How to efficiently perform diagnostic test drives

April 1, 2019
Keeping your diagnostic routine consistent yields many benefits, including fewer mistakes and faster solutions to your customers' concerns.

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.

What other Parameter Identifiers (PIDs) should be I be recording? How about the ones that will allow me to make a Volumetric Efficiency (VE) calculation on a MAF engine if necessary? Some of you may not be familiar with term Volumetric Efficiency. VE is an engineering term that formulates how well a pump can move a liquid or gas compared to its physical limitations. VE is used in a variety of industries like oil drilling platforms. First, we need to know the size of the pump, which in our case is the engine. An automotive engine is an air pump of sorts, so we need to consider its displacement. Pump speed is also critical for the equation and so we use engine RPM. These two things are used in the equation or VE calculator to figure out what is referred to as the theoretical maximum.

MAF and RPM for VE calculation

The next variable is the amount of air entering the engine at maximum airflow, so we will need to capture the MAF in Grams Per Second (GPS) and RPM to do this. In addition, some VE calculators require ambient air temperature and elevation as well. Fuel Trims and Loop Status PIDs are not required to calculate VE, can be a handy reference. So, what can I expect from a known good VE run? It depends on a couple of factors, but at my elevation of 510 feet above sea level I expect to see 75 percent to 85 percent on a normally aspirated engine. My red flag number is usually 75 percent or higher is acceptable on these engines. Obviously, forced induction engines (turbos and superchargers) are much greater and usually surpass 100 percent, due to the fact they are “forcing” air in to the engine via an additional mechanical device. These are my rule-of-thumb numbers, which have worked well for me, but be mindful that your results may differ dependent on atmospheric conditions, calculator choice and your experience.

Choosing additional PIDs

I like to make a custom data list, which allows me to choose the PIDs I want to look at as opposed to the groupings the scan tool software developer choses for me. This usually ends up speeding up the refresh rate of the scan tool by not looking at a bunch of unnecessary PIDs not related to the diagnosis at hand. Once I have my list of the PIDs I want, I then go on a diagnostic test drive and make a couple of VE test runs in which I aggressively accelerate through a shift while recording the snapshot on the scan tool. This is going to give me my max RPM and MAF GPS just slightly prior to the shift.

When I say aggressively accelerate, I mean the proverbial “drive it like you stole it” through a shift. I had a trainer friend that once said if you are unsure of how to make an aggressive wide-open throttle run, grab the 19-year-old kid in the lube bay. They can show you how! (Editor's note: Be sure to follow all local laws and perform your diagnostic test drive in a safe area.)

So on to the diagnostic — sometimes referred to as the “flatrater — test drive. After making several heavy accelerations noting if the upstream oxygen sensors go full rich, pay attention to your five senses and how the vehicle behaves.

Misfires in some conditions may be noticed in the seat-of-your-pants feel and can point you in the right direction. Trust your sense of hearing. Do you notice any unusual noises? Your sense of sight tells if the CEL/SES MIL comes on or is flashing etc. All of these things are part of a process that most of us use regularly and just don’t realize that we do it. It is almost second nature to the seasoned drivability tech. What is imperative is that we have the scan tool setup properly beforehand and are recording the whole diagnostic test drive into the buffer.

Sometimes if I am on a really long test drive, I will pause the recording of the snapshot and save the file. This insures that I have my data successfully stored into the memory or internal drive of the scan tool and it will be there for analyzing later.

Reviewing the data

There are three things I like to analyze back at the shop when reviewing my snapshots or scanner movies. I make my observations under two distinctly different operation conditions; normal cruise state driving and Wide Open Throttle or WOT. In general, this is what I expect to see:

The upstream oxygen sensors should cycle rich/lean in "Closed Loop" and should peg full rich on most vehicles under the heavy load created during my WOT acceleration. I also want to examine my MAP PID as well at idle or steady rate cruise. I want to see it steady and when I accelerated aggressively and deplete the vacuum in the intake manifold, I want to see it go very close to Baro or atmospheric pressure. 

Consequently, what would I expect to see if I were monitoring my fuel trims under the above conditions? I should see my fuel trims in closed loop slightly switch or moving about under steady state cruise revealing the subtle PCM correction to achieve and maintain something close to stoichiometry. How about when I go WOT? How should fuel trims behave then? The answer is “it depends.” But what I should expect is my loop status to change from closed loop to open loop. This is what allows my PCM to change to fixed fuel mapping that allows for full enrichment on WOT on most vehicles.

VE calculation

I can test drive a vehicle in a consistent manner and capture a snapshot or scanner movie of the data. Then, later in the safety and comfort of the shop, I can calculate VE and also observe the three key things; O2, MAP and fuel trims. Using this fundamental data, I can usually gain some significant diagnostic direction as to where to concentrate my testing efforts. Let analyze some different scenarios. My friend and co-Driveability Guys trainer, Scott Shotton, developed this cheat sheet, which we can use to plug the information into to help us find the most probable cause of the drivability concern we're chasing.

