1996 XJ High RPM misfire, all cylinders

I have a 96 4.0 Sport, auto. Short back story, bought it, drove it a few hundred miles, built suspension for it, drove it a bit, took it on a day drive. During the drive, the alternator died. Drove home, 60 miles, with no alternator. Stuck a Used alternator in it from a buddies part jeep. A few days later, started getting cel. Checked codes, Had random cylinder misfire as well as #1,2,5,6. Did plugs, wires, cap, rotor, coil. Standard champion copper plugs. Cleared codes. As soon as monitors reset, codes started coming back. Decided to check the used alternator, had AC voltage. Put a new alternator on it. Cleared codes, came back. With the scanner, monitoring misfire counters, I have nothing, as I raise the rpm, about 2000rpm, every cylinder starts counting misfires. At 3000rpm it goes into overdrive counting misfires. If I drive around town, its dead smooth. It only throws the code pulling a hill on the freeway above 2500rpm or so. Unplugged the rear o2 sensor and monitored misfires, no change. Checked for vacuum leaks and seafoamed the engine. I checked fuel pressure, 39psi, spec is 42 +-5, So its a bit low. But not sure low enough to cause a problem. Fuel filter is new. I can not feel it misfire ever. Got plenty of power. Decent fuel econ. Just throws the codes. I dont want to just throw parts at it, so wondering if anyone has had a similar experiance. No one had our shop is quite sure which way to go. Its every cylinder randomly. oh, tps is new by previous owner.
Thanks

CPS may be going bad.

I would get a good look at the DC voltage at those RPMs (because of the alternator problems), as the voltage regulator is in the PCM and not in the alternator on 96-01, so there still be a problem there.

HV coil is getting loaded more at higher rpms, and might be breaking down, thermally, but the lack of power loss, and good fuel economy make fuel pump and clogged cat unlikely.....and maybe rules out the HV coil as well?

Coil is new msd blaster coil. Did it with old coil as well. If i drive ir to work, its 30 miles home, 2500 elevation gain, several 6% grades. Pulls 70mpg no probem up thise grades in 3rd (auto), at 3000 rpm on 35s. I see around 15mpg in the same mountain highway driving. It pulls hard all the way to 4000rpm, so i dont think cat either. Charging voltage is good at all rpm. I also tried a different battery, i know some dodge trucks do strange things with a weak battery. Im not thinking intake or valve issue, since it effects every cylinder equal and randomly. And it just starter suddenly. I also checked and cleaned all the grounds. Im sure ckp and cmp sensors may be a possibility. But i hate parts throwing.

Are you sure about that fuel pressure spec? I thought that the spec for a 96 was 49 psi, plus or minus 5. If that is the case, you're 10 psi low.

But 96 was a year of change for the fuel system on the XJ. You may be right, but I'd double-check.

Opps my bad. Typo, yes spec is 49 +-5. Im at 39-40, any rpm, including full load against coverter stall. So im 5psi below minimum. I am in process of fixing fuel pressure, but have my doubts, due to how well it runs. But i havent ruled it out yet.

If it were my 96 first thing I'd check is the dark green and orange wire at the coil, if you get low voltage, then alternator, then splice, then ASD relay in and out. Remember this is a *relatively* high amperage circuit, it may read good volts and have low amperage. Low amperage usually shows up as heat, at the ASD relay (hold your finger on the top) a splice or a connector. It also shows as low voltage, but in the beginning (before something fries completely) the lower voltage may be within tolerance and hard to notice.

2500 RPM is where I start looking at the O2 sensor. Best mileage (lean) is near peak torque, if the O2 sensor leans it out too much you are going to get a lean misfire and/or bucking.

Crossfire is also a possibility, the distributor cap for the XJ is small. Though with new cables less likely. It really doesn't take much to get the park to jump between poles inside the cap. One very good reason a hotter coil and more spark isn't necessarily a good thing. If you have an after market coil on there you may need a ballast resistor. Too much can be just as bad as too little spark.

There are other things that can cause this, fuel being one. I've taken a fuel pressure gauge and tapped it tot he windshield wiper and gone for a drive. Pressure isn't the only issue, volume also counts, you run our of volume and the pressure drops.

