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New generation fuel trim diagnostics

The basics haven’t changed, but the strategies have. Are you up to date?
Wednesday, May 31, 2017 - 06:00
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When were you introduced to fuel trim and the power it gives us as technicians in the drivability diagnostic process? For me, it was quite late in life, since I didn’t get into the car-fixing business until 1992. Trust me; there is a huge difference between this business of fixing cars and the business of cutting, skidding and loading logs.

Sometime in 1998 I found the fuel trim data on a scan tool and with the help of some very good friends, I learned how to use those numbers. Well, using the numbers was rather confusing and these friends introduced me to the concept of using a scan tool to graph engine data. I thought it was pretty cool to be able to use a scan tool on a drivability problem and tell whether the problem was an oxygen sensor, low fuel supply, restricted exhaust or even a MAF or MAP sensor that was not reporting correctly.

Has fuel trim changed since its inception in the early ’80s? I guess that depends on a lot of things, although my answer would be yes. Please let me qualify that answer — the basics of fuel trim have not changed. The way it is reported and many of the strategies that are used have changed, and all of these changes have been for the better.

The scan tools we have today do a much better job of getting this information, since the speed of the data is many times faster today than in yesteryear, and to make the deal even sweeter, today a scan tool will graph that data and put it in a format that can be easily understood and used.

Taking full advantage of the info

In the last few years, some new things that made their way into the fuel trim data PIDs are front and rear oxygen sensor trims. Equivalency ratio is another rather new PID that will give us some great information about what is happening with the fuel control.

2010 Subaru Forester — 2.5L engine, CVT automatic transmission, 100,361 miles on the odometer
Figure 1 - Instrument cluster with many warning lights turned on

The vehicle of the day is a 2010 Subaru Forester. This vehicle is powered with the 2.4 4-cylinder engine and the power runs through an automatic CVT transmission. The customer complaint was noisy brakes, and several warning lights were illuminated in the instrument cluster and by illuminated, I mean this instrument cluster was lit up like a Christmas tree (Figure 1).

With the engine idling, the cluster had warning lights for the tire pressure monitoring system, transmission oil temperature, traction control, cruise control and the check engine light was on.

My first step with problems like this is to fire up my favorite scan tool for the job and do a full system scan. This is a quick way to get an overview of any DTC that is stored in the modules and possibly get a quick direction. In my last 20 years of diagnosing problems like this, I have never found the problem with the first test I have done. I always start by using the tool I think will gather the most information in the shortest amount of time and in this case, my tool of choice was the scan tool.

I found there were diagnostic trouble codes stored in three different modules — the ECM (engine control module); ABS (antilock brake module) and the EPB (electronic parking brake) (Figure 2).

Figure 2 - DTCs from the vehicle module scan. Where do we start, with the engine DTCs or the one in the ABS?
Figure 3 - Service information for the DTC P2097. The page gives the code description, the code set criteria and the test procedures.

With DTCs stored in three different modules, where do you start? This is a tough question, but in this case, I started with the engine trouble codes, the P2097. The code description (Figure 3) and code set criteria shows the DTC is for “post catalyst fuel system rich.” In days gone by, we only had system rich and system lean codes.

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