Growing your diagnostic skills

Jan. 1, 2020
We all had to start somewhere. No one was born a drivability expert, though some are gifted with a natural talent for reasoning through even the most difficult of diagnostic challenges.

We all had to start somewhere. No one was born a drivability expert, though some are gifted with a natural talent for reasoning through even the most difficult of diagnostic challenges. I, for one, was not so gifted and had to work at developing my diagnostic techniques. And if I can do it, so can you.

Don’t Be Afraid!
In days gone by, carburetors dumped a poorly atomized air/fuel mix into a common chamber in the intake. From there, the hungry cylinders drew in the mixture through the intake valve (each cylinder had only one) into the combustion chamber. A mechanical switch called “points” was used to interrupt the flow of current to the single ignition coil, and from there the secondary voltage was routed to each cylinder by a distributor rotor that was mechanically driven. The spark ignited the compressed mixture, and it exited through the exhaust valve. All the valves were opened and closed by a single camshaft. Not a lot to it but these early designs had one unique and common factor. They didn’t need a computer to run.

But they weren’t that good at utilizing the energy released during combustion either. Today’s engines are much more efficient, producing more power on less fuel than ever before. Computer controlled systems allow precise control of factors that impact efficiency. And often it’s that word “computer” that scares techs away from drivability issues.

Don’t be one of them. No matter how involved or complicated the technology might seem to be, it’s still an internal combustion engine (ICE) that needs compression, ignition and fuel in just the right amounts and at just the right time to perform properly. Sure, there may be more complex ways of controlling it all, but with a little thought, a good service information (SI) source, and a bit of continuing education, you can learn to adapt with the change.

A Foundation To Build On
Before you can tackle a drivability problem, you first must have a solid core understanding of how an engine works. No, you don’t have to have an engineering degree, but you do have to know more than “suck, squeeze, blow and go.”  

At the heart of the engine management system is a computer. If the computer controls only engine related functions, it’s called an Engine Control Module (ECM); if it also controls transmission functions it’s called a Powertrain Control Module (PCM). You might see these terms interchanged, especially in generic uses like this story you’re reading.

The computer is not as smart as you might think it is. It has to be told what we want it to do, and what is going on around it. These are the input sensors. Once it has the information, it can act only within the confines of its programming. If an input is confusing to the computer, the computer won’t know what to do and might even start shutting things down in a panic. The computer is so paranoid thatit even checks itself by monitoring the outputs it commands to see if they did what they were told to do.

Should the computer “see” something it doesn’t like, it can’t fix it itself. Instead, it turns on the Malfunction Indicator Lamp (MIL) so you can fix it. Which, by the way, you can if you understand the roles the different inputs and outputs play and how to test them. Information you can learn by spending time reading up on the “theory and operation” of these systems in your SI system and attending a few classes from time to time.

Most of the cars on the road are fuel-injected. In these systems, the computer opens and closes the injectors for a precisely calculated amount of time in order to deliver the exact quantity of fuel needed for the air drawn into the engine. Did you know that there are two ways that calculation can be made? Diagnosing drivability complaints are approached differently depending on whether it is a speed density system or mass airflow one, and this kind of knowledge is exactly the kind you need to be successful in your diagnostic endeavors.

What about the ignition system? Yes, they all still use a spark plug but can you identify what type of ignition system you’re dealing with without looking it up? Do you understand how it functions? More foundational knowledge you need to know.

Think of it this way. Remember that game where you had two seemingly identical pictures in front of you? The object of the game is to find what is different in the second picture from the first, and the more you knew about that first picture the easier it was to find the differences, right?

Same thing. The more you understand how these systems work and interact, the easier it is to see when one isn’t acting the way it should.

An Example Of The Process
In addition to a solid knowledge base of the engine and its operating systems, you need a solid process to keep you on track during your troubleshooting. Whether you’re dealing with a Check Engine light that is on or just a symptom with no codes, using the same basic process and making it a habit will keep you focused on the task at hand — finding the root cause of the problem.

Follow along with me as we tackle an older Chrysler with a simple “MIL is on” customer complaint. Hey, you have to learn to walk before you can run, right? The same process applies to any diagnostic issue you will face. As you’re reading the rest of this story, consider what steps you would have taken and why. And if you read something about a system or function that you don’t understand, take the time to look it up for yourself in your SI system and seek the training you obviously need to improve your diagnostic skills.

Verifying the complaint on the Chrysler is the easy part. The MIL is obviously on and stays on long after the bulb check function has been completed. Connecting with a generic scan tool retrieves a code P0401 that, according to the SI, stands for “EGR (Exhaust Gas Recirculation) system failure”. That definition is not quite enough, though, to begin our troubleshooting.

Remember, we are after the root cause of the failure and before I can find out what it is, I need to know why the ECM is logging the code. And I do that by reading up on the code’s enabling conditions and setting requirements (more typically referred to in the SI as “criteria”). There are two advantages to taking the time to study this information. First, it will help me gain a more thorough understanding of how the system works and why the ECM turned the MIL on and second, it will help me test in a way that mimics the ECM tests so I can be sure the MIL will stay off once the repair is made.

In the case of the Chrysler, the ECM will test the function of the EGR system when the engine temperature is over 180°F and the outside air temperature is above 40°F. In addition, the engine must be in closed loop (fuel control) and under a steady load with the throttle open. If the conditions are met, the ECM will momentarily close the EGR valve and look for a change of state in the oxygen sensor signal. If it sees no change or too much change, it will assume there is a problem and record the failed test.