Sometimes it gets really crazy, I had a bad spice near my anti theft module. Voltage would get low, windshield wipers, headlights, heater and whatever on and the motor would crap out. The ASD relay would get starved or juice. I found and fixed that one in a snowstorm in a foot of snow and went right to the problem in a bundle under the dash. For those people who don't believe in ESP, if that little voice is talking to you listen. I will never know how I found and fixed that one in fifteen minutes. Sometimes you just have to have a hunch.

Trust me, i know crazy. I work in a truck repair shop that specializes in it. We get the stuff other shops cant figure out, every tech that has worked here is amazed by the strangeness of the porblems we see. Weve been told by dealers that that never fails, they have never seen it, but it happens here.

I did check voltage to coil, but not sure an amperage. Im going to be doing pump and strainer later today on my lunch, just because the pressure is low, and id rather do the pump now, and have a spare, then have it die on the trail.

Coil is aftermarket msd, but problem exsistes before the new parts.

The *voltage in* wire for the alternator and the coil is the same wire and comes from the ASD relay. The wire is anemic (in size), I've looked at it many times and never found a fault, but I've been expecting to find one.

Just like the starter solenoid wire, way to small for that application. I have seen places were the wires vibrate and the individual strands break under the insulation from fatigue. The injector feed (+) is known for this.

Ecomike called it; crank position sensor. Go with OEM and skip the aftermarket headache

So drove it wirh the fuel pressure gauge on. Was dead on 40psi everywhere. Decided just for the heck of it to get a second opinion, and tried a different gauge....50psi. Our shop gauge is reading off by 10psi. I trust the newer snap-on gauge over our shops older gauge. Im going to try a crank sensor first, even if it doesent fix it, at least.i have a trail spair.

Absolutely agree with buying a Mopar crank sensor. Aftermarket crank sensors are very "hit and miss" with a whole lot of miss. I won't install aftermarket with this part. Too many failures. And even if the aftermarket part tests out good electrically, the "spacing" can sometimes be off to a point where it won't work when installed. Spacing is important with a hall effect sensor which is what the crank sensor is.

As for your MSD coil, as you mentioned it is likely not your problem but I've seen many premature fails with the MSD coil. I prefer to use an OEM style coil. It is all the 4.0 needs and you don't gain anything with the others.....

The msd coil was simply because i was doing a coil anyways, and it was only a few dollars more than a standard one, and for years ive had good luck with the standard round msd coils. Wasent looking to gain anything.
As for crank sensor, i got aftermarket because its was i could afford and get today to see if it fixes it. If it goes bad, its easy to change, and a phone call and they wil come to tue shop, pick up the bad one and give me another.

Thing is, the aftermarket can cause ghost misfires. Seen it been there done that. Same symptoms as yours.. High rpm misfire counter going crazy... 3 aftermarkets on it, thought not the problem and after swapping every other part including a new ECM, an OEM CPS fixed it.
My cost on OEM was 68 bucks.... learned my lesson. Never again.

Cleared the codes at lunch and drove it. Put a crank sensor in it after work, drove to the gas station, then home 30 miles. The later a 60 mile drive code free. Im hopefull so farm it seems to fell smoother upper rpm and pull just a hair better. But that coukd just be me.

Hmm, if it comes back, if the new CPS does not solve it, you may have a carbon build up and or pre-ignition at that point of high rpm, high load on the hills with those steep grades and large tires, and maybe a little too hot engine temp all mixed in there.

Ecomike said:

Hmm, if it comes back, if the new CPS does not solve it, you may have a carbon build up and or pre-ignition at that point of high rpm, high load on the hills with those steep grades and large tires, and maybe a little too hot engine temp all mixed in there.

Click to expand...

Radiator is brand new, and its been happening driving in the snow. Engine has not been hot. And it was more rpm related, not load. I could watch misfires count like mad running it in the shop above 2k rpm, no load. Im hopefull the cps fixed.it, but will need a few more drive cycles to be sure.

Had similar probs with my '96 Sport, ends up being a worn out distributor. Take the cap off and wiggle the rotor shaft, if it moves at all, it needs replaced.

I was wondering about the distributor too, but wouldn't that show up more at idle than high rpm? There is a cam sensor in it for firing the injectors, as well as the spark...

I only had one bad dizzy only once, and it got oil up into the cap and rotor when it when it went bad.

Ya. Thought about the dizzy early on, its tight.