Idiot light, altitude, octane, gremlins?

I went on vacation last week to Colorado and did some wheeling in my XJ. Did great. Except for that damn Check Engine Light.

96 XJ, 4.0, 150k miles, stock.

Light came on and stayed on for a day, even with multiple times turning it off and restarting. It seemed to run fine the whole time.

The day it did it, I had
a) 20 miles of high speed teeth rattling driving,
b) a gas tank with half 88 octane and half 85 octane
c) mild swimming (not over front bumper, low speed with no appreciable splashing)
d) high altitude (varying from 10,000 to probabaly 11,200).

The next day it did fine, and has not come on since.

So, the question is, what is the probelm? Factory owner's manual is not much help. Is it the altitude too much for the O2 sensor? Octane? Emissions in need of attention (cat?) Just one of those things?

Did you check you codes?

Octane at elevation is normally about two points lower than flatlanders are used to - that's okay. I'd say pull codes as well to see what the problem might be. http://allpar.com/fix/codes.html

I don't have the equipment to read the codes, and was hoping for opinions on whether it was worth paying somebody to do it for me. (Or is this one of them that the 'code reader' and book of codes is cheap?)

I thought rocky mountain jeeps came with a different CPS for the higher altitude and had a different advance ???

If you have a key, you have the eqipment to pull your codes. Drive it around for awhile and CLICK the link I sent you, there's a page in there describing codes for the Jeep. It's an ON-OFF-ON-OFF-ON sequence that will start flashing the 'Check Engine' light. Those flashes are a series of two-digit numbers that correspond to the error codes stored by your computer.

The high altitude CPS is a stock Mopar item. If you bought a rig here in Colorado, it came with it.

Yucca-Man said:

If you have a key, you have the eqipment to pull your codes. Drive it around for awhile and CLICK the link I sent you, there's a page in there describing codes for the Jeep. It's an ON-OFF-ON-OFF-ON sequence that will start flashing the 'Check Engine' light. Those flashes are a series of two-digit numbers that correspond to the error codes stored by your computer.

Click to expand...

Thanks Yucca-Man. You learn something new every day. I'll give this a shot tonight and see if it gets me anythign useful.

old_man said:

The high altitude CPS is a stock Mopar item. If you bought a rig here in Colorado, it came with it.

Click to expand...

Rig was originally out of Salt Lake City Utah. Does that get me the high altitude? Any way to tell from the factory window sticker? (OK, I'm at work now and don't have the sticker in front of me, so if it says 'high altitude emissions' or something like that, be gentle on me!)

I could not get any trouble codes at all. Is it supposed to give 55 when everything is OK (in which case I'm just not fast enough on the sequence) or do I need to wait until the light comes on again?