Determining a logical process for diagnostic concerns

May 1, 2017
I always try to have a logical approach to diagnostics whenever I am working on something that I have very little firsthand knowledge or experience in working on. I am always harkened back to elementary school science class, when I was first introduced to the Scientific Method.

How many times have you run into a situation like this? The vehicle that is in your bay has been to a couple of shops and still has problems. Maybe it has an issue that stubbornly hasn’t yet manifested itself. Or perhaps the system or vehicle you are working on you have very little firsthand knowledge of its operation and/or you have never had the “privilege” of working on one. I think if most of us are being honest, we would admit these situations are presenting themselves more and more frequently. So what do you do? How do you proceed at this point? Do you have a set plan of attack (POA) in place or a process that will help guide through these increasingly tough “diags”?

The diagnostic process has to start at the service desk. The questions of who, what, where, when and how need to be part of the original or triage process. The customer is often times the best source of information about the problems with his/her vehicle. They operate it numerous times a week in many different conditions and usually know its many quirks and where and when the problems arise. Our jobs as technicians is to be the diagnostic detective and put all the pieces together to come up with the “why.”
This is the internal diagnostic flow chart I use to keep myself on track. It’ll work for you, too!

So what’s your POA

Do you use Google or some internet "silver bullet" site? Will you use the results of that search to start replacing parts? Unfortunately there has been a trend in our industry to do just that! I get calls or emails from time to time seeking advice on diagnostics. When I ask questions like “what have you tested?” or “what do you know?” I usually get responses like, “we replaced this,” or “Google said to put this part in” I actually don’t have issues with Google searches or pay diagnostic sites — both have a place in the diagnostic process — but use the results to gain some direction and do some testing. 

I always try to have a logical approach to diagnostics whenever I am working on something that I have very little firsthand knowledge or experience in working on. I am always harkened back to elementary school science class, when I was first introduced to the Scientific Method, a four-part process that involved the following steps with a focus on how we can apply them to our logical diagnostics: 

  1. Form a hypothesis — Oftentimes we have an idea or silver bullet or TSB that we “think” may be the cause of our problem. For example, a 2002 Malibu with 200,000 on the odometer is towed in with a “crank-no start” condition. Based upon your experience, pattern failures and internet searches, you “think” or surmise that possibly the fuel pump may have died. This is your hypothesis. 
  2. Design an experiment — Based upon your hypothesis, design an experiment to test or prove or disprove what you think is the cause. In the above example where we suspect a fuel delivery issue, maybe the experiment we design to test the fuel delivery system is to place a fuel pressure gauge on the fuel rail test port.
  3. Analyze the results of your experiment. Using the above vehicle, lets say I have fuel pressure that is in spec and yet the vehicle still doesn’t start. How do I know if the fuel is reaching the cylinders? Make note of the results and design another experiment as to how to test that.
  4. Modify your hypothesis based on the results. Design another “experiment” if needed. The above example may seem a little simple, but I believe it illustrates a logical plan that we as techs use regularly, possibly without necessarily realizing that we are employing a logical diagnostic method. I would implore you to use the same method to diagnose systems that you may not have “time in type” in. We test the no-start fuel delivery system in a logical path like muscle memory, yet if we are confronted with a network no-communication issue, we are often challenged on how to proceed.

Another method someone showed me a few years ago was to try to put ideas on paper and do some brainstorming. Try this sometime: take a piece of paper and divide it into 4 squares:

In the upper left corner, label it “things I know,” and list them. Maybe the vehicle is a No Start (NS), yet it has spark. Make a note of that.

In the upper right corner, label it “things I don’t know,” and list them. Do I have adequate fuel pressure? Does the injector and spark plug fire at the correct time?

In the lower left corner, label it “things I want to know,” and list them. So say I have fuel and spark yet the engine still doesn’t start. Is the fuel reaching the cylinders? Is the engine mechanically in time?

In the lower right corner, label it “design the experiment,” and list ways you could prove the “thing you want to know.”