(Image courtesy of Scott Shotton) VE cheat sheet

Understanding the data

Say we have naturally aspirated vehicle that has a hesitation under load/low power complaint. The condition can be duplicated and during the test drive the oxygen sensors go low instead of high during WOT, the VE calculates at 77 percent, the MAP reaches very close to Baro, and the fuel trims were overly positive at cruise, going into Open Loop under heavy acceleration. These things should lead you to the following conclusions. First, the engine can breathe properly — the VE reflects this — so no clogged cat. Second, the intake is not restricted — the MAP reflects this — so a dirty air filter is out of the question. Third, the fuel trims point to a lean condition. And finally, the oxygen sensors not going full rich point to a fuel delivery issue. This is where I want to concentrate my testing efforts. Perhaps a fuel pressure and volume test as well as current ramping the fuel pump would be the next diagnostic steps to take. 

Good MAP making Baro - poor fueling

Here's our second scenario. A similar vehicle to the one above with a similar complaint of a hesitation. The test drive reveals the oxygen sensors are falling short of full rich, the VE is calculated at 58 percent, the MAP makes Baro and the fuel trims under cruise are erratic and appear to” follow the throttle blade” with their positive trim trends. While one might find the vehicle may exhibit some of the same drivability symptoms, what is it that differentiates this failure from the previous example? The answer is VE. Does a failed fuel pump affect the engine’s ability to breathe? It does not. So, what could cause a hesitation compliant that mimicked a fuel delivery issue and reflected itself as low VE. 

Good MAP making Baro

The answer is an air measurement error. What measures the air entering our engine and its PID is mission critical to the VE Calculation? The MAF sensor. It has been my experience that most MAF sensor failures tend to overestimate airflow at idle and underestimate airflow under load.  While a failed MAF doesn’t affect the engine's ability to breathe, its does affect the VE calculation. It’s the old Garbage In = Garbage Out (GIGO) formula.

Bad MAF fuel trims

Now let’s switch things up a bit and look into some issues that may very well exhibit some of the same drivability symptoms and are revealed in the flatrater test drive by low VE. They are issues that deal with engine breathing restrictions. The first is relatively common to low power/hesitation complaints — the exhaust restriction.

After duplicating the complaint and analyzing my snapshot, I calculate my VE and it's low at 65 percent. Looking at my other three key data points, I want to be mindful of the oxygen sensors going full rich and making almost 900mv. Second, my MAP sensor at WOT reads very close to Baro at 97kPa. Finally, my fuel trims appear to be irregular but less so than the bad MAF example mentioned earlier, trending towards the negative rather than the positive. These all point me in the direction of a restricted exhaust/clogged cat, which I now can focus my testing efforts on, using either a backpressure gauge or an in-cylinder transducer.

Restricted intake

Again, VE is calculated and appears to be low, say 58 percent. The review of the other three things show that the O2 went full rich over 850mv revealing no issues with the fueling of the engine. The fuel trims are very close to normal, in this case total trim being +3 percent. But the what was the third thing we wanted to analyze? If you are thinking the MAP sensor’s behavior under WOT, you are absolutely correct! This is the telltale; the MAP usually falls short of making Baro or may make it for a brief moment but then falls away from it. I like the description the MAP will “ratchet away” from Baro. These things and low VE all point to a breathing issue with the engine but on the intake side. You may be asking how can the intake get restricted? The obvious cause is a SEVERELY neglected /clogged solid air filter. I have also seen those supermarket plastic bags get sucked up in the air cleaner box and cover the air filter, thus restricting the intake. 

Good Snap-on enrichment capture

I tell techs repeatedly in my classes that the most powerful tool in their diagnostic arsenal doesn’t rest on the tool box — it rests on their shoulders. Analyzing scan data is doing just that, using the most powerful diagnostic tool you have. It illustrates the old adage that states “Work smarter, not harder!” I think that we as techs have been test driving cars ever since we have been working on vehicles, the crux of my argument is if you are going to test drive a vehicle do so with a scan tool hooked up to it. Moreover, if you are test driving a vehicle with a scanner hooked up RECORD A SNAPSHOT! Our scan tools have gotten so much more powerful than years passed, and few techs tend to take advantage of all their features. The scan tool allows me the garner as much as info as I can with the least amount of effort. As with anything in automotive there are exceptions, you can’t say always and works on everything. However, speaking from my experience, the flatrater or diagnostic test and recording a snapshot and calculating VE and be mindful of how my O2s, my MAP (if equipped) and how my fuel trims behave has served me well over the years.  I hope you are using these techniques or are open trying them and it helps you streamline your diagnostic process and point you in the direction you need to focus your testing!

Sponsored Recommendations

Best Body Shop and the 360-Degree-Concept

Spanesi ‘360-Degree-Concept’ Enables Kansas Body Shop to Complete High-Quality Repairs

How Fender Bender Operator of the Year, Morrow Collision Center, Achieves Their Spot-On Measurements

Learn how Fender Bender Operator of the Year, Morrison Collision Center, equipped their new collision facility with “sleek and modern” equipment and tools from Spanesi Americas...

ADAS Applications: What They Are & What They Do

Learn how ADAS utilizes sensors such as radar, sonar, lidar and cameras to perceive the world around the vehicle, and either provide critical information to the driver or take...

Coach Works implements the Spanesi Touch system

Coach Works Uses Spanesi Equipment to Ensure a Safe and Proper Repair for Customers