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How to efficiently perform diagnostic test drives

Monday, April 1, 2019 - 06:00
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How many times has this scenario played out at your shop? A regular customer comes in late in the day, perhaps close to closing time, complaining about a driveability problem that you have been chasing intermittently. Moreover, they exclaim “it’s doing it right now. Do you have a minute to go for a ride with me?" Your extinct as a consummate service professional kicks in — you grab a shop towel, wipe your hands and grab a floormat — and away you go. If you are lucky, you get to duplicate the customer’s complaint, at which point you use your power of observation and keenly honed senses to make a judgement on what ails the vehicle. Perhaps you suspect a misfire and use the old “I feel it in the seat of my pants” so it has to be ignition adage…and so on.

Wasted effort

We have all operated at one point or another this way for many years. However, there is one distinct flaw in the methodology that was overlooked. Anyone who has attended any of my training classes has heard me rail on this. There are few things in drivability diagnostics that I feel are more useless than driving a vehicle on a diagnostic test drive without having a scan tool hooked up to the DLC. The second thing that drives me crazy is when a tech test drives a vehicle for a diagnostic issue and fails to record and save a snapshot. Our scan tools have gotten so much more powerful than they were in years passed.

The buffer size, the memory inside the scan tool, can store immense amounts of valuable diagnostic data that can aid in improving the diagnostic process. This article will examine some diagnostic test drive techniques that will hopefully help techs gain valuable diagnostic direction and help eliminate certain possible causes by using scan data analysis to “take them off the table.”

My friend Scott Shotton once stated something in a class that resonated with me, “There is a fine line between efficiency and laziness…I choose to be efficient.” I can think of no greater way to maximize efficiency other than using a scan tool to garner as much information with the least amount of effort! This method involves diagnosing a vehicle by simply analyzing scan data and creating a plan of attack (POA) and “designing the experiment” to test the system(s) believed to be at fault. Furthermore, if I have the snapshot saved, I have valuable sales tools and documentation to share with the boss or the vehicle’s owner. In addition, we now have a pre-repair movie that we can use for comparison post-repair.

What's the point?

So how do we leverage the technology of the scan tool buffer and combine it with a diagnostic test drive? First, I want to be able to use scan data to learn a couple of fundamental things; how well is the engine being fueled and how well is the engine able to breathe.

Good fuel enrichment

We are going to use the oxygen sensors and fuel trims to tell how well the engine is “fueled” and learn if the Engine Control Module (ECM) is in control over the fuel system. We can accomplish this by performing a Wide Open Throttle (WOT) acceleration or by an aggressive brake torque in reverse. On most vehicles, when we go WOT the ECM drops out of closed loop operation to a fixed open loop fuel map that allows for maximum fuel enrichment that should reveal itself in the upstream oxygen sensor(s) going full rich, or well north of 800mv on a traditional zirconia style (Lambda) sensor. If I have a hesitation under load complaint and I aggressively accelerate and see the upstream sensors going to 850-900mv, what does that tell me about the fuel delivery system and the ECM’s ability to fuel the engine? I can, with a fair degree of certainty, eliminate the fuel pump, fuel filter (if equipped) or restricted/dirty injectors as a probable cause of this.

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