This is the best kind of post. Too many uninformed people just throw a lot of parts at their car hoping one will stick and fix the problem, but they're really just inserting a whole new bunch of variables. Also, lots of valuable information to mine for clues. Thank you.
Although I'm not a driveability expert, I can share that the oxygen sensors in the exhaust system only measure unburned oxygen; they do not detect unburned fuel. The computer normally commands the fuel mixture to switch from rich to lean to rich many times per second, then it watches the readings from the O2 sensors to see how they respond. An inactive, (contaminated) O2 sensor won't detect any oxygen. The typical diagnostic fault code would be something to the effect of "O2 sensor not switching properly".
You would think a perfect air / fuel ratio would be ideal for best performance and lowest emissions, and that was true on older cars with carburetors. Catalytic converters in the exhaust system need the lean cycles to introduce extra oxygen that is stored in the material. That oxygen mixes with the extra fuel during the next half cycle, then it is burned to clean up the exhaust. While a perfect mixture is best for emissions, the engine computer would be unable to monitor it without the rich / lean cycles.
As you drive, the engine computer notices that for a proper air / fuel mixture, it has to continually add or subtract a little fuel from the expected value. It puts this information into its memory called "Short Term Fuel Trim". These numbers are continually updated as you drive. After a while, the computer notices that it is always making the same modifications, so it puts the learned values into its "Long Term Fuel Trims". The next time you drive the car, it uses these modified numbers for the various set of conditions as the starting point, then again adjusts fuel delivery around those numbers as necessary.
What's happening in your case is the computer isn't seeing any extra oxygen during the lean part of the cycle. Something is causing extra fuel to be introduced. The computer is trying to subtract fuel from the expected values but is still unable to achieve a lean condition. That's when it sets the diagnostic fault code and turns on the Check Engine light.
The MAP sensor monitors engine vacuum which is directly related to load. Lower vacuum / higher load means the engine needs more fuel. When this sensor reports an incorrect value, it can lead the computer into thinking more fuel is needed. On older cars, this could also be caused by a cracked or deteriorated vacuum hose going to the sensor, but now most of these sensors are screwed directly into the engine. As these sensors fail, they will eventually send a signal that is outside the normal limits. That's when the computer detects the problem and sets the appropriate fault code. Since you didn't have a code related to the MAP sensor, (yet), it is reporting values within normal limits even though they could be incorrect.
To add to the confusion, very specific conditions must be met for the computer to set a code. Besides a simple defect, it also reconciles one sensor reading with the others. For example, a very low voltage from the MAP sensor indicates heavy acceleration. A low voltage from the throttle position sensor indicates the engine is idling or coasting. Even though both readings could be normal, they can't both occur at the same time. The computer has various strategies to figure out which sensor is wrong.
It is also important to understand that the fault codes very often do not specify which part is bad. They indicate the circuit with the problem or the improper operating condition that has been detected. Too many people think the mechanic just hooks up a computer and it tells him which part to replace.
Some fault codes will not be set in memory if other codes are present. The computer compares one sensor to another under various conditions. When a code is present, the computer knows it can't rely on that sensor's reading to compare to other sensors. For that reason, sometimes a defect won't be apparent until the previous problem is fixed. That can lead to additional needed repairs beyond what the original reading of the stored codes found, and that can lead people to think their mechanic is taking advantage of them. It can be a tough position for the mechanic to be put in.
Getting back to the rich condition, a leaky injector can allow too much fuel into the engine. Chrysler has had very little trouble with injectors over the years. GM has had a real lot of trouble. An additional clue to a leaking injector would be an unusually long crank time before the engine starts. The fuel system will normally maintain pressure for weeks without being started. That pressure is needed for the engine to run. A leaking injector will bleed the pressure off. The fuel pump only runs for two seconds when you turn on the ignition switch, (you might be able to hear it), in preparation for starting, but that might not be enough time to build up pressure if it was completely gone. The pump will turn on again when the computer sees engine rotation, (cranking or running). This system is related to Chrysler's very effective and reliable safety shutdown system if a fuel line is ruptured in a crash.
If one cylinder is the cause of the rich condition, a leaky injector for example, the computer will detect that in the exhaust, but it can only control all four injectors as a group, not individually. As a result, it would be subtracting fuel from all four cylinders, including the three cylinders that are working correctly. That is one possible reason for the low power. Three lean cylinders might also explain the rough running condition.
Remember, this is all speculation based on theory. My Auto Electrical course took eight weeks, 20 hours per week to teach electrical theory. Two weeks of that was spent on sensors and that was just normal operation. Later, a different course dealt with diagnosing these circuits, so you can see it's impossible to cover all the potential problems here. I've presented just the tip of the iceberg, but at least what your mechanic tells you might make a little more sense. Hopefully some other people will add additional replies that can shed more light on what to look for on your car.
Saturday, December 12th, 2009 AT 6:23 AM