This is kind of a “brainstorming” technique that I think makes techs critically think and put to paper a logical plan of attack. Another great piece of advice I picked up from a fellow Motor Age contributor that I call the “Manna Mantra” is to “do the extra test.” In other words, if you think you have proved or verified something, find an additional way to prove it to yourself. This will make you a better tech and is a great way to cross check your results.

The "flat rate" test drive

I believe that a logical POA should start simple and have the ability to garner the most amount information with the least amount of effort. This, in my opinion, is to always starts with a scan tool and test drive. Systematically leverage the information in the scan tool. Look at and record codes, both current and historical. Don’t necessarily limit yourself to the enhanced side; use the OBD II generic data to check for pending codes and take a look at the Mode $06 data, if applicable. Make note of the PCM calibration, too. There are times when calibration updates have been issued to correct drivability issues. It’s hard to tell if the module needs to be recalibrated if you don’t know what calibration is in the module. None of these tests are time consuming. It is merely using the scan tool to its fullest ability and having a logical progression or POA. After recording any codes and freeze frame data and comparing that to the customer’s “what, when and how” description, a systematic test drive while recording a snapshot or movie can usually give you a “go/no go” of the fuel system and the engine's ability to breathe. This is sometimes referred to as the “flat rate” test drive.

Too often I see techs test drive cars for drivability issues without having a scan tool hooked up, or if they do they do not have a snapshot set up to record the test drive. This is a classic case of not leveraging technology to your benefit. Most quality modern scan tools have large recording buffers and have made great leaps in graphing scan data. Set the PIDs up in the data stream to include those needed to do a Volumetric Efficiency (VE) calculation and include O2 sensors and Fuel Trim (FT) to get an idea of engine fueling. Next take the vehicle out and drive it with a couple of wide open throttles (WOT) through a 1-2 shift while recording a scanner movie.

When dealing with drivability issues, one of the first tests I make is the relative compression test with sync. An aggressive WOT acceleration of most vehicles with traditional O2 sensors should cause the upstream O2 sensors to go full rich, easily making 800-900mv. Think about this: WOT is usually when the engine requires the greatest amount of fuel to enter the cylinders. Basically the PCM sees the load increase via several inputs and commands a full rich or “all hands on deck” fuel strategy. If you see the upstream oxygen sensors “latch” or flat-line rich, what logical deduction can you usually make in regard to the vehicle’s ability to fuel the engine? What does that tell you about the fuel pump, fuel filter (if applicable) and the PCM control of the injectors? If the O2’s latch full rich is reaching 800-900 mv on WOT acceleration, I usually can remove fuel delivery as the cause of my drivability concern. I can list it in my “what do I know” column of my cheat sheet. 

Furthermore, if I set up the scan tool’s data PID list to make sure I have FT, O2, Load, Loop Status, IAT, RPM, MAF (grams/second) , I can gain a lot of insight as to the engine’s ability to fuel and breathe. If I have driven the vehicle in a manner that captured a couple WOT accelerations through a 1-2 shift and recorded a snapshot movie, I can calculate my VE (MAF-equipped vehicles only) when I return to the shop and analyze my data. VE is a measure of the engine’s ability to flow air. A simple VE calculation can give an idea of the engine’s ability to flow air in and out. This usually is an indication that we don’t have any restrictions in either the intake or the exhaust causing a drivability problem. VE testing is a discussion in itself and Motor Age contributor Scott Shotton covered it in “Asthmatic engines,” March 2016. Googling “VE calculator” nets some free online calculators as well as apps for phones and tablets.

If the VE calculates low, I must ask myself if this is a “calculation error” or if the VE is actually low. The MAF reading is one of the most essential inputs and is coupled with RPM for the VE calculation. A skewed MAF reading could cause my VE to calculate low. It’s the old “garbage in equals garbage out” theory. A rule of thumb I sometimes use is the 40x rule. Most of us have heard about or use the 1 gram per sec of airflow per liter of displacement for engines of 3 liters, which are usually pretty close. The 40x rule of thumb is that in a healthy breathing engine you should be able to achieve 40 times the displacement in liters of the engine on WOT. In other words, a 5.3 liter engine should be capable of producing over 200 grams/second of airflow at WOT (5.3 x 40 = 212 g/sec).

