Verrucktwolf

I just bought a 98 Cherokee last week. Every time I start it up and actually drive it, it will run fine for about a minute, then it will sputters and knock for about a minute with the RPMs rising and falling between 1500-2500 for about a minute, then it will run fine. No codes or anything else. Any ideas what could be causing this? Has 165k on original engine. Thanks all and extremely happy to be in an XJ again!

tjwalker

I would start with a thorough cleaning of the throttle body and idle air control. Removing it and bench cleaning is best, but here is a more abbreviated technique you could try. If this doesn't resolve it, I would next test the throttle position sensor. You may also want to put a fuel pressure gauge on it and watch it closely when it is symptomatic. Your 98 4.0 engine needs to have a steady fuel diet of 49 psi, plus or minus 5 psi.

Also, with a "new to you" XJ, wouldn't be a bad idea to replace the plugs with Champion coppers (gapped to .035) and a quality distributor cap and plug wires. Evaluate each and every cooling system component and when you may need to change ALL fluids. If you have any maintenance records (that you can believe), that is really valuable here.

Good luck and keep us updated! 1998 was a good year for the XJ.

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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

BlueRidgeMark

Well, Not much to go on there. So.... let's look at the basics. I'd say start with something cheap and easy - clean the Idle Air Control (IAC).

Some throttle body cleaner (don't use brake cleaner) and some Q-tips, maybe a pipe cleaner or two, and make that thing shine.

Wouldn't hurt to test your TPS, too, but that's a long shot. Still, testing is easy and free. That is, if you already have a multimeter. If you don't, it's time to get one anyway.

EDIT: Mr. Walker and I were posting at the same time!

Verrucktwolf

Thanks for the responses, I'll clean the IAC when it gets back from the ship and MD's manditory inspection with all vehicle sales, if the shop doesn't find anything else more dire that needs attention first lol.

Waynerd

Is the check engine light on? I also have a 98 with mileage almost the same as yours. It starts fine, sometimes cranks a bit longer. Some days it’ll idle fine and drive fine. Most of the time it’ll idle good for a minute and then starts skipping and maybe the idle will fluctuate. Also it starts sucking a lot of air like the IAC opens fully. Sometimes I can turn it off and restart and it’s good. Other times I need to restart it twice before it’s good. Sometimes also if I try to rev it it’ll hesitate and maybe backfire.

The codes I’m getting are p0135 O2 sensor heater malfunction and p0455 large Evan leak. I don’t believe the veal leak would cause this. I’m leaning toward O2 sensor. Also believe my cat may be on its last leg. Sometimes it gets the rotten egg smell. I had an issue with a bad spark plug a while back and I think some fuel may have entered the exhaust and messed the cat up. Also the exhaust sounds like high pressure air when revving. Also the O2 and catalyst monitors will not ready. At some point I’m going to try a new O2 sensor.

BillyBoy

Search Cruiser54 on Google. Go to his page. Complete his steps for a new to you Cherokee. These jeeps are HEAVILY dependent on grounds and connections. Welcome to the addiction.

Verrucktwolf

Got it back from inspection and they found nothing major thankfully. Just need an exhaust hanger welded by the cat and need to have the drum brake reinstalled correctly (apparently whoever did it last installed them wrong). My neighbor is going to weld the hanger for me and take a look at the brakes hopefully this weekend, so can get it reinspected and then fully registered. I'll pull the IAC tomorrow and clean it as well as check the plugs (new plugs, wired, and call will have to wait till next week.) Last week it threw a code reading upstream O2 sensor, so bought a new sensor and light went away before new sensor could be put on, but that's on the list to replace as well since I have the part.

fb97xj1

If its not NGK, return it. The XJ can be quite picky with certain parts. The o2 sensor is one of those parts.

Verrucktwolf

It's NGK lol.

Waynerd

So the O2 is probably what’s causing it to run that way. Mine would never pass the emissions testing here in CT. First thing they do is plug it in by the data port. If the CEL is on or any of the readiness monitors are not set then it ends there with a fail. And NGK/NTK are the brand of O2 that should be used. It’s Bosch that should be avoided.

Verrucktwolf

In MD they won't even connect to the ODB port if your CEL is on.