I have a 1999 Jeep Cherokee that has had a problem for several years. After descending a long hill in gear (it’s a five-speed), it will spit, sputter, and buck upon resuming acceleration. After a brief period of this, it resumes running smoothly. Also, each time I shut it off during a typical day, it starts right up and runs smoothly for a few moments, then spits and sputters briefly before smoothing out again. If I just let it idle upon restart, it runs smoothly for about 45 seconds, then runs rough for 30-45 seconds, and then it will idle smoothly once again.
Early on the check engine light would come on, so I took it to a local Jeep dealership. They could not find any problem and told me to replace the oxygen sensor. I have done this twice, yet the problem continues. I have changed the distributor cap, rotor, plugs, and plug wires. I have added a PowerAid throttle body spacer and a cold-air intake. I use Lucas fuel treatment at each fill-up, which seems to help, but the problem is still there. Any help you or your readers could provide would be appreciated.
A surefire way to know what is going on would be to pull the codes that are causing the check engine light, but it sounds like it is not throwing codes all the time. This might seem odd, but we would check the fuel pressure first. The 4.0L engine in your Cherokee needs just three things to run beyond all the obvious stuff (distributor cap, rotor, wires, plugs, fuel, air, and so on). It needs a functioning crank position sensor, a functioning cam position sensor, and fuel pressure.
The crank position sensor is mounted near the top of the bellhousing right on the back of the engine. They are known for going bad, and that’s why you should have one in your glovebox. But they usually either work or they don’t, so they are often the culprit of an engine that won’t start rather than one with a rough idle or a miss. The cam position sensor is located in the distributor under the cap and rotor. Though they are less prone to failure, we have heard of situations where they become intermittent once an engine is warmed up. But again, they usually either work or they don’t. The crank position sensor controls the ignition, while the cam position sensor in the distributor controls the fuel injectors (go figure). You could replace one or both of those (again, it would be good to carry spares), but before you do any of that, rent or buy a fuel pressure gauge.
There should be a port right on the fuel rail for the injectors that makes checking pressure easy, and the specification for your Cherokee is 49 psi plus or minus 2. We’ve had weak fuel pumps do very similar things to what you are describing, especially once the vehicle has been running for a little while, and it rarely sets a trouble code. Low fuel pressure can also aggravate vaporlock, which is another common problem among 4.0L-powered Cherokees. Check the pressure under a variety of conditions but especially when the engine starts running rough; the pressure should always be pretty steady regardless of the conditions. If it’s weak or drops briefly and then pops back up where it’s supposed to be, the fuel pump or regulator is bad. We’ve had weak fuel pumps last for a long time, sometimes years, before expiring. And again, a weak pump won’t always set a code. Replacing the pump requires dropping the fuel tank, and we would recommend replacing the entire pickup assembly. Though expensive, the assembly includes the pump as well as the regulator, which is built into the assembly and not serviceable.
If fuel pressure checks out, then it could be a situation where the engine sputters going from open to closed loop. The most likely culprit for this would be a bad oxygen sensor. You mentioned replacing it twice, but are you aware there are two on your Cherokee? There’s one before the catalytic convertor and one after. Problems of this nature should be causing a trouble code, which in turn should take you right to the culprit.
If the oxygen sensors check out we would move on to the IAC and throttle position sensors, but it sounds like both of those are functioning as they should. Again, if these or other sensors are malfunctioning, there should be codes. If everything else checks out then you might look at replacing the ECM, but we would not do that until absolutely every other possibility has been explored.