98 XJ problems, ready to give up (long)

Ok some here may have read the threads Ive posted on my strange bad running problem. Ive piecemealed them together and tried many different things, but still no luck. I need to get this jeep running before hunting season. 98 XJ 4.0 Auto around 170K miles. Rusty's airtube and Flowmaster Catback if it matters. All installed almost 2 years now.

The beginning- Last fall, whenever I restarted my xj while it was still hot and put it into reverse, I would get a CEL, and the temp guage would max out, then drop back to normal. The CEL was Engine Coolant Temperature Circuit Low Voltage.
Tried cleaning the CTS, no luck. Finally went to the dealer and replaced it. Did not fix the problem. Then I read on here about how prone the Alt. is to failure after water crossings. Of course I had just been over the tires in water a couple weeks ago. Replaced Alt. with a new grand cherokee 136A, problem went away. Replaced belt at that time as well. Also did a fall tune up, new plugs, wires, cap and rotor button. All was well.

The middle- A little over a month after the beginning occured, I started having problems with poor idling. It got progressively worse over spring/summer until I am now here at Fall again, if anything further behind than I was before.

What happens-
1. Start XJ. Engine Temp does not matter. Problem happens when engine is warm or when engine is cold.
2. Runs fine for a couple minutes, almost always until it goes into closed loop mode.
3. Engine starts skipping, bogging, revving, shaking, making a racket in general. Can hear IAC trying to let in a lot of air. This happens no matter what I am doing, Idling, driving down the road at 40, it's not picky. HOWEVER if I hold it to WOT, problem goes away until I let off gas.
4. Problem just stops, usually with a little bit of a rev up, and jeep runs perfectly until I shut it off and let it sit off for more than 10-20 mins. Never actually timed it.

What I have tried-
New CTS, New TPS, New MAP, New Plugs, New rear O2 sensor. Cleaned IAC housing. Tried running it with IAC unplugged. Tried running with Upstream O2 sensor unplugged. Various types of fuel/system cleaners. Cleaned all the grounds I could find. Almost seemed to help, but may be wishful thinking. Tried unplugging Injectors, all appear to be working. Tested battery and Alt., machines all say OK.

Things suggested but didn't try-
Fuel pressure at the rail, but I would think i would be having problems across the board if there was a pump/line/pickup problem. Also havn't tried crank sensor, didn't think that could be involved but was recently suggested, could that cause this?

The worse news-
Today after wrenching on it trying to find anything wrong I go to put it in reverse with a warm engine....and the CEL came on with the same code that I thought I had fixed with the new Alt. Bloody Hell.

I've given up, my family's given up, and even my mechanic has given up. My options left are dealership, which I really really really don't want to do. Lack of trust. Or help from all the great people on here. I know it's rare to come on here and have an XJ problem that no one can help with, and I really hope that my jeep isn't beyond help and destined to be scrapped.

Any and all help will be greatly be appreciated, thanks and sorry for such a long post. And if theres anyone semi local that can help I am more than willing to pay in pizza/beer or whatever vice the help may indulge in.

couple things come to mind. i'd check the fuel pressure both while it's missing/stumbling, and after you can get it to clear up a bit. change the fuel filter, if you haven't done so. see if you can find a "known good" ecu to swap in as a test.

Fuel Pressure Regulator??? I had one test out great but leaked. Caused all kinda problems.

yep, that was also my thinking about the fuel filter/pressure regulator...

this is gonna sound really dumb but i will say it anyway, I had similar issues and threw the same parts and money at it and got no where, not until i finally got so pissed i was ready to blow the thing up. upon really close inspection of the loom that connects to the comp and goes thru the firewall right behind #6 injector i found a bracket had rubbed thru the loom and insulation and was causing my issues, i moved the bracket and retaped the loom and nothing since then.


Good luck

Hm I haven't checked the wiring thru the firewall yet, thats definately worth checking.

No changeable fuel filter on a 98 , just a screen on the sending unit. If there was problems with this I think that there would be problems no matter what I did, and they wouldn't go away after a couple minutes. Im almost 100% sure its a sensor/electrical issue because it performs flawlessly at WOT, when most sensors are ignored. But I will look into finding a way to get my fuel pressure tested. Is there a way to test the pressure reg?

And no, no quick way to test with a known good ECU.

Thanks for the suggestions so far!

there is a replaceable fuel filter. it's on top of the pump module. #4798301, it's a pressure regulator and filter together. too often overlooked on the later models.

