misfire under load, tach jumping wildly?

This is kind of a new one for me, I've got some ideas and will be following up as I go in order to possibly help future searchers.

Background: new to me '88 renix/auto/242 that I've been working on, which has been running great for the last 700 or so miles that I've been driving it, starting misfiring pretty badly last night on a long, steep highway grade.
after messing around with it, I have determined that it only happens under a sustained load of more than a minute or two. Cruising flat streets, I can't get it to happen no matter how many times I punch it. As soon as you hit a grade and sustain a load on it for a couple minutes, it'll start to miss heavily and randomly. The more you push it, the worse it gets and the slower you go.

What I think is a clue, is that when this happens, the tach jumps wildly from 3k down to 500-700 and back and forth 1-2k rpm in a split second. You can hear it pop lightly out the exhaust while it's happening. If you ease off the throttle it gets much better, but continues even on the downhill side for several more minutes. After that, it's fine until a sustained load again.
Fuel pressure stays good the whole time. All grounds were refreshed after I bought it, all sensors test good. Cap, rotor, wires all *look* good.
I feel like it's most likely a bad ign coil, bad icm, or possibly cps.
Going to swap to a known good coil and icm this evening and see what happens.
Anyone ever go through something similar?

well... Swapped the coil and icm to a known good pair out of the mj, no change. Although after taking the original coil off the module, I found the u-clip deal that contacts the coil + terminal was quite cooked and the surrounding plastic was melted. Runs somewhat peppier than before, but as soon as you spend a few minutes at freeway speed and put a load on it, it misfires and starts losing power.
Tried flooring it, and it momentarily cleared up and pulled hard, then started missing again.
Anyone think a bum cps could cause this? Seems unlikely to me, but I do have a new spare. Just such a b#tch to change...
Going to pull the plugs this evening, see what they look like. Maybe pick up a new set of wires tomorrow.

I had intermittent cps failure that manifested itself in a similar fashion.

When you try the cps, notch the bottoms of the holes out. Makes things much easier.

Mine was doing something similar, I called it the 2200 RPM buck, that sucker would go chunk, chunk, chunk. Turned out to be mostly the O2 sensor, it would lean the motor way down near peak torque cruising or mildly accelerating. May be related to your issues.

I had some other issues at the same time, don't know if they were also a factor. but replacing the O2 sensor cured most of it. One thing I did notice was it happened on a warm motor, it rarely to never happened on a cold motor.

My CPS was slowly dying and I had TPS ground issues at about the same time and pretty much fixed everything all at once. The TPS seemed to cure my motor hunting at idle (rhythmic RPM changes while at idle) and the CPS made my starts a lot more dependable. I also found some loose intake bolts at about the same time. my vacuum was poor.

Another thought. EcoMike says you can disconnect the O2 sensor and run it on default (pre-programmed fuel curve in the ECU). Might be worth a try just to troubleshoot some. I've never tried it, if you do let me know how it works.

The O2 was something I was going to change as maintenance anyway, going to do that tomorrow.
I believe it just got easier to diagnose, as it just died completely. Was just cruising along at 40 and it started to miss and sputter for a minute, then just shut down.
Got good fuel pressure at the rail, but doesn't even attempt to start. Did the infamous "unplug the cps and plug it back in again" while waiting for the tow truck... Started right up ran for about 30 seconds, started missing and died again. Tried it again, nothing now.
Sounds like no spark, but it's hard to check by oneself.
Going to change the cps tomorrow after work, and see what happens.

For the curious, it seems that a dying crank sensor will cause the symptoms the xj has been having.
Decided not to be a baby and just go ahead and change the cps tonight. I put in the hi-altitude oem one I'd been saving, and voila! Now she starts and runs fine.
I'll check back in on this if it turns out to be a temporary victory, but it appears to be good to go.
Thanks for the help. atriot:

Click on the link below and do posts 1 through 5, and possibly 27.

http://www.cherokeeforum.com/f51/cruiser54s-mostly-renix-tips-153657/

Cruiser, I've been following your renix tips for a long time.
The only things I have left to do are the headlight harness, c101 elimination(already cleaned all the goo out), and the distributor indexing(have a brand new distributor from crown for that).
I've cleaned/applied deoxit to every connector in the engine compartment, done the add-on grounds, retorqued the manifold bolts(every single one was loose), and all the other tips.
Since I got my first renix, I've made that list the to-do to any newly acquired jeep. As a result, I never have any "renix troubles". She runs like a raped xxxxx(insert wild animal of choice )
Your tips are an enormously helpful utility.... Thanks!

Cool. I have become an advocate of C101 elimination, post 27. CPS wiring runs through it. Make sure your CPS wiring is routed properly and not contacting the exhaust manifold. Post 7 in my Tips should have a photo.

The cps wiring is routed the opposite direction of the illustration. Try as I might, I could not find a way to get to the clip "g" in the photo. my hands are WAY too big to fit anywhere near the cps.
I routed it back toward the firewall, and towards the driver's side. It's held in several places with zip ties, well clear of the header.
The next oppurtunity I have enough time, and stuff out of the way, I'll route it "properly". For now, it's safely out of harm's way.
I don't trust the c101, and will be hardwiring around it sometime in the next month.

Excellent plan on the C101.

Because I hate it when someone has an issue, and never follows up...
After changing the crank sensor, the jeep has run flawlessly the last couple days.
Apparently, a dying cps can act pretty strange.

Had you tested it for both ohms and AC output voltage?

Thanks Hellbent for the follow up and solution. I am also saving a high altitude cps. Cruisers tips are printed out and placed in my FSM

My suggestion would be to grab up the PC307s from RockAuto while you can.

The ones I got were OEM Siemens brand.

I still drill the upper hole though.

i had tested it, and it tested good when cold(i wasn't able to test it hot). although when i got it out, i discovered that the wires/insulation were pretty nasty, chewed up/falling apart right where they go into the sensor body.
i had a factory hi-altitude sensor i'd been saving for a rainy day, so i used that.
i still have another factory renix cps(non hi-alt) that i'll probably modify by sliding the sensor over on the bracket.

How did you test it? Ohms test, AC output test, or both?

I was only able to test ohms, didn't have a helper to crank the motor while I read the voltage.
The really odd thing was that up until it totally died, it always started perfectly-every time. It only had an issue when hot & at higher rpms.

That woulda been great to know. Dang!!