Odd bogging problem, really need help

Ok I have a 98 XJ 4.0 Auto getting close to 170,000 on the odo. Also a flowmaster and rusty's airtube, if it matters. What is happening is whenever I start the vehicle, hot or cold, raining or dry, it will run fine with the typical higher idle that you get at startup.
However when the idle drops down to the just under 1K rpm like normal, the engine starts stumbling, bad enough that the whole vehicle shakes. Its already rattled my exhaust loose once.
Giving it gas is no help, it will usually cause it to bog, with the occasional quick rev up to 2k then dropping back to 300-800 rpms and jumping around in that range. If it happens when Im driving it will not go over 1k while its doing this and I usually have to pull over to let it stop.
Now after all this happens it will suddenly just stop after a few minutes and run perfectly normal. It does not throw a CEL light when this happens, so I havn't been able to pinpoint a sensor.
I have tried a new downstream O2 sensor, new TPS, new MAP, new CTS, cleaned IAC. The only direction I have now from searching is the upstream O2 or a new IAC, worn timing chain, or a clogged cat perhaps? Its been in and out of the garage several times since this started happening last fall, can't figure it out. I am trying to avoid driving it an hour to a stealership, but if I can't find help on here its gonna have to be my last resort.
Thanks for any help and sorry for the long post. Im almost ready to give up and sell it.

I'm guessing blindly here, but I'd look at temperature sensors, if the problem seems to happen predictably as it warms up and then goes away again. If it happens when it heats up and does not go away again, I'd look for wiring defects, though I'd be surprised if a bad harness didn't set a code.

It happens predictably, warms up and goes away. I have a new coolant temperature sensor. It was throwing a code late last summer for that sensor whenever I started it up and put it in reverse, a new sensor and alternator fixed that one. Are there any other temp sensors that could be the culprit? I was also wondering about O2 heater but they are not seperate from the sensor on an xj correct? I also noticed that it started to do this to me yesterday after it had been running 20 mins after I climbed a steep bank. Fuel pickup line?

98XJSport said:

I have tried a new downstream O2 sensor, new TPS, new MAP, new CTS, cleaned IAC. The only direction I have now from searching is the upstream O2 or a new IAC, worn timing chain, or a clogged cat perhaps?

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The problem is probably something simple. It's just a shame you've spent a lot of good money throwing parts at the problem without posting here first.
You said that when the engine bogs while you're driving, it won't go above 1000rpm. Did I get that right? If that's so, it sounds like you have an intermittent fuel delivery problem and the most likely candidate is a bad fuel pump. Check the fuel pressure at the injector rail when the engine's idling and see if it fluctuates. You might also wanna check the vacuum in the intake manifold while the engine's idling.
These two simple diagnostic tests will yield very useful information so tell us what you find.

Try replacing the fuel filter. Check the fuel pressure at the injectors. You may have a fuel pump going bad or just water in there.

This is actually something like my 4th post on this in the past 6 months, been trying to use the info ive gotten to narrow down the problem.

Ive tried new plugs, wires, cap, button. Ive also tried many bottles and brands of dry gas, injector cleaner, etc. There is no fuel filter on a 98 if im thinking correctly? I checked today for a fuel pressure guage, the parts store didn't have anything other than an expensive kit, which they were out of stock of. However if my fuel pump was bad wouldn't it act up pretty much all the time?

And to try and add more info whenever im driving and this happens I feel kinda like a clack when im on the throttle, like the butterfly snapping shut harshly. At all times when this happens I also hear the IAC letting in a large amount of air, making me think running rich? And now that the rain has stopped and ive driven it a couple times I noticed a bunch of drops have covered the tailgate, and its not water. I plan on washing it to find out if its something new or leftovers from a bad tcase output seal. To thick for antifreeze to thin for oil I think.

To top things off I blew a tire on my DD today, looks like im gonna hafta either fix the xj or drive er till she dies lol.

Thanks for the help.

Ive also searched for a vaccum leak but haven't been able to find one as of yet. But again if it was a leak, should do it all the time?

98XJSport said:

But again if it was a leak, should do it all the time?

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Not necessarily.... although as cars get newer, vacuum problems are less likely. Vacuum changes with temperature.

I'd agree that a fuel filter issue seems pretty unlikely... and yes, the 98 has a fuel filter, but it's built into the fuel pump and is in the gas tank.

This is going to sound idiotic, but I'm guessing it's a "computer issue". I'm not sure what the equivelant is on a newer rig, but on a carbuerated rig I'd be looking at the butterfly valve opening too soon. That would cause it to stumble, run rough, and go away once it warmed the rest of the way up. I suppose the computer could be opening it too soon. I'm not sure if the coolant temp sensor feeds the computer the info for when to adjust the air/fuel mixture or what though.

