bigdadys97zj

I had the exact same problem with my 97 Zj . The fix for mine though wasnt the most common for most people . After replacing the crank sensor, cam sensor, and doing a full tune up it didnt get any better. The problem ended up being the ECU. The ignition side of the computer was blanking out and caused all the same issues you are having . i replaced it and its been fine for the last 7 years. I know its not common but it happened to me. Good luck

F1Addict

If you're sure you've got spark (checked at the plugs for bright blue snapping spark) then it's not your CPS or any other ignition related component. If you're sure you've got fuel (checked pressure at the rail with a gauge) then it's not your fuel pump. That leaves 1. your injectors 2. your fuel filter (if pressure drops significantly while cranking) 3. your ECU but despite BigDady's experience, ECU failure is rare or 4. massive failure of an internal component but that's something you'd probably notice so stick with 1 and 2 for now.

I'm pretty sure you can do the key on/off trick to get codes from a '91. You might start there.

SgtP_USMC

Starting to think it's the PCM (for 1996 its no longer ECM/ECU...i know...who cares?)...

Gonna do a spark check at the plug tomorrow night.

SgtP_USMC

Ok Fellas, Sorry I haven't been on in a while...hopefully my original posters are still with me here.

So I just recently tested for spark and I got nothing. Not even a weak spark. I tested from the distributor AND from the ignition coil. I also tested my CPS and it is good. My ASD relay is good because the AC compressor (which works!) relay is the same and I swapped them out and still nothing. That leads to one thing in my book....PCM.

Anyone have any feedback on this?

Potatowalker

What kind of coil did you put in there when you replaced it? Maybe it was defective or one of the contacts came loose? I've heard of people getting cheap PCMs from the pick-n-pulls after finding a donor of the same year/model, etc. if that's something you wanna try. Did you search for any codes? sometimes there is a code stored that doesn't trigger the CEL. Also, personally, I'd check all the wiring and contacts, like alt.-to-coil, grounds, etc. first before going to the PCM though.

SgtP_USMC

Yeah I am going to a salvage yard tomorrow (today) to get a PCM from a donor. I have checked all the wiring for bad grounds/corrosion/elves and I haven't found anything suspicious. I used my OBD II scanner/reader to see if maybe it was throwing codes that werent tripping the CEL (unlikely I know) but also I had a sick feeling that maybe the CEL light was burned out, but I replaced all those annoying little bulbs back behind the instrument panel about a year ago and have not had a problem...scans came up clean "no faults detected". I called Actron and asked if the scanner would pick up a faulty PCM and the guy said no. Before I checked the BRAND NEW ignition coil I thought maybe I needed distributor parts, but after seeing absolutely no spark from the coil I knew it wasn't the right approach. I used my multimeter AND my Dad's (which is a super awesome FLUKE) and neither one of them detected low resistance in the CPS connection. From terminal B to C there is supposed to be infinite resistance from what I understand and if it read anything at all that would be considered low resistance than it was the problem. The CPS is fairly new also, so Im done beating that horse. If the PCM doesn't fix the problem, than I will start doing the unlikely and costly options. I'll test the ignition coil, or put the old one back in, and I'll just replace the CPS for fun. Then I'll put new 'guts' in the distributor, including the shaft and gear (cap and rotor, wires, and plugs were replaced about 2 or 3 months before problems started.

We shall see.

Potatowalker

Good luck with the new PCM, sounds like you've ruled almost everything else out. On a side note, I know in my old 96 and in the 99 I have now, if you turn the key to "On" and leave it there and don't turn it to "start", the CEL will come on for a few seconds and then go off under normal conditions (along with other things like the gen light and airbag light if equipped). If it doesn't come on for a second or two when you do that, then I'd suspect the bulb is shot.

kennyWA

Testing the CPS and it being "good" doesn't mean it's good. Problem with that is when the CPS is nice and cool, the resistance values and continuity all check out. But as soon as the motor gets up to operating temp and the CPS heats up, funky things start happening. Suck it up, buy a CPS, and go from there. My .02 cents.

SgtP_USMC

Kenny,

If what you say is true, that the CPS malfunctions when it gets hot and works fine cold, then how does that explain the restarts I described in the beginning of the thread? It would start back up and run fine, and hot, all day. But now, it wont start at all. And it's cold all the time. I don't doubt that the CPS could still be screwed up somehow, even though my testers say otherwise. But if it works fine cold and gets bad hot, then I should be able to start the thing...and get spark from the ignition coil.

