JeeperJase

I have a 2000 jeep grand cherokee laredo 4.0. I was driving it locally to talk to a customer, had the vehicle approximately 4 months. Was driving perfect, went to my location, got out and talked to my customer. After about 20 mins or so got back into the jeep and fired it up. First thing I noticed was it immediately was running rough as hell. Had to keep gas on it to keep it from stalling and managed to limp it back to the house after a few stall outs approaching stop signs. Put the code reader on it and it was showing a misfire in cylinder 3. I replaced the spark plugs, noticed the coil pack rail was worn with a cracked boot and replaced that as well. Fired it back up and still had the same exact issue, only now it wasn't throwing ANY codes. Long story short, with a bunch of research online, and over the course of 2 months, I ended up replacing my MAP sensor, TPS, CPS, (can't remember the name of it, but the sensor you access through the floorboard near the gas pedal), pulled the throttle body and thoroughly cleaned it, replaced the idle air control valve while I was there, replaced the muffler, and finally after more research changed out all 4 o2 sensors. I thought that one was going to nail it, as one of my o2 sensors (upstream one next to the firewall) had burnt up wires being next to the manifold. All 4 wires were well exposed with about 3 inches of wire with NO insulation around them whatsoever. The other upstream sensor was messed up by the factory, after fighting like hell to get it out I ended up having to re-tap the hole for the sensor to mount flush. (Upstream sensors are Bosch) I also had to extend that wire harness as the sensor wires themselves was too short. After everything was said and done, the end result was the jeep running SLIGHTLY better, at least I could fire it up and it would run without completely stalling out (before all this I was having to keep a rock on the gas pedal to keep it idling &#128534 But overall the main symptoms remained. The entire time there has been a very rich smell to the exhaust with some black soot and liquid coming out of the end, and noticed today there was a gas smell to the oil. I hooked my fuel pressure tester up to the fuel rail and noticed with the ignition on fuel pressure sat at 51, with the motor running it was fast bouncing at 59 and 60. I saw spec called for 49 +/- .5 and after doing some research saw where a faulty fuel pressure regulator can cause the same symptoms I'm experiencing. Also I'll note that before the fuel pressure test I hooked a vacuum gauge directly to the intake manifold and my gauge was fast bouncing rapidly between 0-12 hg, when I increased throttle it was the same fast bounce between roughly 12-18 hg. So far I've been unsuccessful in finding a vacuum leak. Looked for vacuum leak first when I discovered white smoke/steam with relatively no odor originating from somewhere under the intake manifold but could never pinpoint where it was coming from as it's extremely intermittent. Online research said it could basically be either something or nothing 🤣 But my main question is, could it actually be the fuel pressure regulator causing all this? (I'm still showing NO codes whatsoever, stored or pending)Thanks a million guys!

syncview

if its still missing, first thing i would do is check which cylinder or cylinders are causing it.. then proceed any further. Fuel pressure regulator cannot as such cause such symptoms, but a fuel pulsation damper can do.. i dont remember if the xj had a fuel pulsation damper on the rail or not.

BruceXJ

We had what sounds like the same issue with our XJ and we went through many of the fixes you've detailed here. We even took our car to a Jeep 'specialist' 200 miles away who couldn't identify any codes either and the car left the garage exactly the same as how we took it in. Eventually we managed to find an old school mechanic who had a good understanding of older engines, he rebuilt the electrical connector that plugs into the throttle body and it idles/runs a lot better. We also replaced the oxygen lambda sensor to reduce the emissions as our exhaust was kicking out a lot of soot/vapour and steam, that seemed to do the trick. Hope that helps!