Those of you that have come to know me are aware that my experience with diesel drivability concerns is extremely limited. For good reason though. I’m not properly equipped to take on the job efficiently and don’t see a need to take myself in that direction. With that being said, when an opportunity arises to get involved with a job like that, I’m happy to provide a set of eyes for evaluation. I’m always in mindset to learn something new, to broaden my skillset and to allow me to aid and support others. For years, I’ve had the attitude that if I have fundamental knowledge of how a component is designed to work, information supporting how a system strategizes to carry out a goal and a thorough understand of the limitations my tools/test procedures provide, I can solve any issue.
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ENTER CODE : ART30 AT CHECKOUT
|2008 Ford F-250|
The unfamiliar adversary
This case comes to us from a great friend and co-instructor of mine (with CarQuest Technical Institute) Brent Delfel, of Advanced Diagnostic Consulting in Snohomish, Wash. Brent was called to a jobsite with the complaint originating from a 2008 Ford F-250, equipped with a 6.4L diesel engine. It seems upon initial start-up, there is a noticeable hesitation upon acceleration. This fault vanishes shortly thereafter but symptoms will return each time the engine is shut-down and re-started. Brent gathered some data, first with a basic scan tool and then the result of the PIDs led him to pinpointed scope testing. Brent had a theory and asked for my input. He sent the captures to me for evaluation and well…. here we are.
The elementary approach
For me, it always starts at the beginning. Particularly, when I’m dealing with the unfamiliar. This is what I’ve learned about this common rail diesel fuel system:
Diesel fuel is carried from the fuel tank through a fuel conditioning module where fuel is filtered and pressurized (to about 3-8 psi). It is then sent to the engine compartment where it is once again filtered before entering a camshaft-driven, high pressure fuel pump assembly. The HP fuel pump is capable of outputting pressures exceeding 24,000 psi! Some of the diesel is used to simply cool and lubricate the pump assembly. The pressure is sent from the pump to parallel standing injector rails (one per bank of the engine). The pump’s pressure output is grossly controlled by a fuel volume control solenoid and curtailed further with a fuel pressure control solenoid. Both are controlled by the PCM to maintain swift and accurate control of the pressure within the fuel rails. When pressurized diesel leaves the pump, i'ts routed to the injectors. Pressure in the injectors is equal on either side of a nozzle needle. When the injector is commanded to fire, a pressure differential takes place within the injector and a control piston initiates delivery of a highly vaporized/atomized mist of diesel to the combustion chamber. The small dose of fuel being displaced to generate the pressure differential is referred to as “return fuel”. This is because this volume of diesel is recirculated back through the low-side fuel system to start the process over again.
I apologize as that system description/operation was a bit lengthy but there is a method to my madness, and it will all come together in the end.
Let the games begin
Now, having a brief overview of how the system is constructed and what components are operated as a strategy to control the diesel injection system; It’s time to come up with a game plan. This game plan will vary on any vehicle or problem you are encountering but the object of the game plan is ALWAYS the same. How can I test most efficiently to give me as much information with as little time invested as possible? This depends a lot on how accessible components are for testing, what tools I have at my disposal, and how capable my available scan tool is, along with the robustness of the PCM’s software (what information is it willing to give up in a PID list).
So, recalling that I’m viewing this from the other side of the country, I’m limited to viewing only what data has been captured. Let’s begin at what I call “the low hanging fruit.” Information like this is very easy to obtain, usually right from the DLC and answers a few basic questions for me. In this case:
-What am I “feeling” from the driver’s seat (the symptom/ customer-concern)?
-What is causing the concern (incorrect fuel delivery)?
-Is the problem being seen and compensated for by the PCM (control issue or mechanical fault)?
The significance? The answers to those questions can be seen easily in the PID list on even a basic professional scan tool. More importantly, the answers to those questions will point me in the direction of testing in which to deploy. Every step taken from this point forward will be justified and will lead to yet another conclusive answer on which component or part of a system to test next. The PIDs chosen to monitor were:
-Desired Fuel Rail Pressure
-Actual Fuel Rail Pressure
-Fuel-Volume Control Solenoid
-Fuel Pressure Control Solenoid
As mentioned above, these PIDs will tell me if what I’m feeling as a symptom is due to a fuel delivery issue and why. It will also tell me if the PCM is seeing what I feel and trying to fix the issue. Looking at PIDs in a graphed format offers some distinct advantages over numerical forms of data analysis. The graphs not only allow the viewer to see a history of the PID but also allows for a comparison of multiple PIDs simultaneously. This provides for the action/reaction point of view. A means to see a fault present itself and a PCM’s response to the fault. Unfortunately, the available scan tool chosen isn’t capable of graphical formatting; meaning, we can only look a numerical value at a single moment in time. This provides for a disadvantage. But all is not lost.
“Excel” to excel
Although the scan tool isn’t capable of graphing, Brent simply took the data from each individual frame captured. He then uploaded these PIDs over time into a Microsoft Excel graph program. If you refer to Figure A, the fault as well as the computer’s reaction is visible. The graph clearly displays a severe lack of fuel pressure along with the PCM’s command to increase fuel volume to the rail and boost pressure. This means the rail lost pressure and the PCM tried to fix it.
So, referring to Figure B, we can see the lay of the land regarding the entire HP fuel system. HP pump assembly, I have surrounded by a red square. Within that is the pressure control valve, surrounded in the green square and the volume control valve surrounded by the blue square. Both these components are self-contained within the pump assembly. The highly pressurized diesel leaving the pump travels in Red through the rails and to each injector. Pictured in Figure C is the return fuel system. Light blue represents the lube/cooling fuel from the HP pump, the return fuel from the left cylinder head and the return fuel for the right cylinder head. They join together in a “T” and enter a return fuel cooler. This cooler has a test port making pressure samples available to us.