Do you understand what EGR is and the basics of how these systems work? After reading this description, how would you proceed with your diagnosis?

The Picture Gets Some Detail
Not every OEM monitors their systems in the same way. By reading up on how Chrysler monitors this particular variant, I know now that the problem is related to the amount of EGR flow when the EGR system is supposed to be on; that is, during a steady state cruise. That could mean a failure of the EGR valve or a restriction to flow somewhere in the plumbing. Time to take a closer look at the hard parts that make up the system.

The 2.5-liter V6 in this Sebring uses a vacuum-operated EGR valve that attaches to the cylinder head on the front bank. An internal passage feeds exhaust gasses to the valve, so excessive carbon build-up here might be a potential cause. The vacuum source has to be controlled some how to prevent the EGR valve from staying open all the time, so a closer look upstream uncovers a weird looking device that has three lines and two electrical wires attached to it.

This is the vacuum transducer. One line is the vacuum source from the engine’s intake manifold and is connected to an electrical solenoid that opens and closes the vacuum path to the EGR valve’s vacuum port. The solenoid must be turned OFF to open that path. A third vacuum line, though, runs from the base of the transducer to the base of the EGR valve. What does that do, and can that be a potential cause of the failure? Time to do a bit more reading.

Sometimes, you won’t have a full description of all the nuances of the system you are working on. Diagnostic flow charts can help fill in the gaps if you take the time to read them from that perspective. Reading over the troubleshooting steps for the Sebring added a bit more clarity as to how everything works. The third line is exhaust backpressure, feeding from the valve to the transducer. It operates a bleed valve internal to the transducer, and when there is no backpressure the bleed valve is open.

In order for EGR to flow when it should, then, there must be sufficient backpressure to the transducer to close the bleed valve and the ECM must turn the solenoid off. Vacuum is then free to move to the EGR valve itself, where it raises the stem off of its seat, allowing the burned remains to flow through a connecting pipe to the intake manifold at a point just past the throttle body assembly. Partial backpressure will limit the vacuum that reaches the valve and reduce the EGR feed to the intake.

This more complete understanding, however, only adds to the list of possible causes, doesn’t it? The code can’t tell if it’s a matter of too much or too little flow, only that the flow of EGR is not what it should be. Our list of potential suspects includes damaged vacuum lines, a failed transducer, a failed EGR valve, and clogged passages at the very least.

Another important step that is often overlooked is a search for Technical Service bulletins (TSBs) that might apply to the problem at hand. There is a bulletin related to the EGR system for this car, but it has to do with an EGR valve that sticks partially open. This causes intermittent misfires, a rough idle, even a hard start condition; depending on how much the valve is open. There are additional online resources available that I routinely check as well. Sources like iATN and Identifix can often help point you in a diagnostic direction. Just don’t fall prey to that easy practice of using these resources to find that “silver bullet” you need to fix the car. Always test to verify that the problems outlined and identified in these resources is the same as the one you’re facing before you blindly apply the repair.

What Is Most Likely?
Diagnosing a drivability issue involves the whole car, and not just the system that logged the code. Now that we understand the system and what caused the code to set, its time to do a little visual check of the car itself. It doesn’t take long to see that there are several oil leaks and the lines attached to the transducer and EGR valve are in rough shape. The car also has some miles on it, so the possibility of a clogged EGR passage is going to move closer to the top of the probable list. There has been a lot of work, unrelated, done to the car over the past several months so odds are the car had a rough life before its present owner took over.

Based on what we know so far, wouldn’t it be a logical first step to see if the EGR valve works? How would you do that and how would you know? The valve is visible from under the hood, but not visible enough to see the pintle move with all the oil that is on it. Remember the TSB we found? It said that an EGR valve that was open at idle would cause the idle to be rough, maybe even stall the engine. That makes sense, doesn’t it? The already burned gasses aren’t going to burn again and the air charge at idle isn’t that much so adding this dirty gas is going to make it harder for the engine to run.

Let’s hook up a hand vacuum pump to the EGR’s vacuum port with the engine idling and at normal operating temperature. Applying vacuum, I immediately noticed two things; one, the idle didn’t change and two, the vacuum wouldn’t hold. What does this test result tell you?

The lack of idle change could mean that the valve wasn’t opening or that the passage leading from the valve to the intake manifold was clogged. The immediate vacuum loss, however, was the final piece of the puzzle. Inside the valve is a diaphragm that attaches to the valve’s pintle. Vacuum applied above this diaphragm creates a pressure differential that causes the diaphragm to deflect towards the vacuum source, raising the pintle off of its seat. The immediate loss of vacuum is a pretty good sign that the diaphragm is damaged and not sealing, allowing the vacuum applied to bleed off to outside air. It’s all we need to pull the trigger on the EGR valve as the cause of this P0401 code.

Time To Fix It
Not the car, you. If you found this too easy a problem to solve then you’re not the guy I want to talk to. If you read this story, and found yourself asking, “What is he talking about?” anywhere along the line, I want to encourage you to pursue those answers. Spend time reading the “Theory and Operation” sections of your SI system, attend that next training session in your area, join us for our free web-based training, take advantage of the over 100 videos we alone have produced and shared online. The knowledge you need to move from general service to drivability specialist is out there. It’s just waiting for you to take it.

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