The three-legged stool approach to vehicle diagnostics

Oct. 1, 2018
As technicians, we are faced with jobs or situations that challenge our abilities. Do you have the right strategy, tools and attitude to tackle them?

With the proper training, the correct tooling and a thorough understanding of how our opponent (the subject vehicle) “ticks,” the odds are high that the fault can be identified, the root-cause pinpointed, and vehicle repaired in a reasonable amount of time. That is, of course if we can identify what the customer is concerned with.

Lesson #1 – The three-legged stool

I want to take a moment to bring up a very valuable lesson I learned years ago from AutoNerdz founder Tom Roberts. Tom is an invaluable contributor to the automotive industry and is known for his diagnostic and scope expertise. He once described the ability of a technician to perform his/her duties as being perched upon a three-legged stool. The three legs represent a technician’s competency, capable tooling and adequate information.

I believe Tom used the analogy because, just like a three-legged stool missing a leg, a technician who lacks one of the three items would soon find himself toppling over in a crash-landing. We must fortify ourselves from those three angles to be consistent and successful diagnosticians and technicians.

Lesson #2 — The 85/15 rule

There is another valuable lesson that I was taught years ago. About 85 percent of every action that occurs on an automobile, occurs on all of them because it must. It is a matter of physics. For example, we can energize a fuel injector by completing the path to ground, by providing a voltage source or even by providing both a voltage source and ground path. The point is the injector must open to allow a cylinder to be fueled properly. The 85 percent is that very fact…the 15 percent is how the manufacturer designed that function to be carried out. This very lesson is the basis for this topic of discussion.

The initial encounter

The vehicle in question is for a very loyal fleet account of ours (Figure 1). It seems their 2013 GMC Sierra Diesel 6.6L (LML) with 186K on the odometer has been experiencing a loss of coolant level for some time now. I was issued the vehicle for evaluation along with some basic routine maintenance. The vehicle was well maintained and in fine shape. I noticed the degas bottle exhibiting a very low level of coolant and in the engine compartment the unmistakable, sweet smell of hot antifreeze lingered over the hot powerplant. A visual inspection of the hoses was carried out and the leak was easily pinpointed to the lower radiator hose. I received authorization from the fleet manager to complete the repair and the vehicle was ready for pick-up later that same afternoon.

Figure 1

It was about 3 p.m. when the driver of the truck returned to the shop to retrieve the vehicle. He was excited to get his truck back but left me with some concern when I met him in the parking lot. It seems the driver had failed to mention upon drop-off that he was experiencing some difficulty starting the engine from time to time. I certainly noted no such symptom each time I started the truck and I asked him if he could demonstrate the erratic behavior for me. The driver attempted to duplicate the strange concern but to no avail. The truck’s engine repeatedly started without hesitation or struggle. I reassured him with visual confirmation of the battery’s condition, as I had left (on the passenger seat of the truck), a print out of the starting/charging system test we perform as a courtesy during routine maintenance. Satisfied, he took the vehicle and assured me that he would return if the symptom were to present itself again.

Lesson #3 — The interviewing process

Sure as can be, the following Monday, the driver returned to the shop with his truck and a complaint of the hard-start concern. This time I asked him to spend a few minutes with me so that I might ask him a few questions regarding the nature of this erratic fault. In my experience, it’s always been a fantastic idea to interview my customers on the nature of the faults and for good reason. Just think how often an intermittent fault arises. We call it intermittent, but the fact usually is that once we figure out how to force the fault to reveal itself we can almost do so at will. Intelligent questions regarding failure criteria include asking about weather and ambient conditions present at the time of the fault, whether the vehicle had experienced a hot or cold soak prior to the fault, the frequency of the fault and overall driving habits.

These can really help narrow the failure down and eliminate a lot of wasted time and energy. After speaking with Ron, the driver of the GMC, it has been determined that the symptom is only exhibited if the truck sits all day after a drive to operating temperature. I asked Ron to then describe the symptom to me. What does he mean by hard start? He responded by telling me, “The engine seems to stutter while its cranking over, like perhaps the starter is failing.” That description brought a thought to the forefront of my mind and I didn’t like what I was seeing. His description of the starter operation led me to believe that the engine became difficult to turn at some point in the 720-degree engine cycle. I do want to reiterate, that the truck was very well taken care of and was relatively young, especially for a diesel powerplant. The thought of a potential mechanical failure didn’t sit well with me but the clues I have before me will lead me down that path for initial testing.

Lesson #4 — Returning to my roots

As mentioned earlier, the 85/15 rule regards mastering the function and operation of all that apply to the 85.

Being intimately familiar with engine operation, engine management systems as well as their components and their functionality, give us the ability to apply testing techniques to monitor their functionality. Because these devices apply to every year/make/model on the road, it’s only beneficial to invest the time to master them. Which brings me to my next point.

