Case Studies

As defined by Merriam-Webster's dictionary, technology is a practiclal application of knowledge or a capability given by the practical application of knowledge. In this time of technological advancements, it is my belief that we as automotive technicians sometimes take for granted, the amount of useful information that is available to us, literally right at our fingertips, even in the driver's seat of the vehicle we are analyzing. The advancements in computer technology allow us to view data in almost "real-time". The benefits are clear that, if we learn to interpret the available data, we can make diagnostic decisions with minimal time and effort invested. This , in turn, helps keep us efficient. 

I think its this very technology that can sometimes serve as a crutch for those that have become so dependant on the fruitful datastream. Losing touch with how "things" function at their most basic level or how sub-systems react with one another during a failure can be costly if having to address sometimes "complex" issues without a good datastream. This case study is written to address the importance of having a solid understanding of system and component funtionality. It also serves to demonstrate how efficient, access to a good datastream can be. The actual "faulty component" is not significant here. The actions necessary to pinpoint the failure are and the order of the testing carried-out is.

                           performed was a simple VE test. What this test allows me to do is scratch "base cam timing" off of my list of failures. FIG #1 demonstrates this. Taking the vehicle up to 5000 rpm/WOT allows the engine to operate in a scenario where it can inhale the most air. A simple equation that puts me in the ball park of "pass or fail" is to multiply the displacement in "liters" by 40. Because this is a 3.0L engine the equation (3.0 * 40= 120gps). If the calculated MAF is much less than my math, i may suspect a breathing issue with the engine and further analysis of fuel trim would be carried out. That would rule out an issue with unmetered air or false MAF input. However, this doesn’t appear to be the case at all, nor did i anticipated finding an issue with base timing.

Referring back to the beginning of this case study, I referenced how technology has really helped us move in a more efficient direction. With a newer vehicle, and more capable scan tool at hand, the power of this technology would’ve been taken advantage of. Monitoring cam/crankshaft correlation as well as actual and commanded camshaft phase via scantool PID would’ve been carried out. Furthermore, through bi-directional control, i would likely have been able to reproduce the fault in my workbay, allowing me to further pinpoint the root-cause without the need for extensive road testing. The scantool I was using to analyze this vehicle had very little capability . I had virtually no bi-directional control for VVT nor did it have sufficient PIDs that directly reflect the status and position of the the VVT system. In fact, because the fault was so elusive, without through understanding the total systems' functionality, any repair attempt would simply be a guess and may prove to be an expensive error.

A strategy to monitor the following PIDs was systematically carried out and eliminated over a handful of roadtesting-techniques.

 I repeated the road test in the same fashion to reproduce the misfiring condition. The MIL remained extinguished and no DTC was flagged as of yet. If you take notice, I always try to use my PID graphs to tell the entire story. If I build them in this manner, they tend to deliver the fault and reaction to the fault on the same screen. This makes diagnostic decisions far easier. Especially after some time has passed and you are no longer in the heat of the moment. FIG #3 shows that cylinder 1 (representing the entire bank) begins to misfire at the left of the capture.  You can see that is appears to be load-dependant. the misfires seem to vanish under heavier load conditions and begin to return as load is decreased. This is providing a valuable clue but it will all come together in the end, with a few more captures.  

FIG #5 was captured after the PCM had gone into an open-loop strategy (fuel trim was not involved). As the fault began to present and bank 1 began to misfire, a definite shift in AF ratio was occurring, bank-to-bank. Again, the AFs for either bank began to mirror/deviate from each other. As the misfires began to diminish , the AFs mA values began to head towards an indication of stotiometry.

So, lets dicuss this further. During the symptom, as the suspected shift in bank 1 intake cam is occurring undesirably...Bank1 is inhaling air at a different rate when compared to bank 2. Unfortunately the PCM will fuel both banks identically according to the MAF sensor input. This means The PCM can only assume that each bank is equal to the other, in regards to air mass inhaled. As a result a specific amount of fuel is delivered equally to both banks. So, because bank 1 is suspected to be advanced (because that is how an intake cam is phased) there is far more valve overlap occuring and all three cylinders on bank one are inhaling exhaust gasses as well as air from the intake manifold. Where as bank 2 is only drawing air from the intake manifold. Worded differently, of all of the air being measured by the MAF sensor, bank 2 is inhaling the majority of it and is being under fueled. Bank 1 is inhaling far less of it and is being over fueled. This is what is being reflected in the Total fuel trims in FIG #4 and AFs mA values in FIG #5. So, why does the fault only present under low load condition and not all of the time? Demand for intake cam advancement is not desirable at idle. This is the same reason why some old muscle-cars idle very rough but perform fantastic under high-load/RPM condition. By phasing the intake cam advanced at higher load conditions allows for a savaging effect. The exhaust gasses being expelled from the cylinder have inertia and help draw the fresh AF chage into the cylinder, causing it fill more efficiently. Because this is desirable under load, the engine performs as designed during this operating condition. 

Through data analysis only, we have determined that a cam phase fault/bank-to-bank breathability fault indeed exists and couple more test will confirm our suspicions. We will monitor correlation of the CMP sensors for both banks and CKP sensors on a lob scope. We will compare them to a capture from a library of known-good captures. This will not only tell us if our camshaft is advanced, but with a little math and the use of cursors, we can determine how far out of phase the camshaft is. Sometimes it is far easier to perform this test in lieu of acquiring in-cylinder pressure wave forms (which requires removal of the spark plug ) but bank #1 spark plugs lay beneath the intake plenum and the last thing I want to do it disassemble anything for testing purposes.  

 Now, take a look at FIG 7. This is a capture of the suspect vehicle, acquired in the same fashion, during the fault/symptom. Bank2 CMP sync pulse (yellow) appears to fall where it was supposed to but bank 1 CMP (red) appears to be far advanced. Now, for the math....by spanning the cursors 720 degrees (one full engine cycle) and minding the elapsed time for this event to occur, I’ve successfully determined that 720 degrees of crankshaft rotation took 118.5 mS to occur. Dividing the elapsed time into the 720 deg will yield me how many degrees occur in one mS of elapsed time ( 720/ 118.5= 6.08 degrees in 1ms).