In the last three years, my 1997 TJ 4.0 has hesitated at specific road locations, which was very weird. Some hesitations were on hills while others were on flat ground, and they were always at the same places. I started replacing all of the pertinent sensors, including the upstream and downstream O2, Cam Shaft, crankshaft, and MAP. I then cleaned the throttle body, replaced the fuel injectors, changed the plugs, plug wires, ignition coil pack, distributor cap, and distributor. Next, I replaced the muffler and catalytic converter with no luck. It was an annoyance, but not a big issue until last month when it got way worse. It would idle so bad that it sounded like it was going to stall. It did this for about ten minutes upon the first startup of the day. What was even weirder was that once it stopped hesitating from the first startup of the day, it was fine until the next morning. I could drive it into the office, sit for eight hours, and it started right up that evening with no hesitation. After replacing the distributor, the only thing left was the ECM. I ordered a new one from Flagship One in NYC. They are SLOW but good. It took three weeks until I got the new ECM. Don't waste your money on two-day shipping that costs $35, or one-day at the cost of $70. Finally, we replaced the ECM last week, and it now runs great.
NRA Life Member