a problem I had about 4 years ago when my throttle positioning sensor went bad. Typically when I release the accelerator like that, the rpm gauge doesn't drop as quickly as it does now.
8)Before this issue I remember having a slightly rough idle, but nothing much more than to be expected from a Cherokee.
9)It's difficult to say if this an issue only when it is cold or warm or both. Like I said before, It started the issue in the morning right as I was pulling out of my friends house and I didn't let it sit there and warm up.


I've got only a basic set of tools, and some pretty good know-how, but I'm stumped. I really don't have the luxury of owning the tools or having the funds to test my compression, fuel pressure, injectors, or the likes. My thought right now is either between O2 sensors and a bad cat, or fuel pressure regulator and clogged fuel filter. I'm pretty sure that’s what most of us have jumped to already anyways, though I'm hoping that somebody out there has had these problems and can really narrow it down for me.

I suppose I should note also, that during the drive to my friend's house, I engaged the 4x4 to make it up a slick hill. 4x4 went in fine, had no problems, no sounds, clunks, or loss of power. Took it out of 4x4 without issue and continued on my way. I only say this because I don't use 4x4 a lot (last time I used it was ~3 months ago for snow) and it might just have something to do with this problem - possibly a clogged vacuum line somewhere?
Either way I want to give everyone as much possible information as I can.
If there are any more questions or quick things to test to give you a better idea of the issue just let me know, I will update as soon as possible. Thank you!

 

could be your cat. saw similar symptoms on a intrepid

 

Next time it runs bad like that, try whacking the bottom of the cat and muffler with a 2x4. If it clears up, something came loose inside.

or take it to an INDEPENDENT muffler shop and have it checked. Chain stores will always say it's bad.

 

Smack the cat with your fist. If you hear a rattle inside...it's shot. The catalyst breaks loose and clogs the exhaust, only to clear up again at random times.

In addition to what you've already planned to check- check the TPS function anyway...the aftermarket ones are spotty quality, and check your IAC. A sticky IAC will cause your engine to choke when you take your foot off the gas.

 

Testing will cost you less (even if a shop has to do it) than throwing parts on it. Don't go that route.

1. Vacuum testing can usually snake out a clogged catalytic converter pretty quickly.

2. You definitely need your fuel pressure verified. For a 98, you should have 49 psi, plus or minus 5 psi.

3. If 1-2 above check out, I'd next test the ignition coil and throttle position sensor.
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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.

 

If you have a vacuum gauge here's a neat site that shows several diagnostic scenarios. #14 if for a clogged exhaust.

http://www.secondchancegarage.com/public/186.cfm

 

Ground points
O2's- both of em
TPS
IAC valve
MAP sensor
ECM
In that order... But save a headache and get a good diagnostics computer on it and check over specific functions of the afore mentioned sensors...
Also possibly fuel delivery related. Does it backfire at the manifold? Check the pump/filter...