Stumbling after startup continues...

I have a 98 4.0, few bolt on goodies. For the past month Ive been trying to pin down a problem, and now I better ask for help before I start replacing $80 sensors just for the hell of it in case it might fix the problem.

What happens- I start my jeep regardles of temperature, and it idles perfectly. Within one minute, it starts to stumble, and the idle jumps about 250 rpm up and down. after 30 seconds to a minute of this it suddenly idles perfectly. Like it never happened. It does it any time I let the jeep sit off for more than 15 mins. Ive run it over 3 hours straight after doing this and it ran perfectly. The one thing ive noticed is that when it does this, I can hear a lot of air rushing in through the IAC. Thats the only thing Ive got going right now, but I wanted to ask for help here before I spend $80 to replace it on a slight hunch.

What Ive tried- Cleaned IAC. Replaced TPS. Cleaned Throttle body. Ran through many many bottles of drygas and gumout like products. Getting gas at different gas stations. Different octanes. Cleaned O2 sensor, its less than a year old.

Any thoughts?

No suggestions at all? From what Ive searched, the things ive tried seem the likely culprits, but maybe Im completely misdiagnosing this?

What kind of "bolt on goodies" do you have?

Reason I ask is, a while back I had an Accel coil a while back that caused some similar problems. Of course it could happen with a stock coil too.

Mine would start up fine, and sometimes stumble like you said, or just stall. It would clear up and run fine, or I would restart it , and it would act like it nmever happened.

Something else to check.

Everything bolt on I have has been there over a year, if that matters, airtube, tb spacer. I also pulled the cap and rotor button, all looked fine there, also did new plugs and wires over the summer.

Wow well I guess this isn't a good sign. Might have to get it into the garage this week after all. Goodbye paycheck...

Well the local garage couldnt find anything wrong with it. He did suggest the MAP, however. Ive searched on here and seen that it can be the cause of no starts and stalling, but not mentioned for brief stumbling/surging. But im running out of things to try....

I have the same problem on a 98. I did try a new MAP sensor and no effect. I have 100k on the odometer now so I was considering changing the O2 sensor thinking it was getting slow. Sounds like that will not work. Have you checked the fuel rail pressure during the stumble time?

I doubt it is the IAC because mine does it at idle or while going down the road. Only for 20-30 seconds though and after about a minute of running. I have also noticed I get weird mileage sometimes. I have the overhead console and last night going 70 down the interstate I was getting 21-22 mpg. Today I was getting 18-19 going 65 on the highway. It does weird changes like that all the time that is why I suspected an O2 sensor.

It could be the o2 sensor, mine has some damaged wires that I thought were all right, but they could be the cause. But the things Ive read were moreso leading me to look at fuel delivery/TB sensors etc. Not that makes it right, by any means. And I have noticed that about the mileage, yesterday it took me 1/4 tank from home to work and back, today its almost 1/4 tank and I havn't driven home yet. I was blaming my bad wheel bearing for that, butit makes sense it could be a less noticable trait of this problem.

I had a '99 WJ (with the 4.0) when they first came out and the mileage on it would not jump around that much on the highway. Maybe the XJ is different, but I doubt it.

Ive also noticed that giving it gas when it does this has no effect, it feels like you are fighting the TB with the pedal at times. Mine was doing it worse yesterday in the rain, to me that screams a sensor with a bad connection or cracked housing, just gotta find it...

An update in case anyone else runs across this problem. I noticed that after long trips the XJ would run fine for a few days. That made me really suspect the O2 sensor. So I changed it and the stumbling is gone. Will it stay gone? We'll see. If not I figure something is fouling the sensor, and I will have to figure it out. For now the problem is solved.

BTW the O2 sensor is a pain to change on a 1998. The sensor itself is relatively easy to remove. The plug however is sandwiched between the splash guard and the intake manifold down by the oil pan/block interface. I had to jack the XJ up to get the front axle to drop enough that I could reach the plug from the bottom. I could not do anything from the top.