Hmm write-up on doing that.

http://www.jeepforum.com/forum/showthread.php?t=138580&highlight=fuel

Looks like a b!tch. Damn.

Wierd that everywhere Ive looked lists it as a non replaceable part that you can only get with the whole unit.

Anyone know a part # or source just for the filters?

Not convinced that that is my problem but it's definately a good maintenance thing to do.

well after following the CEL on my 98 all the time, and it always being the same code, PO 306, and new plug, rotor, cap and wires I finally bit the bullet on a new fuel pump/sending unit/screen/filter......well pretty much the whole damn thing, except the srcew cap, I found in on ebay, for $175 shipped, better than ~$375 from the dealer. And being that you are in Maine, just pop for new j-bolts from the dealer, $14 a piece, 3/8" hook bolts might and probably would work and save you a ton.

I did this today and the hardest part is.. anyone........anyone.......anyone..........getting the skid off. And after draining......which I am not sure you can actually do completely without a pump system, I am glad I did, the screen look like sh!t and the jeep runs alot better. I also sprung for a set of year specific injectors.....your 98 comes from the factory with 23.5lbs injectors, don't run what you get from NAPA, autozone, etc. they list the same injector for all 6cly XJ's.

I would definately swap out the fuel pump assembly.......big chuck of change, but if you are like me and have ~165K miles on your engine and a stroker in your future......its was worth every penny.

Not mentioned alot but periodic fuel injection cleaner in tank (Chevron, expensive when compared to others) IMHO does make a major difference in idle/drivability.

Didn't think to start using it till about 50k (116k now) but I definitely feel difference. That, and opening her up in passing gear, 3, 4k rpm (I tend to baby my motor and 3.55/31's tend to keep revs down) also cleaned up alot of accumulated crap.

At times in drive sitting at light I swear the engine isn't running, that smooth after a few treatments and stompin' on her a bit, sorry to hijack thread...

98XJSport said:

Various types of fuel/system cleaners.

Click to expand...

First thing I tried, the gas station I used to go to ended up getting submerged every spring, so I thought for sure it was water in my gas.

And the only CEL I get is a P0117, for the ECT sensor, forgot to add that.

What I think happening is the CTS is getting a low supply voltage from somewhere. This is making the CTS give off false readings at the least, making the jeep run to rich or to lean. But how could the CTS be getting low voltage? I plan on checking the wiring later this week weather permitting.

Try unplugging the vac line from the fuel pressure regulator. If it smooths out you have a low pressure problem.

Why would a fuel pressure problem, or any fuel problem for that matter, cause problems at specific times in the startup cycle, consistantly, and at no other time? And how could that affect my coolant temp. sensor?

Ok,
I just looked up your symptom on the IATN website, for which I am a member, and I ran across something that sounds like the issues you are having.
In this case, there was some backfeed from the O2 sensor heater ckt. getting into the O2 sensor signal ckt. One guy actually recorded 1.98volts on the signal line. This should be .9volts tops.
Anyways, a new #1 O2 sensor solved this guys problems.
FYI, as I understand, the #2 O2, (postcat), is only used to indicate catalytic converter efficiency. Precat is used for fuel trim management. The pcm still gives a crap about #2 heater, and a toggling signal voltage, but after cat lightoff, it looks for a steady median voltage-
When that high voltage gets into that O2 signal ckt, it's gonna drive that fuel trim lean as all hell. try enrichining during your crappy idle by spraying carb clean/starting fluid/gasoline and see if your motor smooths out- to see if you should pursue this avenue

Hm that does make sense, and that does sound like a good way to test. I haven't been able to get the damn sensor out yet anyways, so testing is about as good as I can try right now. Thanks!

soarhead25 said:

Ok,
I just looked up your symptom on the IATN website, for which I am a member, and I ran across something that sounds like the issues you are having.
In this case, there was some backfeed from the O2 sensor heater ckt. getting into the O2 sensor signal ckt. One guy actually recorded 1.98volts on the signal line. This should be .9volts tops.

Click to expand...

wow! that's a good find. by the way, you are correct about the after-cat o2. it's only purpose is to monitor the cat. doesn't affect fuel mixture in any way.

Did you change the Crank Position Sensor already?

What engine code was that?

I didn't mess with the CPS, Ive changed the CTS coolant temp. sensor, with a code P0117 I think it was...

Ok got the O2 sensor out, new one in, and its been run 3 times without having a problem. And its running better, more throttle responce, etc. So I think that did it. Thanks everyone for all the help, this is the best my Jeep has run in a long time!