We begin the saga at Assured Autoworks in Melbourne, Fla. This reputable shop is owned and run by my good buddy, Brin Kline. Like many times before, he acquired the subject of this month's article as a “tow-in” from another local shop. The original complaint from the owner of this 2011 Chevrolet Aveo — with a 1.6L MFI with an automatic transmission and 59,661 miles on the odometer — was that the vehicle lacked power output and had DTCs pertaining to an exhaust cam sensor fault. Upon further investigation, the original repair shop found a broken reluctor tooth on the exhaust cam. The cam was replaced, along with the timing chain, guides and both the intake and exhaust cam phasers. That repair, although obviously required, led to disappointment and self-doubt as the MIL remained illuminated and the vehicle still performed unsatisfactorily. The original repair shop disassembled the front of the engine many times to double and even triple-check the cam timing only to find everything still appeared to be indexed as intended. The original shop also pointed out several clues that led them to believe the cylinder head had been replaced with a used one. That included the heat-sensitive "button" designed to indicate if the engine was ever overheated. It was noted that the vehicle was purchased wholesale and that no further history of the vehicle was known. This is where Brin jumped in to apply his skillset as a diagnostician, a skillset his shop has developed a reputation for.
Grabbing the low-hanging fruit
The vehicle was scanned for DTCs (Figure 1). Brin and his tech quickly ruled out the cause of the first two DTCs as both solenoids were left disconnected from the previous shop’s diagnostic attempts. All DTCs were cleared and a road test of the vehicle was carried out…at least, the attempt was made. At an idle of 735 rpm, the little Aveo’s 1.6L engine generated a manifold vacuum just slightly stronger than 11 in/Hg (Figure 2).
From experience, we expect vacuum levels closer to 20 in/Hg (at sea level) from a healthy engine. Sometimes the engine struggled to idle and other times, it simply couldn’t get out of its own way. During the road test, the faults for MAF and MAP were obvious. As stated above, the vehicle had performed very poorly, almost as if the cam timing was set incorrectly or it was starving for fuel. Brin monitored some key inputs regarding the breathability of the engine, considering the exhibited symptoms, to help pinpoint the nature of the fault. These PIDs will determine if the engine is being fueled properly or not and if the incoming air is being weighed properly. They included:
- ACTUAL and DESIRED camshaft positions
- MAF
- MAP
- ACTUAL and DESIRED throttle angle
- LOAD
- Fuel Trim
The vehicle again failed to accelerate properly. Although it lacked power, the throttle position indicated the PCM’s intent to accelerate. The car, however, did so reluctantly. The throttle position displayed approximately 48 percent when the APP indicated a request for acceleration (perhaps a state of “limp” or “default?”). This made it impossible to perform a proper Volumetric Efficiency test. The fuel trim showed no signs of correction indicating that the engine was fueled properly, given the amount of air it was inhaling. What’s contradictory, though, is the camshafts’ DESIRED and ACTUAL PIDs reflect no deviation from one another. Meaning, the timing components were installed correctly and phasing as commanded (Figure 3).
Justified exploratory surgery
Reviewing the scan data, the PCM seems happy. It’s breathing right… but it’s not breathing right, if that makes any sense. Proving what is good with the engine holds as much diagnostic value as discovering what is wrong with it. One way to verify that the PCM is correct in its interpretation of camshaft/crankshaft correlation is to scope the signals over an entire engine cycle. Brin did just that with his scope. He then used the scope’s vertical cursors to determine how many degrees each camshaft signal transition was from a common point of reference. Like many elite technicians, Brin is one for sharing information to better hone his skills and so that others may learn as well. In return, when Brin calls on his pals for some needed info, they are happy to oblige. Reaching out to friends across the country, Brin had access to the same capture but from a known-good vehicle. He simply compared his capture to the known-good and determined that (according to the camshaft/crankshaft signals) the engine was timed properly (compare Figure 4 and Figure 5).
