OK, look, before you go firing up the hate-mail generators, we know that picture isn't of a fuel-injected Jeep, and we aren't going to tell you how to take apart your IAC solenoid, replace the diode and capacitor inside it, and then put it back in your Jeep good as new.
Even if there were a cap and diode in there, why in the world would anyone pull it apart with intents of reinstalling it in the Jeep in the first place? It would likely just die again the first time humidity got above 50 percent, much less crossing a stream. Just go buy a new one and enjoy the quasi-reliability of a new parts-store electronic part.
We've all been to the point where we want to just beat on it with the biggest hammer we've got, but what we are trying to accomplish is to familiarize everyone with the widgets and gee-gaws that make the 4.0 H.O. engine work, explain what they are, and talk about possible symptoms when they go bad. We aren't going to cover all the gizmos, just the ones that are common throughout the 4.0 inline-six cylinder's run.
If you've got something other than a 4.0 H.O. engine, don't fret. The gizmos we talk about show up on most every multiport-injected Jeep engine, so this applies to you too.
Alternator Whether or not...
Alternator
Whether or not you like it, agree with it, or accept it, the bedrock of a modern fuel-injection system is the alternator. It is located on the front of the engine in various positions, and if the voltage drops too low, nothing in the system is going to work correctly. Even though most sensors operate at 5 volts DC, if the system voltage is low, things start going wacky. Make sure the alternator is in good shape and properly charging the battery.
C-101 Connector OK, you got...
C-101 Connector
OK, you got us. This one only exists in two or three years of the XJ and MJ. It can be found in '87-'89 vehicles, so it's not really pertinent to most of us, nor is it really a sensor. That said, it is evil. Pure evil. When the Frenchies designed this thing, they had nothing in mind except the torture of future generations of Jeep owners. It is found on the driver side of the engine compartment and high up on the firewall. All the wires coming out of the interior go into one side of this, while the other side goes out to the body and engine. In theory, it made it easier to assemble the wiring harness in the vehicle. In practice, it is the devil! The connectors become fouled with road debris, oil, and other contaminants - not to mention the metal bolt holding the plastic part together. If you know someone with this and they ever have any problems with the Jeep running, suggest they jiggle the C-101 connector.
Crank Position Sensor (CPS)...
Crank Position Sensor (CPS)
Also known as the crank position sensor, this one is important. OK, all the sensors are important, but this one possibly the most. It is located on the bellhousing near the engine either at the 11 o'clock position or the 3 o'clock position. Without signal from this, the Jeep just won't run. That means if there is a short in the wire, it will die. If you unplug your buddy's CPS, his Jeep will never start. It senses (as the name implies) the position of the crankshaft and allows the engine computer to figure out when to fire the spark plugs and simply let it know it's running.