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From time to time, being a mobile diagnostics and programming company, we are faced with challenges that give us pause when we receive the initial service request. I remember this also to be true in many of the brick-and-mortar shops at which I have been employed in my 35-plus year career as an automotive service technician. The challenge usually starts at the service counter or via a phone call with a Star Wars-like request, “Help me, Obi Wan Kenobi — you’re my only hope!”
The pleading is usually accompanied by a sad story or narrative of a strange driveability issue, the number of folks who have tried to repair it, the parts that have been replaced, the money that had been spent, and of course, the detailed internet search for the root cause of the problem.
Oftentimes, the vehicle owner is a friend of a good customer, a referral from another shop that respected one’s diagnostic abilities and/or tooling, or a Google search that listed you as a specialist or someone with great reviews.
While the adage may be true that discretion is the better form of valor, sometimes we find ourselves in the untenable position of having to work on these vehicles. I made my bones years ago as a diagnostic tech by figuring out vehicles that had been to other shops and/or were referrals. Looking back, I was either too stubborn to admit defeat or saw these challenges as an opportunity to gain a favorable reputation for myself and the shops that I was employed by. This also may have been the genesis of my mobile diagnostics company. Since starting it years ago, I wanted it to be known that we were a results-based company, or as my friend Kirk Holland from Gladney Automotive Services aptly put it, “You don’t want to be known as a company of NO.”
As the story goes
The phone rang and it was the familiar voice of the service manager at a fleet shop we service. We exchanged pleasantries, and a plea for help soon followed.
The vehicle, a 2010 E450 Jayco Motorhome with a 6.8-liter V10 and 24,000 miles on the odometer, was super clean and the owner had initially complained of an intermittent misfire. He researched it and found that many folks in online forums recommended replacing the coils and spark plugs due to a pattern failure.
Someone had put all new COPs, spark plugs, and plug boots in this vehicle recently. The owner had also noticed a lack of power and a smell that occupied it under heavy loads, such as going up hills and merging on the highway. The owner told the shop that that the exhaust manifolds had “gotten much hotter than they should be.” Someone had informed the owner that this was most certainly a restricted catalytic converter issue that was most likely caused by the ignition component failure, so the cat was removed and replaced.
In addition, the MAF sensor had been changed, along with scoping the injection and ignition events and a thorough injection/engine decarbonizing process. Moreover, our client (the shop) hadn’t replaced all the parts or performed all the services, although they did have a lot of hours in verifying the failure, the parts replaced, and if the problem was related to parts that were changed/services performed or if it ever got any better along the way.
We were told that the weird part was this thing ran like it was dragging a boat anchor at times, but there were no codes present. Furthermore, we were told that “timing was checked” and the engine was in time. Fuel pressure and volume were also reported to have been checked and were within spec. I knew from the jump it was going to be something overlooked or off the beaten path, as my client has some competent techs; they are usually pretty thorough before contacting us. My client said the motorhome’s owner was a would-be great potential customer for them, owning a company that had a large fleet that needed servicing. They were emphatic about getting to the bottom of this issue and requested our help. A solid plan of attack and logical path would be required to get to the bottom of this.
The appropriate technician
I set the appointment and dispatched one of the best and brightest techs, Bryan. He is a sagacious professional who has an uncanny ability to drill down on these weird driveability dilemmas. As a company, we strive to be systematic and logical in our diagnostic process, although this doesn’t mean we never go down the rabbit hole and have an occasional misdiagnosis. My company regularly attends training, both outside and in-house, to help the techs make educated, informed, logical decisions, rather than making decisions from silver bullets, pattern failures, or Google searches. We strive for and emphasize the need for a systematic and common-sense approach to diagnostics.
Bryan arrived and quizzed the service manager as to what had been done to the vehicle, and what — if any — parts had been changed. He started his diagnosis by gleaning as much information with the least amount of effort by hooking up the scan tool. The complete code scan revealed no PCM codes active or CMDTCs. The shop thought the low power could possibly be caused by defective COPs that were recently replaced, so a misfire power balance was checked along with the Mode 6 data relating to misfires. Neither the test nor the data indicated any misfire activity. This vehicle is OBDII-rated, so one would think there would be “breadcrumbs” or something that would have coded indicating an issue. From the onset, this appeared to be a “no-fault code” driveability issue.