Hope my random thoughts have given you some ideas....

98XJSport said:

Ive also searched for a vaccum leak but haven't been able to find one as of yet. But again if it was a leak, should do it all the time?

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A vacuum leak usually causes a high idle but the engine generally runs well unless the leak is very big so in your case, it's highly unlikely that's the problem. You might wanna check the actual amount of idle vacuum anyway 'cause a low vacuum caused by things other than leaks (e.g. plugged cat) can also cause driveability issues and poor gas mileage.

98XJSport said:

I checked today for a fuel pressure guage, the parts store didn't have anything other than an expensive kit, which they were out of stock of. However if my fuel pump was bad wouldn't it act up pretty much all the time?

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Try another parts store. You should be able to get one for under 20 buckaroos.
As far as fuel pumps go, they CAN misbehave intermittently especially in the early stages of their impending retirement so don't rule it out yet. At least if the fuel pressure's at a normal stable value when the engine bogs down, you'll have ruled out the fuel pump as the culprit and you won't have to add that to the list of parts that you've replaced unnecessarily.

i was having a hell of a time figuring mine out....doing similar things......ended up being the rubber hose that is clamped on the sending unit inside the tank fell off.......it was only running 13 psi of fuel pressure at idle.....put the hose back on TIGHT and it runs like a top.........wouldnt hurt to check it, after all its free.....good luk, and dont give up.......

I checked a couple more parts store for a pressure guage, only one I could find was around $40 and said it wouldn't work on anything newer than a 96. Do XJ's newer than 96 have the Schrader valve or will i need a different one than this. $40 seems overpriced, but Im running out of options. And to top it off my tcase output now leaks and is throwing ATF all over the place. And I just had that fixed this spring. Great.

Could ATF flying everywhere from the output shaft have any effect on anything? Other than the obvious mechanical problem.

No suggestions? Cause its all soaked under there around the tranny/t-case...

i had a simular problem w/ my 99' but it was also spitting black smoke from exhaust when under pressure at takeoff. changed all sensors but IAC. i was getting trouble codes Po5o, Po171, P0122 and P0123. check your serv. eng. light, bulb could be out. auto zone does it for free. changed both O2 sensors again and so far so good after 50mile trip.
goodluck

No check engine light, I have a small scanner myself, and I had an o2 sensor go out last fall, so I know the light works. Im now wondering about my upstream sensor as well, I connected for the helluvit and pulled a pending 02 sensor/heater/catalyst code.

More info from wheeling yesterday. I am now getting a Temp Sensor low voltage imput CEL. I was getting this last fall, replaced the sensor, it was no help. Replaced the Alt, havn't had the problem since. Until now. Ive checked the immediate wiring, that looks fine. Any other suggestions/ ways a low voltage would be caused?

Im also noticing that if I mat it, the stumbling goes away. If I recall correctly, most sensors are not read at WOT? This might be an indication that it is a sensor problem perhaps?

I am leaning towards the PCM. I am fighting the same woes as yourself and I just cant seem to fix it still. Here is a post that I have been working on:
http://www.naxja.org/forum/showthread.php?t=87377
Lots of money and it still isnt running right. I hope it isnt your PCM, but if it is then you can send it in to a place in Florida to have it rebuilt and back in your jeep in one week for $200 plus shipping. Good luck.

According to the post you linked you had good luck with a new CTS, I may have to try that, the one on there is only like 6 months old but you never know I may have damaged it when I put on a new belt or something stupid like that. The new belt was the last thing I did before it started going to hell on me.

Did you check the fuel pressure as I suggested? You haven't ruled out a problem with the fuel pump yet.

98XJSport said:

I have tried a new downstream O2 sensor, new TPS, new MAP, new CTS, cleaned IAC. The only direction I have now from searching is the upstream O2 or a new IAC

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Replacing the downstream sensor is pointless as it's only there to see if the cat is working or was removed - it does not affect the engine. The computer only uses the upstream sensor for determining rich/lean. If you're on the original upstream O2 sensor, it's probably time to replace it anyway.

Instead of blindly replacing the CTS sensor, get a cheap meter (shot for something in the $20 range) and measure the resistance when cold and hot to see if your is in spec. Check the voltage on the grounds as well to see if you might have a grounding problem. You do basic checks on the TPS, CPS and MAP as well. It just bugs me to see people throw $200 worth of sensors at a problem without trying to diagnose the problem first.