Hi. I have an 01 Grand Cherokee 4.0 I just got. Once it gets up to temp, it starts to miss bad. When it's cold, it's runs pretty smooth. I thought it was the coil pack so I put a new one on. Same deal. It also had no oil pressure and had a top end noise. I changed the oil pump, but I didn't prime it by opening it up before installing and filling it with petroleum jelly. However, i pulled the electronic ignition sensor on the block and spun the pump with a drill and got the oil pressure up, and reinstalled the ignition gear unit. It ran quiet and smooth til it warmed up and then started missing again. After additional research, I think it may be the upstream O2 sensor causing the rough running when it warms up. Also, the fan wasn't turning on when it does warm up. Soooo, I decided to temporarily hard wire the fan to run whenever the key is on. The trouble is, I was having a hard time finding switched power under the hood. I unplugged lots of sensors and connectors, INCLUDING 2 OF THE THREE CONNECTORS ON THE COMPUTER ON THE PASSENGER FIREWALL, and used the red fan wire as a tester, and popped several fuses in the process, but eventually, I found switched power feeding the wiper motor.
After I was all done, I replaced blown fuses that I could find, and proceeded to start the vehicle, which was driven into the garage running pretty good before I started clowning around with the wiring. Now it cranks but won't start, and the yellow security key light comes on, goes out momentarily, and then comes back on steady. That's where I'm at with this thing. I figured on possibly changing the O2 sensor once I saw it staying cool, but I want to see it running again first.
After I was all done, I replaced blown fuses that I could find, and proceeded to start the vehicle, which was driven into the garage running pretty good before I started clowning around with the wiring. Now it cranks but won't start, and the yellow security key light comes on, goes out momentarily, and then comes back on steady. That's where I'm at with this thing. I figured on possibly changing the O2 sensor once I saw it staying cool, but I want to see it running again first.