1987 Jeep Wrangler XJ - The Invisible Gremlins

Camshaft Position Sensor (CPS)
Technically, this one could also be called the CPS, but in the normal usage of things, it is ignored and the CPS is the crank sensor, while this is the red-headed stepchild of the group. Located inside the distributor of early injected engines, it provides the computer with input of the valves' location and typically runs like a top. However, loose connections or sometimes older units have been known to keel over, resulting in intermittent Jeep operation. For those of you who need it spelled out: failure of this sensor will often result in intermittent stalling and dying of the Jeep.

Coolant Temperature Sensor (CTS)
This sensor is located on the driver side of the block - at the top of the head, all the way in the rear corner. We'd really like to tell you that the Jeep will die a horrible death with this unhooked, but the most you might see from the driver seat is a check-engine light. The theory behind this one is that the computer uses this to see how hot the engine is running and adjust the idle speed and fuel mixture accordingly. In the real world, the computer takes more input from the O2, MAP, and TPS than it does the CTS.

Fuel Pump
So it isn't really a sensor, nor does the computer rely on it for input as to how the engine is running. What it does - and does well - is provide the high-zoot, fuel-injected engine with the proper volume and pressure of fuel that is required to run. It is located in the fuel tank (go figure), and a common misconception on the fuel pump is that it is either all or nothing. Well, consider that myth busted. The fact of the matter is that a non-H.O. engine needs to see 35-40 psi, while an H.O. motor needs to see from 40-50 psi of pressure at the fuel rail. Consult your factory service manual for the specifics for your year, but suffice to say if you are running 5-10 or more psi under the specified value, the Jeep will buck, spit, sputter, and generally run horribly. Attach a fuel-pressure gauge to the port on the fuel rail and check pressure at idle, under load, and full throttle against specifications to make sure that's not the problem.

Idle Air Controller (IAC)
More commonly known as the idle air controller, this electrical solenoid controls the amount of air passing into the engine at idle. It is found on the throttle body and through the movement of an electrically controlled cylinder. Even if the butterfly in the throttle body is fully closed, air gets into the engine and it still runs. When the Jeep is first fired up, the engine's rpm should go up to around 2,500. This is a sign that the IAC is working. It allows for a fast idle when the Jeep is cold, and also allows for more air (and, thus, fuel) to get to the engine when there is no throttle input. If you've ever crawled your Jeep, it seems to have a mind of its own when encountering an obstacle - it just doesn't want to die. This is due to the IAC. When the IAC fails, often the Jeep will die at stoplights or at trail obstacles. It could also idle high or erratically. We hate to be a sellout, but consult the FSM pertaining to your particular Jeep for proper testing procedures.

Throttle Position Sensor (TPS)
This could possibly be one of the most maligned sensors in the modern world. The throttle position sensor's purpose is relatively simple: provide the computer with a quick input as to demands made on the engine. If you mash the throttle down, this sensor is the first to tell the computer to advance the timing and dump the fuel; kind of like the secondaries on a carburetor, but way more sophisticated. Look at it as vacuum secondaries in a tuxedo. It is always located on the opposite side of the throttle body from the throttle cable. At heart, the TPS is simply a variable resistor and just alters the 5-volt input signal. In practice, it's a nasty, temperamental sensor that will keel over if it gets wet or too hot. Most common symptoms of a dead TPS are hesitation and backfiring.

Manifold Absolute Pressure (MAP)
Unlike what you might find at your local hiking shop, this MAP provides input to a computer that isn't made by Garmin or Magellan. Located on the firewall behind and above the engine or on the throttle body, the MAP sensor provides the computer with the amount of pressure the atmosphere exerts on the Jeep. Why do you care? Have you ever driven a carbureted Jeep at elevation without changing the jets? At higher elevations, there is less combustible air per cubic yard than at sea level. What that means to a carbureted Jeep is that the mechanically set mixture at sea level for this much air with that much fuel isn't enough anymore, and the amounts need to be doubled or tripled for the engine to run well. In a carbureted rig, this means at higher elevations, the Jeep becomes gutless, and almost not worth driving. The MAP sensor acts like a jet change without any of the nasty seal mess, fuel spills, or setting of float levels. As the elevation changes, and air pressure varies, this sensor tells the computer to increase or decrease the amount of fuel to the engine through the amount of time each injector opens. While it isn't perfect, it does a good job of compensating for a lack of combustible air at higher elevation.

Fuel Pressure Regulator
Well, we hate to say it, but this is another item that does exactly what the name implies. While the computer doesn't care what's going on with the fuel pressure regulator, for the most part, it plays a big role in things. The fuel pump listed above pumps at somewhere between 60 and eleventy billion psi. If the full eleventy billion psi was to go into the injectors, they'd simply explode. So the fuel pressure regulator cuts the pressure on the fuel line down to the specified 35-50 psi (depending on year/make/model) and is located on the fuel rail near the fuel line input or in the gas tank. This is important because the computer is preloaded with "maps," which allow the injectors to open for a certain amount of time. This amount of time that each injector opens (thus, allowing fuel into the engine) is based on a set pressure. Lower that pressure, and the engine runs lean. Raise it, and it runs rich. Again, attach a pressure gauge to the fuel rail to test for the correct pressure.

O2
This little beauty is the backfield safety of the fuel-injection world. It keeps the rest of the electrical soup in check. Too much fuel because of the MAP sensor? The O2 sensor picks it up and tells the computer to cut back. Too much air due to the IAC freaking out? Again, the O2 sensor tattles to the computer and brings everything back to what it should be. "What it should be" is often referred to as "stoichiometric" by the scientist types, which, of course, is a mixture where there is just enough air to completely burn the available fuel. In our case, it indicates a 14.7:1 air to fuel mixture that an engine "should" operate at. If anything else goes haywire in the system, the O2 sensor should catch it and let the computer know to adjust accordingly. Often, if something is out of whack and adjusted due to the oxygen sensor's input, the computer will light the check-engine light. If the O2 sensor dies, often the Jeep will run rich, which can be smelled at the tailpipe, but running rich will also kill the catalytic converter. This sensor should be replaced every third or fourth oil change. Take care of the O2 sensor(s), and it (they) will take care of you.

Want to get more out of your Cherokee lift?
Run Grand Cherokee coils - they're slightly longer but share nearly the same spring rate and diameter. They're a perfect fit.