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.
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ENTER CODE : ART30 AT CHECKOUT
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
- ACTUAL and DESIRED throttle angle
- Fuel Trim