JeeperJase

Awhile back I bought my first jeep cherokee, a 2000 with 4wd and the inline 6 4.0. Everything on it is stock. It ran great for several months, one day I drove it about 6 miles down the road to talk to a customer. Drove perfect going down there. Was there about 15-20 minutes and as soon as I started it up to drive home the problem was immediate. Was running really rough, would stall out approaching stop signs. Had to keep giving gas and brake at the same time to keep her from stalling, idle wouldn't hold and it would stall and I'd be lucky if I could get it running again, wanted to just crank. Anyways, over the course of several months and bouncing between forums and you tube channels, this was everything I had replaced: Started with the spark plugs and coil pack rail as the code reader was showing a misfire on one of the cylinders. That took care of the code but not the problem. I replaced the MAP sensor, tps, cps, I pulled the throttle body and thoroughly cleaned it, replacing I IAC valve at the same time. After doing all that I could finally start it and not have to rely on a rock on the gas pedal to hold idle but overall the symptoms were still there. Ran a fuel pressure test and vacuum, fuel pressure was 10 above normal so I replaced the fuel pressure regulator which put it back in normal range. The vacuum was an issue, plugging it right into the manifold showed the needle doing a wild fluctuation at idle bouncing between 0-10 in hg. Very fast bouncing. So I tore the motor down to replace the manifold gasket, put in a felpro gasket and torqued everything to spec after giving the manifold a thorough cleaning and inspected for cracks, etc. Also changed out my 02 sensors while they were exposed with Bosch sensors on the upstream. Had to extend a harness to make it work as one was too short. (Note on that: One of the upstream sensor wires had completely burned through, all 4 wires were bare at the sensor for about 2-3 inches and sensors themselves covered in soot). Also while my manifold was down I inspected my injectors, they were grimy as hell and a few were cracked on the housing. So I replaced all 6, also installed a new heat sheild along with fuel rail and injector wrap to fight any future heat soak. (Old sheild was beyond deteriorated). Gave her a fresh oil and filter change, oil was kind of gunky and reeked of gas, fired it up and at first it was beautiful. Smooth start up, smooth run for about 5 seconds then as the idle dropped down started back to the same symptoms. It'll stay running, however rough enough to shake the jeep, runs very rich as in my eyes started to burn (in the garage, not in the jeep), vacuum doing the hard bounce between 0-10 in Hg. Read somewhere about a piece of paper at the exhaust which resulted in it being pushed away and suking back up over and over. I'm at a loss and can't afford sprinkling money all over this thing, any one have anything similar? And one more note lol.... damn thing isn't throwing a single code and hasn't since the original misfire code at the beginning. And I have no idea if this is related or not, more than likely not but at this point I wouldn't be surprised if a loose will lug is causing all these problems LMAO. Heater isn't working properly, as in you can't switch between the fronts lower vents or defrost. I feel like a hobbit lost on his quest to Mordor

exasemech

It's always risky throwing parts at something without doing diagnostics, especially given the bad replacement parts that are around. With that said, in no particular order, I would:

Check the compression

Use your scan tool to check fuel trim. Is the fuel you're smelling because it's being driven rich or because of a mechanical caused misfire
Use your scan tool to see if the oxygen sensors are actually working... If they were shorted when the insulation burned off that could cause a lot of problems

Look for vacuum leaks in the HVAC system

JeeperJase

Thanks for the reply, The compression test is the only test I haven't done as I don't have a compression tester yet, no misfires from the cylinders. I did a vacuum test which a buddy confirmed pointed to the intake manifold gasket and also had codes pointing to the 02 sensors, which I replaced with Bosch sensors as well as the manifold gasket. All the bolts were torqued back to spec.
I haven't learned my way around the code reader enough yet to learn what the fuel trim specs need to be, any suggestions on that? I did do a fuel pressure test which was high which is what led me to replace the fuel regulator. The pump is fine. The injectors were replaced because they were original and all kinds of gunked up, they needed to be replaced regardless

JeeperJase

Oh and what do I need to look for on the scan tool to confirm the 02 sensors are working?

bluejeep2001

Disconnect both front O2 sensor connectors. It will throw a code but take them out of the picture.see if it smoothes out. Have you done a compression test. The fuel regulator on a 2000 is in the fuel tank...you changed that?