98 XJ 4.0 running issues

My 98's been having some issues for a while and they are starting to get worse, so I figure I should probably fix it. It's a 98 XJ 4.0L/AW4, nothing else is relevant but it's got an NP231, a d30, and a 29 spline 8.25 for the curious. Around 150k miles on the motor.

Temps: fine. takes a minute or two of driving to come up to 210 and stays there.
Oil pressure: fine. 40psi cold idle, 20psi hot idle, 40+psi hot running.
volts: around 14 running, and the factory gauge usually lies on the low side so I'm not afraid of issues here, it's been like this for months so the battery would have gone flat by now.

What it does - it idles extremely roughly, and cylinders 3 and 4 are effectively doing nothing. I can unplug the injectors and the idle doesn't change at all. Unplugging the other injectors causes it to run even worse. If I goose the throttle I get some pretty awesome loud backfires through the intake. Trying to drive it, it basically has no guts at all, makes some interesting pop/backfire noises that almost sound like loose control arm or track bar bracket bolts until you hear it do it a couple times in quick succession. It strains and bogs down and runs horribly till around 2500rpm, at which point it picks up a little, then completely falls on its face at around 2700-2800 and you have to floor it to get it to downshift so it picks up the revs and runs again. If you can baby it downhill till it gets to 3000rpm it'll take off and go, and almost feel normal, but it still lacks power and sounds extremely rough/throaty. Check engine light is sometimes off, sometimes on, sometimes flashing :scared: before my code reader decided to take a vacation I pulled the codes and there were so many I couldn't really tell what the hell was going on.

I've searched till I'm blue in the face (kinda like that scared guy a line or two up...) and it looks like it could be O2 sensor (replaced a few thousand miles ago), cam sensor (need to check this), fuel pump (it was fine a while ago, need to find my gauge and check this but I'm expecting it to be fine since it runs ok above 3000rpm), MAP sensor (doubt it, but might toss my spare on to verify), plugged catalytic converter (I have a new one on order, going to cut the junk muffler off it tomorrow and shine a light into the cat to see if I can tell if it's disintegrating, bad head gasket between 3 and 4 (I hope not, I don't feel like doing another motor swap right now), burned/cracked valves (see above), wiped cam (see above)... basically "hey dude it's something to do with the motor and the sensors attached to it!"

What say you? I am going to try the cat first since I already have the replacement on order and the muffler's beyond junk anyhow, no loss there. I don't think it's the cat though because that generally causes bogging down ABOVE 3000rpm, not below it... If it's not that, fuel pressure then probably the sensors. At some point I need to check compression but my crappy gauge's adapters won't screw into the 4.0 head so I have to make one myself. I really don't want to turn this into a "throw expensive sensors at it till it runs again" session, even if it just means swapping them off parts vehicles. If it's the motor I have a spare in storage but will be sorely tempted to toss a gas can and a road flare in it and collect my insurance money on the damn thing, it's been fighting me since last October or so. Every time I thought I had it fixed and went to get it inspected it would flip the CEL back on, or burn a bulb out, or otherwise fail me, and I've gotten tired of explaining to the police how many things I've replaced on it and why I'm still test-driving it around with a rejection sticker from last fall on the windshield.

Oh yeah - this all started when I replaced plugs 2-5 but not #1 (since some chucklehead previous owner had mickey moused the heater bypass tube screwed into the water pump and I was afraid to bend it till I had a new one sitting around spare, last time I did that it broke and I was pretty much boned.) It started giving me a lot of check engine lights, replacing the last plug (checked the others and they are still fine) and resetting the computer improved things for about 5 minutes but then it went back to its new tricks again. It's been all downhill from there.

EDIT: holy crap, wall of text. Hopefully someone actually feels like reading that mess.

KAS,

Since I didnt read that you tested them; I have to ask.
Have you tested the voltage pulse to injectors 3&4 ?
Have you tested to see if these injectors are functioning properly?
I replaced the factory fuel injectors with a set of FiveOMotorsport Bosch four hole injectors and noticed quite a difference in acceration and idle....

