As time passes and technology continues to advance, we suddenly realize that what was once referred to as new technology has become the norm. The days of sitting back on our haunches and waiting for the next four-wheel brake job to enter the shop have become fewer and further between.
With that being said, our diagnostic approach to drivability symptoms hasn't changed all that much. What I mean by that is simple — we still have to deal with the same physics, and that is not likely to change anytime soon. However, it has become increasingly challenging, as the added layers of complexity of these once simple systems are ever evolving. We, too, have to continue to evolve our diagnostic skillset.
Gracing the pages of Motor Age in the not-so-distant past was my good friend, Technical Support Specialist Chris Martino of Opus IVS. Chris’s seemingly bottomless pit of cleverness once again shines through as I take you through one of his recent solutions.
The over-achieving Audi
Chris was recently challenged with a 2013 Audi A4, 2.0L (CPMA). This vehicle was presented to him with the complaint of "High fuel rail pressure." After speaking with the shop harboring the vehicle, it was stated that before contacting Chris, the High-Pressure Fuel Pump had been changed not once, but twice. When neither of those attempts rectified the issue, a fuel rail pressure sensor was changed as well. That attempt was all for naught.
Once Chris was assessing the vehicle, a scan for DTCs corroborated the story provided by the client
(Figure 1). This vehicle was producing more fuel rail pressure than the PCM desired. Typically, when a fault within the fuel system occurs, it's the more likely one producing less fuel pressure than desired. So, let's see what could be crossed off the list of suspected-faulty components, simply due to the nature of the fault:
Low-pressure fuel pump
Fuel Pump Control Module
High-Pressure Fuel Pump (unlikely, since it's been replaced multiple times)
Fuel Rail Pressure Sensor (it, too, has been replaced—not "high" on the list of suspects)
The next question should be one derived from what is already known above. What could cause fuel pressure to be higher than desired? To answer that question requires understanding how the Gasoline Direct Injection System (GDI) functions—more importantly, how fuel rail high-pressure is made.
For this vehicle, the High Pressure Fuel Pump (HPFP) is driven by the back end of the exhaust camshaft. Four lobes will operate the HPFP, similar to how conventional camshaft lobes operate the valves (Figure 2). It can be seen that the pump will be stroked four times each engine cycle (each full rotation of the camshaft). However, the pump cannot produce pressure unless the fuel it is attempting to displace is trapped. For this to occur, a Fuel Regulator Valve solenoid (FRV) is attached to the pump assembly. When the valve is closed, the pump traps fuel and can squeeze the fuel to build pressure. If the FRV is open, the fuel is just displaced back into the low-pressure supply line, from which it came.
There are two differently designed FRVs available. Depending on which one is present for the vehicle being addressed will determine the diagnostic approach. A “normally-open” FRV allows fuel to vent when it is not energized. Said another way, the FRV has to be energized to allow the pump to generate fuel pressure. The other design is a “normally-closed” FRV. At rest, this FRV allows the pump to generate pressure. When this FRV is energized the fuel is vented and the pump cannot generate pressure.
So, how can one swiftly determine which design FRV is present on the vehicle? That is as simple as monitoring the fuel rail pressure PID on the scan tool with the engine running and unplugging the FRV (Figure 3). Because the fuel rail pressure sky rocketed, it proved the FRV to be of a normally closed design.
What we should now realize is that the PCM has to energize the FRV to limit fuel pressure. Is it possible that the FRV is not functioning correctly? Or perhaps the PCM is not sending the correct signal? That is as simple as watching an amperage trace when coupled to a lab scope. The trace will not only prove the command is sent but also that the solenoid is physically shuttled. Take my word for it—it happened.
A stretch of the imagination
One other thought comes to mind and it's of significant importance. The HPFP is driven by the camshaft, which is assumed to be in proper time with the crankshaft. Well, at least it is assumed by the PCM. How did we determine that assumption? We know that the PCM will set a DTC if the camshafts are not properly timed.
Keep in mind that the pump begins to generate pressure only when the FRV is de-energized/closed. It’s the timing of the FRV’s closing that determines how much pressure will be generated during a particular stroke of the pump. If the FRV closes earlier, more pump stroke could be capitalized upon, generating more fuel rail pressure. A delay in the FRV’s closing will create the opposite effect. Less of the pump stroke could be capitalized upon and would result in less fuel rail pressure generated during that pump stroke. Let’s consider what effect late cam timing will have on this process. Chris Considered this arbitrary scenario for discussion purposes:
A pump is beginning its stroke and the PCM commands the FRV closed at about 50 percent of the completed stroke. The pump will build only about half of its maximum capability. If the cam timing were late, FRV would still be commanded at the same time (relative to the crankshaft angle). However, the camshaft-driven pump would not be at 50 percent of the stroke. It would be delayed and may be approximately 25 percent of the stroke. This would allow the pump to travel the remaining 75 percent of the stroke and would be generating pressure the entire time. This would leave the fuel rail under higher pressure than intended. This would all be visible through the eyes of the fuel rail pressure sensor and the DTC would soon follow.
The theory sounds plausible, so Chris decided it was the next logical test to perform. A lab scope capture displaying the camshaft position sensor signal and how it correlates to the crankshaft position sensor signal (CKP/CMP correlation waveform). This of course would infer cam timing.
Capturing the signals on the vehicle and using the measuring rulers of the PICO scope software, Chris first determined 720 degrees of crankshaft rotation (Figure 4). He then used the cursors to measure the difference (in degrees) when the signals transitioned (Figures 5 and 6). He performed this test in the same area of the captured signals on the suspect vehicle as well as a known good file he had stored in his library.
The proof is in the pudding! Although not quite an entire "tooth-off," it appears the chain has indeed stretched. This would certainly cause the symptoms he has experienced as discussed above. But why not a timing DTC? Referencing service information, the code set criteria for a timing DTC indicates there must be at least an offset of 11 degrees. The 7-degree offset this vehicle has is not enough to set the DTC. It is, however, certainly enough to affect the pressure output of the HPFP.
The point is, it’s justifiable to advise the client that they must disassemble the engine to access and inspect the timing components. When the engine was disassembled, the evidence was clear. The view of the timing chain tensioner showed it to be extremely extended when compared to a known good (Figure 7).
It’s a given — technology will always improve. That is just the nature of evolution. We can run from it, but we can't hide from it. You have a decision to make as a diagnostician: stagnate and hope for the best or continue to grow your skills and open your mind to newer concepts. As I always say, you should reach for the stars; you'll find it's never crowded up there.