Of course, the possibility of a damaged locator or shear key could certainly have one of the shafts out of time, while the reluctors remained in time. He could certainly have removed the valve cover for a visual inspection to prove or disprove that theory. However, Brin values his time and through networking with other like-minded technicians, Brin has learned and practiced (many times over) using his mind and tools to force the fault to surface, rather than chasing down the fault. In lieu of disassembling the top of the engine for visual inspection, a more efficient approach was chosen initially. Using a series of easy-to-perform test procedures to justify more involved forms of analysis likely leads to the most efficient diagnosis. With the use of an in-cylinder pressure transducer and the scope, Brin observed the changes in pressure for one of the cylinders while cranking the engine over through multiple engine cycles. This allows for an evaluation that will yield him a reason for disassembly. Like many others, Brin doesn’t wish to disassemble anything unless he knows he will find a fault. That is exactly what this type of testing will provide for him — a reason to disassemble.
Multiple arrows in the target
Like any other type of testing, practice and acquiring a library of known-good captures will give you the trained eye required to spot bad because you’ve become confident in what good looks like. While viewing the in-cylinder compression capture acquired from our poorly performing Aveo, the waveform exhibits some very strange characteristics. When compared to the known-good captures Brin has access to, the peak compression is significantly lower in the suspect vehicle. What’s very strange is the fact that a visible exhaust plateau is present during cranking. This is a very atypical characteristic of a cranking compression event. The presence of this plateau is due to the in-cylinder vacuum of over 16 in/Hg being generated (Figure 6).
What’s equally as revealing is the late closing of the intake valve. The late intake cam is the most likely cause for the deep in-cylinder vacuum as well as the low compression (see Figures 7 and 8).
So, lets recap for a moment:
- no timing correlation DTCs, but breathability DTCs present
- engine breathes improperly and performs poorly
- fuel trim doesn’t indicate a skewed air mass input
- timing signals indicate proper cam/crank correlation
- in-cylinder pressure testing exhibits a very late intake camshaft
There is no doubt that a mechanical fault is present in the engine. How can this be? Perhaps a damaged keyway on the intake camshaft?
The fireside chat
A discussion was carried out via FaceBook Messenger among a group of enthusiastic diagnosticians. We analyzed the information that Brin presented to us and weighed the facts. Due to the indication of the used cylinder head installation, we used logic and determined that perhaps an incorrect intake camshaft was installed (difference in application). With this information, the confidence grew and Brin found a known-good in-cylinder capture from a previous model year of the same vehicle platform. Again, the networking provided him with the capture. It reflected, almost identically, the same characteristics of his suspect vehicle. A new intake camshaft was ordered and as you can see by the comparison of the intake camshafts, the proof is clear (Figure 9).
The reluctors are configured the same. It’s the actual clocking of the lobes that is significantly different, model year to model year. It conclusively explains:
- why no correlation DTCs were present
- why the cam/crank correlation waveforms reflected proper timing
- why the engine performed so poorly
Fishing with a net instead of a pole
Regardless of your level of experience in any realm, including automotive drivability analysis, there is always someone who has some experience, information or data that you don’t currently possess. Said another way, it's beneficial to network with others. With the compound knowledge and experience, everyone involved prospers and grows. The automotive industry as a whole continues to move forward in a positive direction and you will build friendships that can last a lifetime.
I want to take a moment to not only thank all of the participants who took the time to offer their input but also indicate where the participants of this case study were actually located. It still amazes me that we can be thousands of miles apart and still have a conversation like we are all elbows up at the bar, enjoying a few beers together!
Special thanks to these gentlemen for their contribution: Brin Kline, Assured AutoWorks in Melbourne, Fla.; Brian Culotta, Dave's Auto Care in Chardon, Ohio; Justin Miller, JM Diagnostics in Herriman, Utah; Matt Wallce in Clover, SC; and Scott Brown, Diag.net in Fontana, Calif.