The vehicle baseline and plan of attack
The base data PIDS were looked at KOEO and then the vehicle was started. Nothing seemed to jump out there, either. The driveability complaint needed to be verified with a test drive. Many who have attended my classes have heard my rant regarding test-driving a vehicle for a driveability complaint without a scan tool hooked up. My second pet peeve is when a technician test drives a vehicle with a scan tool hooked up and doesn’t record a snapshot. Failure to do this is simply laziness and poor diagnostic technique. Scan tools have grown so much more powerful and the buffers have gotten so much bigger not to leverage this technology to your benefit.
The techs who work with me know this is important and have seen the diagnostic value in doing so. The vehicle is driven in a manner to gather data that will be useful in determining how well the engine is being fueled, whether the PCM can control fuel delivery (and to what degree), whether the engine has fuel-related misfires and the engine’s ability to breathe, just to name a few. All with a simple test drive, a scan tool hooked up and a snapshot recorded. The data is saved and evaluated after the test drive; this is for safety — it is dangerous to be watching a scan tool while test-driving a vehicle. I wrote an article for Motor Age published in April 2019 that detailed the diagnostic test drive process.
The test drive
The process begins by reducing the data PIDs to only the ones you feel are required to gain a solid direction in your diagnosis. I always want to know how well the engine is being fueled and if the PCM is in charge of fuel control, so O2 sensors, fuel trims, and loop status PIDS are a must-see for me. In addition, I want to know how well the engine can breathe — its Volumetric Efficiency (VE) — so I select PIDs such as MAF GPS, Load, RPM, BARO, and IAT. The information from this last set of PIDS will be inputted into a VE Calculator to give us our VE in a percentage. This gives us a window into how well the engine can flow air through it. Remember, an internal combustion engine is a large air pump that creates kinetic energy. These are the bare minimum number of PIDs I use on a diagnostic test drive. Remember the more PIDs the scan tool must incorporate into its snapshot, the slower the baud rate or refresh rate of the tool. Moreover, if you select a preselected data PID group on the scan tools menu, it can make the screen very busy and lead to information overload, often overwhelming the technician and sometimes causing things to be overlooked or missed.
Bryan selected his PIDs and proceeded to attempt to validate the customer’s complaint via the diagnostic test drive. He made several WOT runs through a 1-2 shift. He paid attention to his senses, but he also recorded a snapshot to gain empirical data and diagnostic direction to substantiate his claim. One of his first observations was that the large RV lacked power. His initial thought was that the exhaust may be restricted. He made several snapshots or movies to review back at the safety of the shop.
The forensics of this Ford V10
Upon further examination of the scanner movies, Bryan noticed that the upstream O2 did go rich momentarily under heavy throttle but did not “latch” rich (800-900mv). However, it didn’t trail off precipitously like a lack of fuel delivery, either (Figure 1). The fuel trims also were not drastically out of whack. His initial thoughts were that the fuel pump could deliver sufficient fuel to the injectors and the PCM was trying (at least part of the time) to deliver the proper fuel to the cylinders.
So, what about the low power and the sensation that the engine felt choked back? Time for that VE calculation. VE is going to be calculated using a free online calculator. To calculate accurately, one needs to know at bare minimum the displacement of the engine and the maximum amount of air entering the engine at the greatest RPM. IAT and Baro can help fine-tune the calculation as well.
The scanner movie revealed that the MAF drew 131 GPS of air at 4,540 RPM (Figure 2)! This seemed low to Bryan. These numbers were entered into the VE calculator (Figure 3) — the result was an abysmal 44 percent (my rule of thumb would be at least 75 percent for this engine, at our elevation). So, his initial thoughts were somewhat confirmed. It does appear to be a VE issue. He didn’t believe it to be an MAF issue (which had a shiny new OEM part on it) due to the behavior of the fuel trims at idle and under load. MAF sensors typically fail by overestimating airflow at idle and underestimating airflow the more the throttle is opened. This usually manifests itself through slightly negative trims at idle and increasing positive fuel trims that tend to follow the throttle blade opening. So, if we can trust the MAF numbers are correct, the next hair to split is determining if the VE is an issue caused by restriction or valve timing. The technician decides to diagnostically kill two birds with one stone and go in-cylinder with a pressure transducer. This will give a quick reference of cylinder compression, the timing of some of the valve events, and will reveal any backpressure that occurred and when it specifically happened.
