^^^ i also agree with the above

 

O2 sensors are not monitored during Open loop mode? That dosent sound right. Im sure they are monitored in the Renix year(87-90) 4.0s.

 

scanner probs there is a write up ive read about some scanners not working on xjs did you try the key on key off if it appls to your year

 

It's right. OBDII is totally different from Renix.

 

I sounds like it could be a bad coolant temp sensor that is telling the PCM the engine is cold and to add more fuel. The only way to tell is to use a scan tool, not a code reader, and monitor the live data stream watching the CTS and have an infared temp gun handy so you can compare the CTS reading and the temp gun reading. A bad CTS will not always set a code, so it can get very confusing to diagnose if you're not sure how the sensors work and relay info to the PCM.

 

Ales the most sense. I don't have any better advise. This is my GUESS. Lol

 

Anything ever come of this? I also have a 99 4.0L, and I am having the same problem. Except my IS throwing CELs, for O2, repeatedly & the MAP sensor once. Also, the 15 AMP downstream O2 fuse has blown, repeatedly.

I've replaced both O2s, MAP, TPS. Checked for vacuum leaks. Replaced cap, rotor, plugs, wires. Cleaned the throttle body, ran seafoam through the intake and the gas tank. Cleared the CEL after each, came back on after a bit of driving.

Someone mentioned a CTS, and how to test it, which I haven't tried yet, so I'll look into that. Buddy of mine has a scanner & IR temp gun I can use.

Someone else mentioned the cat being clogged. That's about what it 'feels' like to me, would also explain the O2 fuse. Is there a definitive way to test this?

Been a member of the forum for a while now, done lots of reading, but this is the first I've posted, so be gentle if I f'd something up. Read several threads with similar issues, but most without conclusion.

 

Just thought i'd chime in...I had a similar problem, turns out the harness side of the upstream o2 sensor connector was corroded. Took connector apart cleaned and it hasn't skipped a beat in a year. Also it never turned a cel. chased it for a month, driving with scanners, fuel pressure guages etc.
Good luck

 

Injectors leaking down

 

check to see if your distributor is tight....press down on the distributor cap while it is running and see if it stops runnin crappy

 

FWIW,
The OP hasn't been on this forum since 6-13-2011.

I'm thinkin' he won't be replying to this thread.....

 

Thanks for the tip, checked all the wiring and connectors. And I'd previously inspected all the wiring to the O2s under the Jeep looking for damage, unfortunately for me, I'm not getting off that easy.

Distributor's fairly new, but I checked anyway, just in case I'd fowled it up somehow. Still no luck, but I appreciate the tip.

I think I've narrowed it to one of three things; The fuel filter/pressure regulator, the coil, or the PCM. I've only owned the Jeep for about 6 months, and I'm not sure if the fuel filter has ever been changed, figure I'll start there, 'cause she probably needs it anyway.

Still I appreciate the input. If I figure it out, I'll let you know what it was. If not, I'll be back to ask for help, lol. Thanks guys!

Off to drop the tank . . .

 

Find the short/cut/burn in your O2 wiring.

No it wouldn't. The fuse is on the sensor's heater circuit - not it's sensor circuit. It won't blow that fuse unless there is a short to ground - or the heater in the sensor is shorted.

Backpressure test

 

Did you look at your fuel pressure or are you narrowing down using The Force?

Possible - they can do flakey things once they heat up.

Last resort - very expensive to "try"

I've seen situations where dirt was being sucked into the pump/filter and causing a restriction. When the restriction caused the pressure to drop enough the dirt would backflow. Once the pressure built up again in came the dirt. Rinse lather repeat.

 

A really easy check:
There is a gang of wires coming up the front of the engine that run up around the manifold heat shield to the injectors. (at least there is on my 90). The edge of that shield can cut into those wires, sometimes causing all sorts of trouble. Mine would go a week without a problem, was pretty tough to find.