Firehawk068

I did a search on this subject, and found a few posts...........There are a bunch of suggestions, but none that actually stated what the problem was, or what the end solution was?

I've got a 1996 XJ Country/ 4.0/ AW4/ NP242/ 3.55
I have a 3-1/2" Lift, and 31x10.50/15's, but I doubt this has anything to do with the issue.

The problem I have is that sometimes, at random, when I am at high altitudes (usually over 10,000 ft) the Jeep will start to sputter, and stall....
When I try to restart it, it usually sputters, and backfires a few times.
Sometimes it will catch, and if I feather the throttle I can get it to rev up.
Usually I have to wait 5 to 15 minutes before it will restart and run normal.
When it does re-start, it will run fine the rest of the day...........

The issue happens totally at random..........sometimes it will do it when I am crawling up a trail. Sometimes it will do it on the highway................
The problem happens about once a month, and only happens when I am at high altitude..............(although, it did happen once when my wife was driving it as she just came off the freeway in Aurora. 5500ft)

The frustrating aspect of it, is that the "Check Engine" light does not come on during the issue, and it NEVER throws a trouble code!
Otherwise, I would at least have a direction to look.................

The things I have tested and/or replaced are:

*Fuel pump/pressure- I drove around for an hour during my lunch break with a pressure gauge hooked to the fuel rail, cold and hot, under idle/cruise/heavy load, the pressure was rock-solid at 47-48 psi...
*Map sensor- tested voltage output at various vacuum readings with mity-vac. Compared to readings of a brand new factory replacement, and ended up with the identical voltage outputs.
*Checked for any vacuum leaks at intake/throttle body/vacuum lines- None found.
*Replaced Coolant temp sensor for the computer.
*Replaced Upstream Oxygen sensor
*New Spark Plugs/Cap/Rotor
*Checked wiring and connector for the Crank Sensor- didn't find any issues

Being a '96 I figured it should at least throw a misfire code, or a rich/lean code, but I get nothing.....................I'm stumped?

Anybody ever have this issue, and figure out the solution?

XJwonders

Wiring for upstream O2 sensor, did u try checking that?

CCKen

Did you check the MAP Sensor per the FSM procedures?

The MAP Sensor is the only thing I can think of that would be altitude sensitive and that affects mixture.

Firehawk068

Yes. Wiring is routed correctly, and is not touching/chafing on anything.........Connector looks solid also............


I am just assuming, but I would think I would get an O/2 related code if the sensor readings were not in range..............


A couple months ago I also hooked up a scanner that I could watch live sensor data in real-time........All the sensors looked to be in the normal ranges......With the O/2's toggling between + and - like they should..........Ran it all the way up to E-fan kick on temp, and everything seemed normal.........

Firehawk068

I don't have a FSM, but I found a test article online, and followed that procedure with a hand vacuum pump...............Watched the voltage change with smooth transitions, no spikes or dead spots...........
Wrote down voltages at various vacuum values (5,10,15,etc....) and compared them to a new factory replacement Map sensor, and got the exact same voltages.(I was able to get one of my accounts to lend me a new one for 10 minutes while I tested it)


The Map was the first thing I suspected, due to the issue mostly happening at 10,000 feet or more.............I even checked into seeing if there was a "High Altitude" version available from Mopar or Aftermarket?...............There is not
And I already replaced the little elbow that connects it to vacuum.....

CCKen

I don't have a '96 FSM so...

Is the MAP Sensor on a '96 mounted on the TB or on the firewall, like previous OBD-I years?

Firehawk068

It is mounted to the intake manifold, right next to the throttle body.
There is a short rubber vacuum elbow that connects it to the intake, directly under the throttle body............

CCKen

Did you see this thread?

https://www.cherokeeforum.com/f2/po1...g-fine-200315/

Does your CEL bulb work Test at key to RUN bfore starting).

Firehawk068

I just read the thread.....................I never get any codes.....
Yes the CEL bulb works fine. It comes on at key-on, before engine start, then goes off after start...................

Battery and charging system voltage is good..........I don't have any issues there.........

I am at a loss as to what would cause the issue I'm having, and yet NEVER throw a CEL, or store a code?

CCKen

Do you have a scanner that can check for pending codes. There may be one or more lurking.

Try "High Altitude Gas" and see if it helps. :-)

Firehawk068

None.........My scanner reads pending codes also...........

And all the gas I buy is "High Altitude" gas..........
I live in Denver area...........

CCKen

I see you installed 'New Spark Plugs/Cap/Rotor', did you replace the plug wires as well? Old high tension ignition wires may be breaking down in the rarified air at altitude.

Have you done a compression check?

Firehawk068

I have not replaced the wires, but it has Taylor wires on it that look fairly new..............

I have never done a compression check on it, but I never had a reason to.......It runs perfect most of the time.........I'm thinking if I had a compression issue, I would have issues all the time..........

Do the folks who have Crank Sensor issues, and Distributor issues get trouble codes?

I'm running out of things to diagnose

CCKen

I like your trouble shooting techniques. Especially "that look fairly new". Runs perfect most of the time doesn't include at altitude however. That's why superchargers are used for high altitude vehicles...not enough air. Weak compression adds to the grief at altitude....not enough air.

Crank and Cam Sensors are monitored by the PCM and do have a few codes.

Your issue must be a system that is not monitored by the PCM and that doesn't effect emissions. Any unmonitored system that does have an effect on emissions will trigger a DTC.

Have you hooked up a scanner and watched live data while this is happening? Like watching ignition timing. Advance timing at altitude will cause detonation. Does your engine ping when it's acting up?

Fuel

Air

Ignition

Firehawk068

Actually "Runs perfect most of the time" included high altitudes..............
It runs just fine at high altitudes most of the time................I would say maybe 1 out of 8 or 10 times I drive at high altitude, the issue happens................If I had a compression issue, it would run like crap most of the time? Or I would have tons of blow-by? I don't...........


I would also think that if I had an ignition problem, then I might get a misfire code? Or does that only apply to the newer coil-pack 4.0 engines?


It never pings................I've never heard it ping ever, even when really hot..........
Unfortunately I don't own a scanner currently that will monitor real-time sensor data..............I borrowed one to use for an hour when I looked at all my sensor data............
I would have to borrow one for two months If I stand a chance of having it ready when the issue presents itself.....................


As far as the ignition wires are concerned.....................Unless it is just the coil wire, or the ignition coil itself, I doubt they are the problem.


When the issue happens, it is ALL cylinders.............Not just 1 or 2........


I went wheeling this past weekend at high altitude, and never had an issue............................Also, a few months ago I drove the XJ fully loaded, while trailering my FJ from Denver to northern California and back, with no issues whatsoever the whole 2500 mile round trip.................
(Donner Pass)



The next time it happens, I can try switching one of the shorter plug wires with the coil wire?
But usually the issue rectifies itself after sitting for 10-15 minutes, so in the time it takes me to swap the wires around, the problem may have resolved itself anyway?