Now we have some more variables to confuse the computer. All sensors are different. The computer learns their characteristics as you drive by comparing their signals to other sensors and known conditions. I doubt replacing the O2 sensors is going cause problems, but randomly replacing parts is the least effective and most costly way to diagnose this type of problem. You'll be money ahead to just have it professionally diagnosed.
I would have told you also to not waste your money on a new fuel filter. Except for diesel trucks, they last the life of the vehicle on Chrysler products unless they start to leak like mine did at 180,000 miles. The pickup screen in the tank will collapse or become plugged before the filter will, and even that is not very uncommon. Replacing the filter will not have any effect on the computer. He won't notice any change from that. Also remember we're dealing with way too much fuel. A plugged filter or screen will reduce volume and pressure. The engine would run poorly but the fuel mileage would not get worse.
Two volts on the MAP sensor at idle seems a little high from the experimenting I did when building "bugged" cars for my students to troubleshoot, but it is within the acceptable range to not set a code. The higher the voltage, the higher the load and the more fuel is needed. Since that voltage could be incorrect, the fastest and easiest way to know if it is causing the problem is to pop a new one or known-good used one on and try it. If I had to guess, I would have suggested around one volt at idle would be normal but if I found two volts on a good-running engine, I'd accept it.
How did you "reset" the computer? If you disconnected the battery, that erased all of the stored fuel trim data it had learned during previous drive cycles. Knowing that is what helps it figure out when there is a problem, and why. Once the battery is reconnected, it has to relearn all of the sensors while there's still a problem present. Any fault codes that were in memory are lost too. That can provide real valuable clues. Part of the strategy the computer uses is to constantly compare sensor readings with each other. As an example, it knows that when the engine has been off for more than six hours, the coolant temperature sensor and ambient air temperature sensor, (or battery temperature sensor), had better be reading the same temperature. If there is a fault code stored for one of them, the computer knows it can't rely on its reading to compare to the other one, so it will not set a fault code for it except for extreme conditions such as being unplugged. Every fault code has a long list of conditions that must be met for that code to be set in memory. Every list usually starts with a whole bunch of "xxx code must not be set". THAT is where mechanics got the notion to disconnect the battery to erase existing fault codes but that is counter-productive. Those original codes that set will be related to the current problem. Erasing them allows the conditions to exist that allows the other codes to set. The computer could have been watching the MAP sensor and trying to figure out which related code to set. Now that all the stored data is wiped out, all the computer might be able to figure out is the exhaust gas is staying too rich too long. That does not indicate a problem with the oxygen sensors; they're just the messengers. I used to read a lot about mechanics resetting GM computers by disconnecting the battery, but we never did that at the Chrysler dealership I worked at. Those '90s GM computers had a lot of issues, but on other brands nothing is going to be magically fixed by erasing the memory and starting over. The fuel trim numbers the computer learns over time are what it bases initial fuel metering on. Those numbers will tell you whether it's trying to reduce fuel without success or whether it's adding fuel in response to incorrect sensor information. As soon as the problem is repaired and the sensor information is correct, the short-term fuel trim numbers will begin to update immediately. You don't have to do anything except drive the vehicle.
As another point of interest, MAP sensors are so sensitive, they could be used to measure engine speed by counting the pulses of intake manifold vacuum each time a piston takes a gulp of air. If you watch the signal voltage on an oscilloscope, you would see four little voltage ripples for each crankshaft revolution on a V-8 engine. Those ripples are a part of how the computer figures out which fault code to set. Electrical codes are easier to understand. If the 5.0 volt supply is good and the ground wire has 0.2 volts, but the signal voltage is wrong, the sensor is suspect. But the computer can also detect pneumatic codes. On older engines that meant a leak in the vacuum hose or a defective sensor. On newer engines the sensor's vacuum connection plugs right into the intake manifold. No vacuum hose is used. While experimenting on prepared cars in the shop, we used to disconnect that vacuum hose and run the sensor with a hand vacuum pump to see how the engine responded. It was very common to set a "MAP Pneumatic" code even though we kept the signal voltage at the proper level. The computer set that code because it saw there were no ripples in the voltage and it knew that wasn't proper operation.
The reason your meter reading was bouncing around was due to those ripples. The meter takes a reading, thinks about it, displays it, then leaves it there while it goes back for another reading to think about. That one might have been taken, (like a snapshot), just when the voltage was at a high spot of that ripple. If you were to read that voltage on a scanner you would see it holding steady.
What you need to do is either drive it long enough to get a code to set or you'll need a scanner to view live data to see what's going on. If the MAP sensor is indeed reporting a load higher than actual, the computer is going to request more fuel. THAT one can easily double fuel consumption.
No matter what the oxygen sensors see, the computer can only modify that initial calculation plus or minus about ten percent based on their readings. If it subtracts fuel, you'll have a noticeable stumble or hesitation on acceleration. If it's adding fuel, you likely won't even notice unless you carefully keep track of your fuel mileage. Another clue when oxygen sensors or their readings are related to the problem is they are not even in the fuel metering calculation until they get to 600 degrees. For many years the O2 sensors have had electric heaters that turn on only when the engine is running to get them up to that temperature faster. Until that happens, the engine will run differently for that minute or two.
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Wednesday, January 18th, 2012 AT 11:12 PM