Tuesday, March 13, 2012

ECM misfueling Engine?

In this case customer noticed multiple problems with his V8 Engine car.

The vehicle was misfiring,it was running rough.
There were several OBD error codes retrieved.

But on testing Bank 1 and Bank 2 sensor reading.The readings are different.

As per that the troubleshooting details are as follows :----

This problem is pretty common among some years of GM trucks. What is actually happening is the ECM is misfueling one whole bank of cylinders, in your case the left bank (cylinders 1,3,5,& 7). The reason it's misfueling is the oxygen sensor is degraded and lazy causing incorrect data to the ECM. The fix is rather easy, replace the upstream bank1 oxygen sensor. This is the oxygen sensor on the same side of the engine as cylinder #1, and in front of the catalytic converter. After replacing the sensor you will need to reset the fuel trim. If you have a scan tool that is capable of that function. I recommend using it, if not you will need to remove the battery cables and touch the positive and negative cables together for 30 seconds to perform the reset. I recommend a AC Delco or GM sensor as I have run into problems with some after market sensors.

What you will see in your oxygen sensor data is that the bank 1 sensor 1 (pre-cat on left bank of engine) is slow to respond and that it will likely stick at an average reading over 500mV. This data is actually false and is driving the ECM to subtract fuel and artificially lean out the cylinders on bank 1. The ECM monitors oxygen sensor data (one of many data parameters) to determine the proper amount of fuel to add to achieve proper engine air/fuel mixture of 14.7 parts air to one part fuel. The only way to see this trend is to watch the oxygen sensor data and the fuel trim data over time while driving. Negative fuel trim numbers (-10% or more) indicate the computer is removing fuel to try and achieve that 14.7 to 1 ratio, and of course positive fuel trim numbers indicate the ECM is adding fuel to achieve that same ratio. As far as proof positive that the sensor is failing you could switch the two front sensors from side to side (left to right), what you will see is that the misfire will start to go to cylinders 2,4,6, & 8. I have seen this particular scenario hundreds of times in the last 6 or 7 years in the dealer service department.
All O2 sensors forward of the cat are sometimes referred to as engine monitoring, sensor 2's are catalyst monitoring. On GM vehicles sensor 2 will not cause a engine fueling concern and their primary purpose is to monitor catalytic converter efficiency to assure it's function is above the federal emissions standard.

This details will help.Thanks.

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