Helping a friend in need...

99SJ_Ex

99SJ_Ex

NAXJA Forum User
Location
La Crosse, WI
Okay guys, I have a friend that has a 96 Grand Cherokee with the standard I6, AW4 drive-line in it.

Heres the problem:
Cold starts up to when it warms up, the Jeep misses like no other. The range were it misses is around the 3000RPM mark. Sometimes the Service Engine soon light comes on, sometimes it doesn't. Sometimes it flashes then goes solid.

We've read the codes numerous times and its a mix of 02 sensor malfunctions, cat failure, and misfires on one or all the cylinders.

Things that have been replaced prior to him talking to me about it.
o2 sensors
Plugs
Wires
Cap
Rotor
CPS
Coolant Temp Sensor
IAC
Fuel Filter


For the life of me, I cannot figure out what is wrong with this thing... Any ideas?
 
First inklin was CPS since it causes a lot of start issues... But you replaced it. Hell you replaced everything off my list.

May want to try the engine Harness or something that goes to all these sensors. You may have a bare wire somewhere grounding out intermittently. Could be a bad ground. IDK.

Sorry
 
If it happens at a certain rpm I would think about checking the tps. I knew I've had mine go bad quite afew times and it threw some odd ball codes a few times
 
Rob - Thats what I was thinking, but then its up at the 3000RPM range and once it warms up it goes, which lead me to believe coolant temp sensor, but that didn't do it either...

Brendan - We're going to try that next, just gotta find my multi-meter...
 
check ALL the vacuum lines. my 94 zj did a similar thing. when i first started it, it would misfire and and try to idle at like 300 rpm's(that's not a typo,yes like 300 rpm's) and act all retarded and as it would warm up it would start loping like it was cammed or something and then once good and warm it would go away but would still act up kinda randomly. racked my brain for about a week and a half and it wound up being a tiny little vacuum line(maybe 1/8" big around) down below the fuel injection.
 
shoot a PM to Collin (BIGSLVRXJ) and link him to this thread or fill him in via PM. He worked on a very similar deal with is brothers ZJ...it was something in the distributor I believe.
 
Sent it, I really hope we can fix this, he's interested in going to WF if his Jeep would stop breaking (essentially all the high miles maintenance stuff has been going bad finally...)
 
Hey Austin, the issue Tyler was referring to was this. My brother has a 95 ZJ 4.0. We had an issue with his where it wouldn't even start and it was the Camshaft Position Sensor (the plate sandwiched between the distributor cap and distributor itself). This part is also called the pickup I believe. Anyway, it would be worth a shot if you can score a verified good one cheap. Again though his issue was slightly different than yours, so I'm not sure if that's even a possibility, I just know on his ZJ, we had tried everything else, and that was what it turned out to be. Hope it helped.
-Collin
 
I had a xj that done some thing simmilar to this once turns out it did not have power to the heater side of the o2 sensor. There is a fuse in the fuse box on late model xjs for them.I dont remember what the voltage was to be at the plug down at the o2 sensor but i had none . triped all kinds of code 02s,cat,colant temp sensor. replaced fuse and o2 sensors again and all was good.
 
I was going to bring up the same thing jcshaney did about the fuse. I didn't see you mention that you checked it.

I'm not totally sure if the layout is the same for the '96 like it is on my '98 but under the hood on the pass side is a fuse box. There's a 15amp (blue) fuse that is for the o2 sensors. (there should be a diagram under the lid of the box) This could be blown which is a common problem. If that fuse is blown it won't matter if you changed the sensors or not. My '98 runs just like you discribed if that fuse is blown. It may not be the problem but a couple of minutes will tell you if it is.

edit: I now see its a Grand but that shouldn't make a difference...
 
Thanks guys, I got a list of 3 or 4 things to check out now and luckily, some easy swaps with sensors from my motor might make a quick job of this... Its appreciated!
 
Go ahead and check those items out that were mentioned. But I'm just gonna throw it out there that I think he has a Cat problem.

I see the same kind of thing on Subie's. I know its not a Jeep, but same principle. When the vehicle is first started the computer is in Open Loop(cold). When it warms up and runs all the checks for the computer, it goes into Closed Loop mode(warm). Then it see's the odd readings from the O2 sensor, not enough to throw a hard Code on at that time, but as soon as you rev it up, Boom, it misfires and on comes the Check Engine light.

After you do your checklist that you mentioned earlier, if you have the capability, erase all the codes in the computer. Take apart the exhaust after the downtube, where the Cat and downtube meet, and start it up again. Let it warm up and see if when you rev it up, if it misfires. That will tell you if its the Cat for sure. It will probably be loud, but oh well.

Let us know what you find. Keep us updated.
 
okay guys heres the scoop.

Pulled Cat off to find that the inside has broken free, ran it as such and it still misfired bad.
Replaced Cat and it still misfired bad.

So I set my eye's back onto the ignition system cause that was really the only thing that made sense to me... So I went back through the entire system :D
Spark Plugs - Good. Wires - Good. Cap/Rotor - Good. Etc...

Got down to the ignition coil and unbolted it. Looked good, tested good... Decided that I needed to do some heavy bench testing on this thing. Set in Vice. Hooked up Multi-meter. Broke out Heat gun. Heated it up to approx what it would be when running (verified this temp by running my jeep and measuring the temp in that location).

Lo' an behold, a nice crack appeared on the backside of the coil. Threw on my old coil (had replaced some time ago with a higher voltage one) and boom, misfire gone.

Thanks for the help guys, without it I would probably still be scratching my head...
 
Thanks for keeping us posted on what fixed it. I'll add that to my list of remedies that I might happen to need one day. You never know when something like this will come in handy.
 
Glad you got it fixed but I noticed something......if its a 96 there is no AW4 in it.
 
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