Lesson #5 — Test known-good vehicles

I’m not a diesel tech, and though I’ve worked with them successfully, I don’t currently possess the experience necessary to be comfortable with them. They require me to remain extra focused as they don’t present to me as second-nature like they do to more experienced and properly trained diesel technicians. What is neat about the approach to this diagnosis is that I don’t need to be a diesel tech! I know DC motors and what to expect to see on a lab scope to determine whether they’re operating properly or not. I’ve learned years ago to carry out testing techniques on known-good vehicles. Being comfortable with what “GOOD” looks like means “BAD” sticks out like the proverbial sore thumb.

Figure 2

Because of the fault description Ron provided me, as well as the way the fault presented itself, it led me to believe that the engine was the cause, becoming difficult to turn somewhere in the 720-degree engine cycle.  I was going to capture the fault using a lab scope and a high current amp probe while operating the vehicle under the same failure criteria Ron had described earlier.

If you refer to Figure 2, you can see a capture of starter current, while cranking a known-good vehicle. This trace first exhibits an in-rush current. This is the tall peak you see to the left of the capture. What this represents is a momentary high rate of current flow. This occurs because the starter uses a lot of energy to get the engine moving. Once the engine begins to rotate, the energy required to keep it rotating has diminished. Next you will notice the repetitive peaks. The represent the Top Dead-Center (TDC) locations of each piston’s compression stroke, in turn. The waveform presents in this fashion because the starter consumes more energy trying to compress the contents of the cylinder then it does simply rotating the crankshaft (Figure 3).

Figure 3

Flushing out the fault

The GMC was taken on a road test around town for about 20 miles to get the big 6.6L up to operating temperature, just as Ron described. The vehicle was then parked for the rest of the afternoon and was prepped to capture the fault, red-handed, first thing the following morning. I placed a 600A-rated current clamp around the heavy cable feeding the starter motor.  Although it may not have been totally necessary, I chose this test location to eliminate other devices that will serve as “noise” in my starter current capture. Devices like fuel supply pump and glow plugs may impede upon my capture and hide what I was trying to see.

I connected my current probe to my lab scope and zeroed the probe out. I turned the key and awaited the extinguishing of the glow-plug indicator. I then cranked the engine over and found the description Ron provided to be totally valid. On top of that, my lab scope reflected the fault as a momentary event of high current draw, indicating a high starter effort was required.  Unfortunately, the capture was inadvertently erased and I had to repeat the prep-process with only a couple of hours allotted for me to capture the fault for this article. If you see Figure 4, you can see the second peak from the left is not like the known-good example displayed. It represents the starter momentarily struggling before it continued to rotate.

Figure 4

Because this only occurs upon the initial cranking of the engine, it’s an indicator that the engine is likely trying to compress a liquid in one of the cylinders (fuel, coolant) rather than a mechanical fault internal to the engine, as this would likely be more prevalent. I will point out that that there is no visible smoke once the engine is started.

Who’s the culprit?

I always let the results of my easy tests drive me to more pinpointed tests. This way, every move I make is justified. Every subsequent test will yield an answer. There is no shooting from the hip, so to speak. I also always curtail my testing around the configuration of the subject vehicle. In most cases, I would like to prove (as easily as possible) which cylinder is at fault. In this case, I already know that the fault is going to be time-consuming to repair and I just must see what fluid is filling one of the eight cylinders. This piece of information will determine what the repair will be and how far the vehicle will have to be disassembled for repair.

Figure 5

I chose to remove all eight glow plugs. They are very easy to remove and because they plug directly into the combustion chambers, the removal will yield me the information I seek. The engine was once again ran hot and put away for the evening. The glow plugs were removed and the engine was cranked over. If you refer to Figure 5, you can see a still-capture of a short video. It is showing cylinder #6 expelling coolant from the glow plug port as the piston ascended towards TDC.

Pinpointing the root cause

After gaining permission to begin disassembly of the 6.6L, it was noted throughout the disassembly process that no other components outside of the combustion chamber were wet with coolant. This further corroborated the thought that the fault lay internal to the combustion chamber of the suspect cylinder or at least common to only the suspect cylinder. The cylinder head for bank #2 was removed for inspection and a crack in the coolant jacket within one of the #6 intake valve ports was found to be the root-cause of the “hard-start” concern (Figure 6). The takeaway from all is that a solid foundation of basic testing techniques is what it took to gain a diagnostic direction. One doesn’t have to be a “specialist” of any particular make or model to be successful in performing the diagnosis – but having a solid foundation, capable tools and adequate information is key to keeping your diagnostic balance on the three-legged stool!

Figure 6

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