Fuel Trim

Fuel Trims should also make the list of things we want to know on our PID list for flat rater test drive snapshot movie. This is especially true if you are chasing a low power or a lean for rich code. Fuel Trim (FT) are the PCM’s correction to the Injector Pulse Width (IPW) in closed loop based on the PCM’s inputs such as the O2 and other engine sensors. I like to think of it as a “if /than” correction. For example, if the PCM senses that the engine is leaner than the PCM wants to see it, then it will make a positive correction to the IPW and show it as positive FT numbers.
Dealing with a lean DTC? This matrix may help you locate the problem faster. Conversely, if the PCM detects the engine is richer than the PCM wants to see, it will subtract fuel from the engine by decreasing the IPW resulting in negative FT numbers A couple of important things to note regarding use of Fuel Trim: the engine must be in closed loop, and FT should be checked at idle, at 2000 rpm with no load and 2000-2500 rpm with a load. Be sure to look and note your Loop Status PID. Moreover, using the scan tools ability to graph will aid in your diagnosis immensely and help you “see” the if/then decisions or correlations between the PCM’s inputs and outputs. A cheat sheet can be used for locating lean issues by “plugging in” the characteristics that you deduced from your test drive movie, VE calculation and FTs.

Mechanical concerns

Engine mechanical issues are always something bet found out early in the diagnostic process. There is nothing worse for a tech or shop to find out that there is an engine mechanical issue causing the misfire after calling ignition or fuel delivery components and selling and replacing parts. Diagnosing the cause of engine misfires can be challenging for some. I believe that you have to have a logical and systematic POA for engine mechanical testing will help guide you. 

One of the things I look for is proper response of the oxygen sensor. Part of have a good POA is to have one in place already and have one that will work on a large variety of vehicles you service. It would not do you a lot of good to have a test technique for checking the mechanical integrity of an engine if we had to have a separate test technique for each OE. While each OE engine design does have different characteristics, most four-stroke internal combustion engines share some commonality like compression in the cylinders that must be relative to one another and that the spark plug generally fires BTDC compression. We can use these universals to create a POA or test technique to use a “relative compression” test measuring starter cranking current using a high current probe and a DSO to infer engine cylinder compression “relative” to one another. This test has been outlined many times in this publication and has proven to be an accurate and time saving test over conventional compression testing. Speaking of a logic approach and progression, it is one of the first tests I perform if I break out the scope on a drivability issue. Couple it with an ignition sync and possibly a vacuum transducer like a Pico WPS 500 or a FLS sensor, I now know a great deal about the mechanically integrity and spark plug firing relative to piston position. This is a great “go/no go” test that in relative short order I can take engine mechanical off the list of possibilities causing my drivability problem.

In summary, there has been and continues to be a paradigm shift in the technologies of modern vehicles. No can possibly stay up on the servicing of these systems. Does this mean we refuse to work on them or run for the doors? No! Does it mean we use Google and the internet to diagnose and replace parts based solely on that search? No!

We use a lot of these tools every day, but are you using your most powerful tool – the one between your ears – as effectively as you use these?

What I think it illustrates is the need to use a logical approach and leveraging the technologies available to us to come to a successful outcome. Always get as much information from the customer, harness your scan tool’s power, use the flat-rate test while recording a movie, use the “scientific method” and the four-corner piece of paper brainstorming technique. These will hopeful give you the ability to design the experiment to verify what you want to know. Do the extra test! It will help you to better understand how things work and helps to cross check your results. It will make you a better tech in the long run.

Please remember that “rules of thumb” are generalities and there are always exceptions. The point is to have a simple, systematic and repeatable POA that you execute every time you drive a vehicle. Using the scan tool’s ability to record the event and analyzing the recorded data, you can garner a great deal of information with a minimal amount of effort. This just having good diagnostic technique and then applying a little logical deduction to it. Remember there has never been a scan tool, lab scope or five-gas analyzer that ever fixed a vehicle by itself without your cognitive input. You and your ability to reason are what make it all happen. The most powerful diagnostic tool in your arsenal does not reside in or on top of your toolbox — it rests on top of your shoulders!

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