The in-cylinder investigation
The engine had roughly 160 psi of cranking compression, per the in-cylinder transducer. This seemed adequate for this single-timing chain OHV beast of a V10 (my rule-of-thumb numbers are 140-160 for OHV engines). The compression — and the fact that this RV only had 24,000 miles on it — made it highly unlikely that it was out of mechanical time. A quick in-cylinder running capture with a throttle snap was in order next. This will confirm some of the valve timing events and show if the suspected exhaust restriction will be revealed. Several throttle snaps are performed and the most backpressure recorded is 1-2 psi. How could this be (Figure 4)? Bryan knew this vehicle had low power; through scan data and empirical diagnostic testing, he had systematically removed off the table the major factors of low power, such as a misfire, fuel delivery, ignition timing, compression (valve timing), and restricted exhaust.
He went back to his gut feeling that this was a VE issue, and it felt like a plugged cat. This is where his training kicked in; he remembered a class of mine that dealt with low-power/VE issues that incorporated a VE/Low Power Cheat Sheet designed by The Driveability Guys trainer, Scott Shotton, with whom I train as well. He remembered from class that VE numbers can be skewed due to a calculation error, i.e. “GIGO, or Garbage In = Garbage Out.” This is often caused by a failed MAF — the scan tool snapshots and fuel trims ruled this out. So, what else could it be?
The diagnostic detective work
The great Sherlock Holmes quote by Arthur Conan Doyle often comes to mind when dealing with complex drivability dilemmas — “When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth.”
Hopefully, that is what we do as driveability diagnostic techs every day! Bryan incorporated this and his training into his everyday repertoire. Through a logical systematical process, he narrowed this to a VE issue. So, if it isn’t an MAF (calculation) issue, mechanical valve timing issue, or restricted exhaust issue — what else was left? He remembered a John Thornton low-VE case study involving an intake restriction that had been shared with The Driveability Guys and incorporated in some of my classes. The cheat sheet is examined; three things can cause low VE; and he had MAF off the table. Now the hair to split is which is restricted — the exhaust or the intake path of the airflow through the engine. His in-cylinder testing told him that it is not the exhaust, but what differentiated this in the scan data or on the cheat sheet? If you noticed that it is the Baro PID, you would be correct.
The differentiation
The O2s will go rich under acceleration, and VE will calculate low in both exhaust and intake restriction. The following two characteristics will help you differentiate between intake and exhaust restrictions: the fuel trims are often slightly negative with exhaust restrictions and the Baro will make atmospheric pressure and stay close to there while the throttle is open. Conversely, the fuel trims are closer to normal on an intake restriction, but the often-overlooked Baro PID is what tells the tale. The Baro reaches atmospheric pressure momentarily but trends away from or ratchets away from Baro. This has a very distinct look when this PID is graphed. Upon further review of his scanner movies, Bryan realized this was the case. Indeed, the scan data, his diagnostic tests, and his logical approach empirically proved that the low VE has to be caused by an intake restriction!
The cause: a close examination revealed his diagnosis to be correct. There was an obstruction in front of the inlet to the air filter box (Figure 5). The obstruction was removed, and a verification test drive was performed. The low-power issue was corrected, and the VE calculation almost doubled from 44 percent to 84 percent (Figure 6).
Moreover, I’d ask you to note the subsequent behavior of the Baro PID after the intake restriction was removed; it should now reach atmospheric pressure and stay there the entire time the throttle is wide open, rather than ratcheting away from atmospheric pressure. However, I apologize as I don’t have that Baro PID in the “post-fix” capture to display. Instead, we can see the MAF reaching about 220 GPS at 4,673 rpm (Figure 7).
The moral to the story is to develop a systematic, logical plan of attack to diagnostics that includes a diagnostic test drive incorporating snapshots and movies, calculation of VE, and follow-up with your instincts by designing an experiment to test your hypothesis. Document your work. Trust your training and keep up on it regularly. May the diagnostic force be with you!
