AeroDawg13

Hi guys and gals, first time posting but a long time lurker. You guys have helped me out of more than a few jams!

But I've got a curious "problem." I've got a 98 ZJ 4.0L Laredo 4x4 with 190K miles. In the mornings and evenings after work (my truck's been sitting for >3hrs) my engine will stumble at around 1200rpm. I crank it up in the morning to go to work and with light gas pedal pressure to get the engine to 1200, it will stumble very quickly with the tach either jumping up briefly or the dropping down briefly. But very quick, like possibly up to 3 stumbles per second! Even driving down the road with steady gas pressure, at around 1200 rpm, it'll stumble, maybe multiple times.

I can sit in the driveway and make it stumble or it'll stumble on the road moving, reverse or forward. Above and below around 1200 rpm and the engine is as smooth as it's ever been.

After driving for 5 minutes or so, the problem goes away. What I usually do in the mornings is turn the OD off, that way the engine stays in each gear longer and never actually dips back down to 1200 rpm on my commute. And after 5 to 10 minutes I hit the OD button again, achieve 4th gear, and everything is smooth again.

I have changed out a lot of things in order to find this particular faulty part. Crank Position Sensor, Throttle Position Sensor, Pick Up Coil (cam sensor), Idle air Controller, Ignition Coil, Distributor and plugs with wires. Ive Jiggled the brake pedal to get the Torque Converter to lock and unlock to no avail. I've adjust the TV cable back to spec. I've checked vacuum lines to find a leak. I've checked fuel pressure, should be good(49-50psi +-6psi at rail.) I had the computer harness connector problem where the connectors were pulling themselves off the computer. So I fixed that with a metal square tube and giant band clamps to clamp everything back together.

Other than checking solenoids in the transmission(which I don't think is the problem; it does this stumble in park, neutral, drive and reverse) I don't know what it could be. I do know that the next item to repair is my fuel pump, as the level gauge has a problem reading too low. It's always had a dead spot around the middle reading of the tank (gauge dips to empty and back to full quickly at one particular spot in the range of movement) and now it's reading empty when it still has 5-7 gallons left.

I've searched the forums and internet for this particular problem to no avail. My Jeep does this on level ground, up or down hill, around turns, from stops or going 40mph. Don't know.

AeroDawg13

Well, my problem is evolving. I've had the stumble issue for around 6 months now, and this past weekend the stumble has crept up into the warm engine range too. Still completely random, but when in 4th gear (usually coasting somewhere in the 1-2k rpm range) the tach starts going nuts. The torque converter has started to lock and unlock randomly and the computer tries to increase the rpms to get the tranny out of the troublesome gear. Still no codes, which tells me probably a computer problem. But when I turn OD off, things smooth out, although rougher shifts and higher rpms, like when normally driving with OD off.

I also noticed while driving this weekend, when letting off the gas, the rpms would drop very quickly, then rebound just as fast, then drop again, then rebound, until either the brake or gas was applied again. WEIRD.

Anyone have an idea what the process-logic is from throttle-throttle body-TPS-computer-TCM-transmission? This problem is extremely frustrating. Thanks.

AeroDawg13

Well in the last two days Ive installed and driven 100 miles with a new-ish ECU and the stumble cropped up again. So my one big fix didn't fix the problem. I'm afraid at this point I'm going hunting for a short or frayed wire. Still doesn't make sense that it would only stumble while cold and stop 5 minutes after warming up. I'm open to any suggestions you guys have.

AeroDawg13

My Jeep's slowly killing itself now. It starting stalling at stops and while driving, even in the middle of turns. Still has the stumble/mis-fire around 1200rpm, hot or cold. I've recently replaced the MAP Sensor thinking it was an air flow/fuel mixture problem. $500 in new parts and sensors in the last six months and the problem has gotten so much worse. Now I'm looking at new distributor system (might possibly be timing issue), new fuel injectors (might not be fully closing), trace engine ground wires (due to ground-switching fuel injectors), short in wire harness (faulty info randomly sent to ECU), odd malfunctioning engine temp sensor, odd functioning battery sensor (if that even exists), replace the battery (4 years old), short in ECU harness connector (flat bar stock fixed the engine stall while shaking the connections at the ECU), new fuel pump module (possibly dirty fuel filter, faulty pressure regulator or pump), new plugs & wires (due to flooding/stalling), or maybe even the torque converter or TCC solenoid.
This thing is getting out of hand. Does anybody have any ideas?