Imperial Home Page -> Imperials by Year -> 1990 -> Repairs & Maintenance -> Fuel System
The fuel system on this car consists of the fuel tank, fuel pump, fuel filter, fuel lines, fuel rail, fuel regulator, fuel injectors, and the computer system.
The fuel pump sends fuel from the tank through the filter and lines to the fuel regulator and then the fuel rail. The fuel regulators (both stock and after market) controls the fuel pressure (kept at 60 psi) by controlling the flow of the fuel return line. The fuel return line sends unused fuel back to the tank. The fuel rails are like fuel lines, only they bring the fuel to the fuel injectors.
But how does the car know when and how much fuel to use?
In a "nut shell", the computer takes inputs from a few sensors, and compiles the data onto a preprogrammed data chart. The chart then says that under specific inputs, the fuel input should be "X". "X" is defined by the combination of all those data inputs. 'course its allot more involved then that, but that's the basic idea.
In a more "in depth" description, there are several sensors within this car, a few of which are: the oxygen (O2) sensor, MAP sensor, an air temp sensor, and then the sensors associated with the ignition system. The oxygen sensor is used as a monitor of the A/F ratio of the engine. The MAP sensor monitors the pressure (or vacuum) of the intake manifold. The air temp sensor watches the temperature (and thus density) of the intake air.
The car's A/F ratio will vary with the status of the car.
At idle, the computer system keeps the engine lean. This is to ensure low gas usage, and also low engine RPM's. This is thus an other reason why idling is bad for an engine! During idle, the computer basically uses a tach signal (crank sensor) to determine the RPM's. Then, the computer will know the car is at idle, and will lower rate in which the injectors fire fuel into the engine. The oxygen sensor is used to find how lean or rich the A/F is based on oxygen in the exhaust gases. If things are too rich, the computer will slow fuel input farther. The MAP sensor is used to also determine how much fuel should be used. The throttle body idle control motor will control how much air enters the engine.
At stop and go driving (whenever not at idle or WOT), the computer will vary the A/F based on the O2 sensor. Basically the car switches from rich to lean very quickly. Basically the computer wants things to be rich whenever the gas pedal is pressured, and lean when it isn't. The MAP is also used to determine how much fuel should be used. Basically the oxygen sensor is just a fine-tune for this procedure (but still needed!). The computer will also use the TB position sensor to determine how much acceleration the driver wants. I.e. at WOT the TB is wide open, at idle it is almost closed. During driving, it will vary in between. Different air inputs mean different fuel requirements.
At WOT, the computer will keep the A/F VERY rich. That way there is no chance of engine knock, or a lean condition at WOT (both of which would be harmful to the engine!). The TB position sensor is used to tell when WOT occurs (and thus when things need to get so rich). Since WOT is programmed to be as rich as the computer can make the A/F ratio, the 02 sensor is ignored (though it still operates, its just that the PCM ignores the signals).
People who modify the 3.3/3.8 with boost, or other modifications which effect a lot of the air input, may lower the richness at WOT. This is because the stock fuel and computer system will put max fuel in at WOT, and if WOT starts getting far more air then any stock condition, that fuel won't be enough to stay as rich as the guys at Chrysler wanted. The only result is the need for more fuel at WOT. This can be done any number of ways, from extra injectors, to larger injectors. An other thing to consider with these modifications is boost. The stock MAP sensor will only go to 1 psi of boost. So if your over 1 psi, the PCM will still think that you are at 1 psi and thus you don't get enough fuel. The computer will (with the exception of WOT) see that the fuel isn't enough (thanks mostly to the 02 sensor) and will put more in. And thus a stock fuel system can handle boost for the most part until WOT occurs, or when fuel input to the engine equals fuel input at WOT. Not bad for a non-boost computer! Cool, uh?
This page was last updated on October 2, 2003. Send us your feedback, and come join the Imperial Mailing List - Online Car Club