Vehicle brand challenges can impact diagnostics

What you'll learn:

• Common Subaru repair challenges 
• Tips for proper diagnosis
• Understanding different PIDs across brands and what they mean

Some of you may know I am an industry technical trainer and a mobile technician. I work on almost all makes of vehicles, but there are a few makes I do not see very often. Historically, in my day-to-day work, I have not seen Subaru vehicles regularly. Recently, Subaru's market share has been growing, and more Subaru vehicles may be coming through your door. I have personally been seeing more Subaru vehicles in the last year or so, and this has forced me to sharpen up on my Subaru knowledge and skills. Additionally, I have decided this may be worthy of a class. To do justice to such a class, I had to network with individuals who know more about Subarus than I do. My choice of Subaru contacts was Leo Gilmore. Between the two of us and many broken vehicles, we put together a class. In the process, I learned a lot.  

Our class intends to cover many issues such as differences between other makes, common failures, differences in scan tool data, and diagnostics for said vehicles. As a result, we intend to teach this class at Vision 2022! Here are a few case studies that we have gathered on Subaru vehicles that may or may not be used in our class at Vision. Here we go. 

2012 Outback 2.5L “P0301” 

This vehicle presented itself with an illuminated MIL and a rough-running concern that any of us could feel was a single-cylinder misfire. If we were to look at scan data, we would see some differences if we were used to other makes of vehicles. What I mean is that there is not a data PID named “Misfire Counter,” like in the case of a General Motors vehicle. In Subaru's world, the data PID we would be looking for is called a "Roughness Counter." In this case, we can see that we are counting misfires on cylinder number one (Figure 1). This data was recorded with a Subaru Select Monitor (SSM), which is the factory tool for Subaru. Most quality aftermarket scan tools should be able to see this data. 

Fuel trim numbers were checked next and were close to normal when the vehicle was in closed loop. Fuel trim diagnosis is extremely valuable when the vehicle is in closed loop. Incorrect fuel trim numbers could point us in the direction of a fuel delivery-related issue. However, if the fuel trim numbers are close to normal, we may need to head in another direction.  

The next option was to investigate the potential for a mechanical or ignition failure. For the sake of efficiency, the throttle was held wide open and the engine was cranked over. I know it is old school, but this test still gives us direction. The result is a rhythmic engine RPM fluctuation that can be heard. I think it is time to investigate a mechanical issue. 

A high current probe was connected around the battery cable, the throttle was held to the floor, and the engine was cranked over. The scope capture confirmed that there was a compression issue in a single cylinder of this engine (Figure 2). For you scope individuals out there (I am one of them), I realize there was not a sync present to identify the cylinder with low compression, but we knew which cylinder it was because of the DTC. 

Now we had quickly determined the issue was mechanical. We could use vacuum transducers to narrow down the cause. In this case, the tool choice was conventional. Either way, we came to the same result. Cylinder number one was confirmed to be near the top dead center of the compression stroke and a cylinder leakage test was performed. I was pretty sure that the results confirmed an issue (Figure 3). 

While snooping around to find where the leakage was, a rubber glove was taped over the tailpipe, and it immediately inflated (Figure 4). I think it was safe to say that the loss of compression was related to the exhaust valve.  

The relatively simple task of removing the exhaust manifold was performed. It became visible that the exhaust valve guide had broken loose and dropped from its bore in the cylinder head, hindering the exhaust valve from closing fully (Figure 5). It should be noted that this is a common issue on some Subaru engine platforms. 

2008 Impreza WRX 2.5 L Turbo P0011 and P0021 

This vehicle came in for an MIL complaint with “A-Camshaft Position System Performance (Bank 1)” and  “A-Camshaft Position System Performance (Bank 2)” DTCs stored in the ECM. In Subaru speak, “A-Camshaft” means the intake camshaft is the issue. This particular engine application only phases the intake camshafts. With the scan tool connected and viewing the AVCS (Active Valve Control System) data while varying engine RPM, it was noted that bank 1 did not vary at all, while bank 2 changed only a couple of degrees. 