Charles

I actually haven't. I was at a gas station 50 miles from home when I checked that, simply trying to figure out why my motor had just gone to shit. I'll have to put a noid light on them in the morning unless I can find my oscilloscope.

The Check engine light is the key to solving the issues. The Jeep is trying tell you what is wrong.

As you said, randomly replacing part in the hopes you accidentally fix the problem is a waste of time and money.

Download the trouble codes, write them down, decipher them them, and proceed from there. Start with the lowest numbered codes. Higher number codes are often caused by faulty data from the sensors/systems that are generating the lower numbered codes.

You have kept up your routine maintenance and all the tune-up parts are fresh, right ?

Tim_MN said:

The Check engine light is the key to solving the issues. The Jeep is trying tell you what is wrong.

As you said, randomly replacing part in the hopes you accidentally fix the problem is a waste of time and money.

Download the trouble codes, write them down, decipher them them, and proceed from there. Start with the lowest numbered codes. Higher number codes are often caused by faulty data from the sensors/systems that are generating the lower numbered codes.

You have kept up your routine maintenance and all the tune-up parts are fresh, right ?

Click to expand...

Last time I tried this it basically complained about almost everything and I threw up my hands in disgust.

Yep, it's up to date on most things. Oil, coolant, plugs, wires, cap, rotor (these were what set me down this road in the first place! doh), air filter, upstream O2 sensor replaced a few thousand miles ago, etc. Just remembered, I tried fuel system treatment and that didn't improve things either.

Start with the lowest numbered codes.

free shipping too!!!!!!


X2 on getting the codes, all the tune up stuff on that engine are new if you need them

Another data point, it's getting something like 15mpg even with me revving the shit out of it and putting my foot on the floor to get it to move.

After I check fuel pressure, I am going to check codes (too busy with work right now to even try till sometime sunday.) The way it sorta stumbles at random is making me think it is a flaky fuel pump, still not sure obviously.

Doug - I have one in storage... if it comes to that. I'm really hoping it doesn't.

My wife has a 98 like yours,,,so I,m somewhat familiar with your XJ.

Check the injector plugs for good contact--a loose plug wit kill a cylinder and give a CEL, A blinking CEL will indicate a major miss-fire and will cause a cat melt down.

I have had to replace distributors in both my 96 and her 98 at about the mileage you have -because of loose/worn housings letting the rotor button contact the spark plug terminals.
Plug wires do break down....
I would think a fuel pump issue-filter or pressure would cause all cylinders to be affected...

HTH

Wayne Sihler said:

My wife has a 98 like yours,,,so I,m somewhat familiar with your XJ.

Check the injector plugs for good contact--a loose plug wit kill a cylinder and give a CEL, A blinking CEL will indicate a major miss-fire and will cause a cat melt down.

Click to expand...

huh. It's blinked a whole bunch, I knew that meant bad things but frankly just wanted to get where I was going and and I haven't ever particularly cared about this XJ's drivetrain, bought it to build since it was rust free. I am now very glad I have a new cat on order.

Wayne Sihler said:

I have had to replace distributors in both my 96 and her 98 at about the mileage you have -because of loose/worn housings letting the rotor button contact the spark plug terminals.
Plug wires do break down....

Click to expand...

I have replaced the cap/rotor/wires/plugs (which is what started this whole mess, at least as far as I could tell) but I didn't check for play in the dizzy, I'll do that.

Wayne Sihler said:

I would think a fuel pump issue-filter or pressure would cause all cylinders to be affected...

HTH

Click to expand...

Likewise... but I'm lazy and my thinking is "I have the gauge and it takes 30 seconds to check, so I might as well do that first". We'll see what I find.

I'll X2 the play in the dizzy.

Replaced the one on my buddy's 98 Classic a couple of years ago, not because it was doing the wacky stuff yours is doing (at first...keep reading), but because we heard a knocking sound that wasn't a typical Jeep knocking sound. A few minutes poking around with a piece of wood between my ear and the motor and determined the dizzy was literally moving up and down.

