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The importance of asking 'why?' during automotive diagnostics

Monday, December 3, 2018 - 09:00
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Ok, So I live near the Philly area, born and raised. True, my mom is from Jersey City and dad, from Brooklyn. My friends across the country poke fun at the way I talk but hey, let’s face it, I am a “WHY’s GUY,” through and through (Yep, I spelled that correctly). It’s not what you think, though. When it comes to troubleshooting drivability faults, sometimes we got it made. We simply drive the vehicle and analyze the data we capture, looking for the root-cause of the fault presented at the time. We invest time learning how to understand the relationships between the PIDs and how they reflect underlying faults. We all know, it ain’t that easy all the time.

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Why ask why (or when or how)?
Sometimes, It’s a bit more involved than that. Sometimes we get that vehicle that requires us to dig way deeper than we are accustomed to. Because I spend a lot of my day troubleshooting drivability faults, to remain efficient I had to devise a plan of attack, a strategy. This strategy involves analyzing preliminary data (the low-hanging fruit). I use this data to ask questions of the vehicle, like:

Why are you performing poorly? (Are you lacking sufficient fuel supply, can you breathe?)

When do you perform poorly? (Is it when you are idling, or when I force you to work hard?)

How can I get you to reveal your fault to me?  (Are there certain weather conditions you don’t like?)

Questions like the ones above help me decide which road I will head down. More importantly, it gives me a sense of direction and justifies the next test I will perform. Otherwise, we’d just be shooting from the hip. I do get lucky every now and then, and diagnosis falls in my lap, but I’d take a rock-solid game plan over luck any day of the week.

2002 Buick Park Avenue with a 3.8L (K) engine

The Bum
The subject vehicle of today’s topic is a 2002 Buick Park Avenue with a 3.8L (K) engine and just shy of 130k miles on the odometer. The customer is concerned with the vehicle’s lack of power and a “knocking” noise from underneath the hood. The vehicle was in the shop in the recent past for replacement of spark plugs and ignition cables and had been without fault for at least a few months.

Figure 1

I began the analysis with a scan for DTCs and, to my surprise, none had been stored. I drove the vehicle, monitoring some basic PIDs and within a very short distance, the vehicle began to ping hard and lacked power. This is where the questioning began. If you refer to Figure 1, you will see that I had placed the vehicle under heavy acceleration. Now, at this point, the vehicle wasn’t pinging but the goal was to see if the vehicle had adequate fuel supply. Ruling out what is “good” with the vehicle is equally as valuable as discovering what is faulty. As you can see, both the pre and post CAT H2O sensors reflected high voltage output, indicating a lack of O2 (or otherwise stated, plenty of fuel). The demand for fuel is much greater under heavy load than it is under less of a load. So, the fault doesn’t appear to be related to a lack of fuel delivery. But why the lack of power output?

Figure 2

The next screen capture answers that question, see Figure 2. It’s clear to see that as this vehicle begins to ping, the PCM compensates (through the ears of the knock sensor) and generates a command to retard spark. You are all likely aware that a spark occurring too early initiates a combustion process that hinders the piston’s ascent towards top dead center of the compression stroke. This in turn, places a tremendous force on the piston and can damage the internal engine components, due to the violent collision. So, the question then becomes why is the engine pinging?

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