I have to wait until Friday before I can buy anything for the Jeep, and a CPS is on my list. I know its a waste of money, unless someone out there can offer an explaination for its malfunction that coincides with whats happening. I've tried it all (as far as I can tell), I just hate to drop $300 on a new pcm and another $70 on a sensor without being sure. It goes against my nature as a Marine and as a guy who builds w*****s for the gov't to just throw resources into something without knowing why I'm doing it. Thats more like something Obama would do. Yuck. But I guess I'll have to. I called the Jeep dealer to ask if I can bring my pcm in for testing and he said I'd have to drive the Jeep over so they could test it there. I laughed at him! If my Jeep ran I wouldn't be needing to test the pcm would I? He said I could get it towed, but the cost of towing there and back would almost pay for a new one.

F**k it. I'm ranting now. I'm just going to buy the crap and be done with it. I'll let you fella know next week what happens. I'm not budgeted for Jeep parts this week.

Thanks for reading anyway.

xj_maniac_newb

Check the wires. If the wires are run close to the exhaust manifold you'll get whats happening to yours. Same thing happened to mine at one point, moved the wires and never had a problem since.

kennyWA

That's why I'm saying start with the CPS. Cheaper than the PCM. And I know it doesn't make sense that the CPS would work at certain times and not work at others. I had a 3rd gen Eclipse in my shop that Mitsubishi sent to us because they couldn't figure the problem out. Signs pointed to bad CPS. We put three different brands of CPS on the car, they would all start and run fine. Sometimes for days, sometimes for an hour, sometimes for 2 minutes. But the car would still die eventually. I finally hooked an oscilloscope up to the sensor and watched the signal while it was running. When the car would start to sputter and die out, I noticed gaps in the CPS signal. We finally tried a Mitsubishi, OEM CPS, which is what you would have thought Mitsubishi would have done from the get go, and ran perfect. I'm just trying to give you an example of how finicky a CPS can be. I still say buy the CPS and give it a shot, atleast if it doesn't work, you'll know you won't have to worry about it for a while. Good luck, man.

SgtP_USMC

xjmaniac,

The CPS wiring is only about a foot long and doesn't come close to the exhaust manifold in any way. Its close to the intake manifold, but it mounts to the transmission bell housing at about the 1 o'clock position, too high to be close to the exhaust manifold. I couldn't run it next to the exhaust if I tried.

Kenny,

I went ahead and replaced the CPS, and it did nothing. I tested it alongside the old one with my multimeter and they both gave the same readings. I installed it anyway and got no dice. I've checked all the fuses in the Power Distribution box that pertain to the ASD and ignition and everything looks normal.

Is there something I am missing? Is there some other component besides the CPS, ASD relay/fuses, and PCM that controls the spark to the ignition coil?????????????????????????????????????????????? ?????????????

MtnHermit

Not likely your problem, but . . . .

Have you checked the cable between the engine and firewall? It was the source of my mystery misfire.





In my case it was the sharp end of the fuel manifold. Others had one of the head bolts cut/ground the wires. Cheap fix.

I spent hundreds of $$$ chasing my tail on idiot codes and "professional" jeep mechanics before this simple defect was found.

SgtP_USMC

Hermit,

I wish it was a misfire. You may want to read the entire thread so you can understand whats happening. The ignition system is the problem, but there is no spark whatsoever.

Plugs are new, Distributor isn't getting any spark from the NEW ignition coil, the CPS is brand new as of yesterday and the ASD relay and fuses are all good. All wiring is visually intact and the Jeep was shutting off intermittently until it shut off for good.

Fuel system and air system are not the problem. Fuel pressure is strong, pump is fine.

Read the thread to get the full history, but I've ruled out everything but the PCM, I'm going to order one tomorrow, but if anyone has any suggestions that pertain to what the issue is I'm all ears. Thanks

Radi

Do you have +12V power to the coil with the key on? There is a primary connector (plug) with two wires on the coil. One supplies +12V from the ASD relay, the other wire goes to the PCM. PCM grounds the coil, opening the ground connection momentarily to fire it.
Need to determine which of those two signals is missing so you know which direction to look.