You haven't provided any information that can be used to help find a solution. What makes you think it's running rich? Are you seeing black smoke coming out of the tail pipe? Which sensors did you replace? High fuel consumption is not necessarily the same as running rich. The engine computer will turn on the Check Engine light and memorize a diagnostic fault code related to running rich too long if that's what it sees being reported by the oxygen sensors.
A sticking brake will cause poor fuel mileage but not a rich condition. If you smell unburned fuel by the tail pipe, there are a lot of possible causes. The MAP sensor has the biggest affect on fuel delivery. On older cars, a cracked or leaking vacuum hose going to the sensor will mimic an engine under load that needs more fuel. The sensor is bolted to the engine without using a hose on newer cars. There is a range of acceptable signal voltages the sensor can report to the engine computer. If the sensor is starting to fail, it can report incorrect values, but as long as they stay within the acceptable range, the computer will not detect a problem. The incorrect values result in the wrong amount of fuel entering the engine.
Another often overlooked cause of too much fuel is a leak in the exhaust system before the oxygen sensor. Between each pulse of exhaust flow, the momentum creates pulses of vacuum which draws in outside air. The oxygen sensor sees the unburned oxygen, and reports it to the engine computer which tells the injectors to spray more fuel to go with the air. O2 sensors do not measure fuel, just oxygen, so even though there's too much fuel, the sensor still sees the unburned air. No matter how much extra fuel is requested, the sensor will constantly see a lean condition.
It's important too to understand what happens when you start blindly throwing new parts at the problem without first properly diagnosing it. The engine computer has learned over time the relationship between the various sensors under many different conditions. As an example, the computer knows when the engine is idling based on the reading from the throttle position sensor, and it knows when it's under load based on readings from the MAP sensor. Both could report normal values, but they better not show up at the same time. Similarly, after the engine has been off for a long time, the computer knows the ambient air / battery temperature sensor and the coolant temperature sensor will be at the same temperature. It learns what the voltages are when it compares them to each other. There are a lot of things like this the computer does to constantly verify proper sensor operation.
When an apparent problem exists such as what you described, the computer might make adjustments, and it will try to determine the cause. If it recognizes it's receiving bad information from one sensor, it disregards it and runs as well as possible on pre-programmed approximate values from memory. When you stick in a new sensor, the computer has no choice but to believe the slightly different values because it will not try to learn the new characteristics when it knows there is some other problem. Every part you replace introduces another variable that the computer isn't aware of.
Here's a better example of what can happen. After disconnecting the battery, the engine computer loses its memory. One of the things it forgets is "minimum throttle". Until it relearns that, it will not try to control idle speed, and the engine will stall at stop signs. To relearn minimum throttle, the computer must see high manifold vacuum for at least seven seconds. That can only happen when coasting from highway speeds. When the conditions are met, the computer takes a reading from the throttle position sensor. It knows that's the lowest voltage it will ever see, so from now on, whenever it sees that voltage, it will keep the idle speed properly adjusted. If there is a problem with the MAP sensor or its circuitry that the computer detects, it will not try to update minimum throttle periodically. When you throw a new throttle position sensor on the engine, its lowest voltage at idle might be a few hundredths of a volt higher than the old one. Since the voltage will never reach the "minimum throttle" voltage of the old sensor, the engine will stall at stop signs. Now you have a new problem in addition to the original one, even though there is nothing wrong with the new part.
This is why throwing parts at it without doing the diagnostics first needlessly complicates the problem. If the Check Engine light has been coming on, there will be at least one diagnostic fault code in the engine computer's memory. That code will direct you to the circuit or system with the problem, not necessarily the defective part. Your mechanic will connect a hand-held computer to watch what the O2 sensors are seeing. If they are switching rapidly between rich and lean, they are working correctly. He can also read short and long term fuel trims. Those numbers show how much fuel the computer is adding or subtracting from the base values programmed in at the factory. If the numbers are high negative, the computer knows the mixture is too rich and it's trying to cut back. You need to find out why it's getting too much fuel. If the numbers are high positive, it's adding extra fuel in response to believing there's too much unburned air in the exhaust. Do not disconnect the battery or run it dead because these numbers will all be erased and the valuable information will be lost. Until the computer rebuilds this database, this will be one more variable that could adversely affect engine performance without there really being anything else wrong.
Sunday, February 14th, 2010 AT 10:58 PM