I'm currently fighting with my partner's 2000 XJ, and I finally figured I should ask some Experts for guidance, since I really don't know much at all about cars, but after dropping $2k into her within this month alone, I could really use a pointer on where to look for a solution.

Car details: 2000 Cherokee Sport 4.0L. She's completely stock, and we've been the only owners. Sitting at 230k miles. I replaced the alternator myself in 2019. On my mechanic receipts he's got notes as follows: "2013: Engine has oil leak. 2014: Hard start - ?Starter?oil leaks-multiple, f. pinion seal leaking, rear diff leaking. 2016: No AC only .5 lb Freon. 2019: Recommend coolant flush. 2022: Front drive shaft slip joint sloppy, rear spring shackles rusty, exhaust hanger broken @ rear axel. 2024: Other injectors are original and may fail." The XJ is our only vehicle and we usually get to skip emissions testing each year because we drive it so minimally.

Current issue is that she idles really rough at red lights or when you put her in park, RPM needle hovers right around .75 mark but "wobbles" up and down during these rough idles. We've noticed it happens most often when she's been running for a while or on a second run, like when you go out to the store, and get back in the car to head home.

I took it in a month ago for a similar problem, where the RPMs were dropping to zero while driving, and she wouldn't respond to the accelerator at all for a second before it felt like things "kicked back in" and was fine again. Mechanic pulled back error codes of P0303 Cylinder 3 Misfire and P1494 Leak Detection Pump Switch. Had him replace all 6 spark plugs (they were either stock or from 2012 at the latest), the ignition coil, and the crank sensor. This solved the dropped RPMs while driving issue, but exactly a month later I had it back in because this rough idle issue started up. This time he found the #4 cylinder misfiring (also mentions "Test injector-fail"), saw that the new #4 spark plug was dry, and replaced the #4 injector and replaced all injector o-rings. He had warned me that, as the other injectors were stock, they were also at risk of failing. We thought at first that this solved the rough idle issue, but not a week later and the check engine light was back on and the rough idle was back. Called him up and explained, and he brought it over, scanned for codes (told me the other injectors were all registering misfires a little above what was "normal"), and put what is listed on receipt as "Truck Fleet Cleaner" in the fuel. The tank was really low but we drove it home empty that night, went out the next afternoon and put about a half tank in it. Still idles rough. ETA: Should we drive it more/harder for a while, to pass the cleaner through faster? He didn't tell me to "drive it like I stole it" or anything, just "drive it a bit" over the next few days.

At a bit of a loss here because I feel like we're just sinking money into her trying to figure out what the problem even is. I know she's a 24 year old car, and old cars need work the older they get, but we really do love her to death (we're a Jeep family through and through!). Just thought I'd ask for some advice on where to look, if we're even looking in the right area, etc, in case someone has experienced the same issue before. Thanks in advance for any input you guys can give, and sorry for the wall of text!

It's been almost two months so I thought I'd drop back in with an update.

In June our mechanic re-replaced all 6 spark plugs and the fuel injector he had previously put in, warranty-ing them out for being faulty. He really went above and beyond and when we got the Jeep back, I swear she had never run better in at least ten years! It was like a new vehicle. All the shaking was gone, she was super responsive, just the best outcome we could have hoped for.

Well, I'm here to tell you that the check engine light came back on. This time I took it over to Advance Auto Parts to have them scan the codes for me. He said it was our "Cat system" and also a #2 Cylinder Misfire. The misfire I'm not surprised about, since we didn't replace all 6 of them, and #2 was one of the originals. But the "cat system" has me worried, and also curious: would this cause the rough idling to return? From what I've seen online, it seems like a cat issue, especially if it's just one of the O2 sensors, would just throw a code, not cause the car to run rough. Which would lead me to assume that the rough idle is just due to a continuation of cylinder misfires. I know we're gonna have to take her back in though, it's just really frustrating to think we solved it... just to end up back here.

Another thing that the AAP guy said to me: he asked what gas we use, and then said that getting our gas from Wawa stations means we're using "scab gas"? And explained that "Wawa buys the cheapest gas and then throws a bunch of filler in, and that filler is what can clog your system up." Kinda think he was bull****ting his way through that, but what do I know.

I'd start with cleaning the throttle body and Idle Air Control Valve.

What are the actual codes? What do your fuel trims look like? There are a lot of possible causes for misfires.

I've never heard of any issues with WaWa gas. In most areas all of the gasoline comes from one company's refinery and the different gas station brands add their own additive package. There are legal requirements for those additives. WaWa would not be able to sell gasoline laden with "filler" for very long without getting into a lot of trouble.

I'll have my mechanic take a look at these next time it's in.

The AAP guy didn't give me the codes, but I assume it was P0420 for the "cat system" one, and P0302 for the misfire (which doesn't surprise me, seeing as cylinder misfires were the main issue we'd been having for months). Since we already had the coil pack and all 6 spark plugs replaced previously along with one fuel injector, we're going to get the last 5 injectors replaced next time its in so hopefully those misfire codes go away and she can go back to running a little more smoothly.

Those would be the most frequent codes for those conditions. Hopefully running with a misfire for months has not 'poisoned' the catalytic converter. There are a number of things other than the cat itself that can trigger the code:

https://www.carparts.com/blog/p0420-...ld-bank-1-2-2/

https://www.obd-codes.com/p0420

Do you have one of those crazy setups with two pre-cats in the exhaust manifold? I think that was California-only in 2000, all in 2001.

If there is a problem with fuel injectors that should show up in the fuel trims.

For what it's worth, we only drove it a handful of times once the check engine light originally started coming on, since we didn't want to risk getting stranded anywhere, and we don't go out in it that often anyway. So it's not like we were doing a to and from work commute every day for months with it on.

I have to check because I actually don't know which style of cat ours has. I've got it scheduled for the first week of next month to go back in with my mechanic, and most likely have the other 5 injectors replaced, but I know he'll take a look at everything while he's got it on the lift for me.