2005 Colorado Too Lean, continued

At this point the truck is confirmed to be running too lean, but why is it too lean? This could be a fuel quality issue, fuel pressure problem, injector problem or it could be a base calculation error. The more that can be ruled out with scan data, the less physical testing that needs to be done to prove what is wrong.  Some of the things to think about are what information the PCM uses to make its base fuel calculation. GM uses a mass airflow sensor (MAF), and the software takes that MAF value and calculates the engine load and from there what the injector pulse width should be under those conditions to provide the engine with the right amount of fuel.  While on a road test the accuracy of the MAF can be checked by watching that engine load calculation in scan data. During a wide open throttle acceleration a snapshot of the engine data can be captured and then later played back so that the engine load pid can be examined to see if the MAF sensor reported.

During the road test it was easy to get the MAF to report 100% engine load.  So the PCM should have been calculating the correct base pulse width and turning the injectors on long enough for the engine to get the right amount of fuel. However it was still very lean under all phases of operation and the fuel trims were adding 40-45% to the injector pulse width. One note-able observation was that during a hard acceleration where the PCM commands an open loop enrichment to provide full power, all four O2 sensors reported over .88 volts, and then on a closed throttle deceleration they all fell to 0volts showing good amplitude and reaction. At this point it’s back to the shop for the last phase of testing.

Once back in the shop the fuel pressure was confirmed to be at 63 psi, with enough fuel pressure and fuel volume that mean the only two things left that could be causing the problem were fuel composition/quality or restricted injectors. With the vehicle otherwise operating reasonably well there was an option on how to proceed and its one that the customer chose to try. Instead of doing specific fuel quality testing and manually cleaning the injectors, he was to go fill up at a Top Tier fuel station, and add a very specific fuel injector cleaner, Chevron Techron to the tank.  His travel plans included one immediate trip of about 250 miles to start the injector cleaning process and run all of the old fuel out of the tank. Then we would check the fuel trims and see if anything changed after he refilled the tank.

To be continued…