Not ignoring the basics, the next step was to check the oil level. Lo and behold, the crankcase was more than one quart low. The oil level was corrected, the DTCs were cleared, and the vehicle was taken on a test drive. AVCS data was observed again. Now the bank 2 (left) camshaft was moving, but the bank 1 (right) camshaft still did not phase, even though it was being commanded (Figure 6). During the test drive, the P0011 returned but the P0021 did not. 

Current flow through the oil control valves (OCVs) can be seen in the data, so it was a safe bet that there was circuit integrity and the potential for an electrical-related fault was minimal. The next step was to investigate the possibility of a mechanical or oiling issue that affects bank 1 only. The right bank OCV was removed and inspected. The oil screen was removed and was found to be plugged solid with debris (Figure 7). 

The screen was replaced and everything was re-assembled. The next test drive confirmed that the issue was resolved for now. Most likely the cause of the restricted screen was a lack of normal maintenance, as noted by the initial low oil level, and the issue could reoccur over time. 

2013 Impreza 2.0 L Engine replaced and MIL illuminated

 This particular vehicle had an engine replaced with a used unit. After the engine was replaced, DTCs were cleared, and the engine was started. The MIL illuminated immediately. A DTC scan was performed, and P0366 “Camshaft Position Sensor — B Circuit Range/Performance Bank 1” and a P0391 “Camshaft Position Sensor — B Circuit Range/Performance Bank 2” were both stored. If you remember from our previous case, camshaft A was the intake camshaft. A DTC that calls out “B” is referring to the exhaust camshafts. Unlike the last case study, this engine application phases all four camshafts.  

The Subaru diagnostic information was pretty limited for these DTCs. The trouble chart would have had us go through a series of circuit tests, much like other manufacturers do, that may be time-consuming and inefficient. However, a description of the code-set criteria was found that could aid in our diagnostics. In a nutshell, the ECM will store these DTCs if there are irregular pulses detected from the camshaft position sensors during a relatively low number of crankshaft revolutions. The information also showed a poorly drawn CKP/CMP capture. Although this may not be completely valuable, at least it pointed us in a direction. Time to break out our scope!  

First, we needed a known good capture to compare to. It does not matter what we gather with a scope if we do not know what “good” is. In this case, a known good CKP/CMP scope capture was found for a 2014 Crosstrek with the same 2.0 L engine (Figure 8). 

For reference, the top trace (blue) is the CKP. The bottom two traces (red and green) are the exhaust CMPs for both banks. Cursors were also pulled up on the screen to analyze timing. Again, this was a known good vehicle. The next step was to scope our subject vehicle. Making the same connections with our scope we obtained a capture that did not match the known good. The capture obtained from the subject vehicle showed some anomalies in both CMP signals that should not have been there (Figure 9). 

I think it was safe to say that the CMP sensors were good. They both pulled down to a clean ground, the voltage transition was instant, and both sensors hit a full 5 volts. Now it was time to investigate the camshaft reluctors. The CMP sensors were removed and the engine was turned over by hand to inspect the condition of the camshaft reluctors. Looking through the CMP mounting bore for Bank 1 exhaust camshaft showed an obvious problem (Figure 10). Repeating the process on Bank 2 yielded similar results (Figure 11). 

The cause of the issue was damaged exhaust camshaft reluctors. If we were to play automotive CSI, I would have guessed that the used engine had been apart in the past to repair camshaft carrier oil leakage. Head gasket issues were common on older Subaru engine platforms. Although that problem seems to have been fixed, a common oil leak concern on newer engines is the seal between the camshaft carrier and the cylinder head. My guess, in this case, was that during a previous repair, the reluctors were damaged by the technician during an unrelated repair. I cannot swear to that, but regardless, the cause of the DTCs had been identified and the engine had to come back out again. 

Conclusion 

These three Subaru case studies illustrated a few points. Some show similarities with other manufacturers and others do not. If you are not used to working on Subarus then I hope this information has helped. A misfire is a misfire, but the data PIDs may be different. Mechanical engine testing is still the same, even though spark plug access may be tougher than some other vehicles. Acronyms may be different, such as “AVCS,” but the systems still work on the same concepts and our diagnostics are no different once we know the terminology. 

Assuming that COVID is not going to shut down Vision 2022, and you have an interest in sharpening your Subaru skills, please feel free to attend, and Leo and I will do our best to provide many more case studies and answer your questions to the best of our ability.