Come to think of it, we did that job twice that night. The first reman dizzy we got from (insert name of your favorite auto parts store here) had a bum cam sensor in it. Once we had it in and fired it up, the timing was all sorts of wrong. Blinking check engine light almost immediately, backfiring through the intake and next to no power. Wound up pulling that dizzy back out and swapping it for another at the store.

If you can, pull the codes again and post them up. I'd be curious to see what it's throwing.

Several years ago I had a problem with my '98 Jeep stumbling and the engine shut off a 1/2 mile from the house. I towed the Jeep home and parked it in front of the house. Later in the day the engine started long enough to drive into the garage.
Tested the CPS, fuel pump, relays and several other things and found nothing..
A Jeep tech friend stopped by and asked me to restart the engine. We both heard a funny noise coming from the passenger side of the engine.
When the distributor cap was removed there was A LOT of carbon debris inside the distributor and cap. The rotor was wobbling when the engine was cranked. The distributor shaft bushing had worn out causing the Cam Position Sensor to short out on the curved trigger shield. The ECM would reset and the 5V reference was restored.
Where I am going with this is that at a certain mileage (approximately 95K) some distributor bushings have reached their life span. My Jeep is a early production year (Aug '97) 1998.

worn cam will produce the results you are seeing

Have you checked the firing order on the plug wires? Maybe you have cylinder 3 & 4 mixed up causing the whole problem. Since after the tune up is when it all started thats the very first thing id check.

Sent from my SGH-T959 using Tapatalk

Check firing order, check distributor, check cam sensor, check map sensor, check fuel pressure, check injector harness w/noid light. You could also use an automotive stethoscope on the injector to hear if it is opening or not.

Alright, so, what I've done so far:
* checked fuel pressure. Holding solid at 49psi or so.
* tried pulling the muffler off, it was total junk but didn't improve things.
* heard rattling in the cat, so since I already have a replacement and removing the old one will wreck it anyways, bored a couple 7/8" holes through the core lengthwise to improve flow. Very little improvement. Don't worry, new one's going on before this turd does anything more than take a spin around the block for troubleshooting.
* verified injector pulse to all injectors - I can feel them ticking when they are plugged in, no ticking when they are not. Also used mechanic's stethoscope.
* verified firing order - 153624.
* pulled spark plugs. As expected 1/2/5/6 are normal, white insulator with some light brown haze, terminals not melted with no significant deposits. Also as expected, 3/4 are badly dry fouled, insulators are dark grey with one side almost black.

Haven't done yet, in order:
* check distributor for axial and rotational play, wear, and debris. This is next.
* new plugs - going to grab some at the store shortly. Won't be leaving them in for long, don't feel like fouling a second set, but I figure I need a new set when this is all over anyways, so might as well toss them in for a second and see how it does.
* pull codes, check lowest first
* check sensors, based on codes
* check compression (this would be further up the list, but I have to build an adapter to make my compression tester work, or buy a better one, so I'm doing the cheap easy stuff first)
* based on compression results, check valves/gasket/cam... or just say screw it and throw a new motor in, then check this and rebuild when I have time.

I'm taking a pic of the plugs in case I missed something (I'm not that great at reading plugs):

click for big, click again for bigger, plug 1 is on the left, 6 on the right

welllllll I figured it out!

Not as bad as it could have been (wiped cam or burned valves were my worst fears), but not great either.

Distributor's fine. No play, no debris.
Plugs - didn't bother putting them in, keep reading.
Codes - didn't bother pulling them...
Sensors also not checked.

I bought a compression gauge since they had one for a decent price when I went to get plugs.

Compression test results (dry, all plugs removed, fuel pump relay pulled and pressure released before testing)
cyl 1 - 150psi
cyl 2 - 150psi
cyl 3 - 30psi :explosion
cyl 4 - 30psi :explosion
cyl 5 - 150psi
cyl 6 - 150psi

I also observed a loud hiss from the vicinity of the motor while testing 3/4. Inserting a spark plug in the other cylinder while testing resulted in the loud hiss turning into a pulse from the exhaust.

Gee, I think the head gasket's smoked between 3 and 4. I will probably drop a new motor in it (since I have one I just bought for $100 hanging off an engine hoist in the back yard already) and replace the gasket in the other when I feel like messing around with it.