A PID-o-full diagnosis on a 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.

After analyzing the graphed PIDs, it’s time to pinpoint the location of the fault. We should all understand that the HP system can only function properly if it receives a healthy supply of low-pressure fuel from the low-pressure pump and filter assemblies. This was verified by Brent with a pressure test under load when the fault occurred. Brent “tee’d-in” to the low-side supply line leading from the fuel conditioning module to the filter module. There was no decay (exceeding specification) of low-pressure fuel during the exhibited symptom. So, logic will tell you that we can eliminate the low-side fuel system as a contributor to this fault. The focus will be on the HP side of the fueling system only…and the only test we performed was by placing a gauge on the low-pressure side. Not a lot of time/energy invested at this point. Eluding to the fault being on the HP side left a few items as potential failure-points. The pump itself could surely be at fault, but so too could it be any of the injectors. Now, the nature of the HP leak would tell us if an injector was over delivering, as we should see this reflected in the smoke bellowing from the tailpipe. This vehicle DID NOT experience this fault.  If the injector was leaking to the crankcase, we should be seeing evidence of that in the oil but that too revealed no evidence of diesel contamination. However, the injectors could be leaking excessive fuel back to the return-side of the system. How could we tell? Even if there was no specification found for this type of test?

Let’s scope it out
I’ve mentioned many times before that having a thorough understanding of the tools you use makes that tool an extension of your mind. It allows you to make inquiries of the components you desire and offer you the ability to evaluate an entire system dynamically. With that notion in mind, Brent deployed the scope to monitor the action/reaction display of the system when it is malfunctioning. Looking at Figure D, In RED is the signal from the fuel rail pressure sensor. In green is a pressure transducer connected to low-side fuel supply system. The black trace is a math-channel on the scope. It is a derivative of fuel volume control solenoid command and is displayed in “duty-cycle percentage.” It represents the PCM commanding more fuel pressure generation. The story told by the capture is that under high demand situations that we placed the vehicle under, the PCM commanded a high amount of fuel volume to the HP pump. This allowed the HP pump to produce a steep increase in pressure, as it should. The lack of performance from the engine could be felt, meaning the fuel never made it to the cylinders. Now, the loss of low-side supply typically causes a loss in HP output. It did not do so in this case. (Further explanation to follow). Clearly the HP pump could produce the pressure so why didn’t it make it to the cylinders? This is the question that will be answered in the next justified test.

Look at Figure E. In blue is the injector current trace from injector #1 only. It is used simply as a point of reference along with the firing order. This will indicate which injector was firing at any given point in the engine cycle. You can see I’ve annotated the top of the capture to show the firing order and partitioned the capture eight different ways, to represent each of the eight injectors firing. In red is a pressure transducer on the fuel cooler test port to represent pressure on the return-fuel side of the system. This is a zoom capture to represent one engine cycle and is displays a variation in pressure, on the return fuel side, comparing each injector’s firing to the other seven. What isn’t important is the actual pressure value (not that we found a documented specification, anyway). What is important is the fact that the injectors don’t return the same amount of fuel when they de-energize.

 It’s the combination of the captures that tells the entire story. The PIDs reflected a lack of fuel pressure when it was commanded and that the PCM was trying to compensate. A test of the low-pressure system was carried out and displayed the ability to delivery pressure under high-demand situations, allowing the HP pump to do its job. Although the low-pressure side of the system dissipated under load, it stayed within specification. The big increase in HP pump output (to pressurize the rail adequately and overcome a leak to the return-side was like turning on a faucet) caused the depletion in low-pressure supply.  A lack of performance proved the fuel never made it to the combustion chambers and the final capture told us why. Under high demand situations (when the symptom occurred) the return fuel pressure increased for all injectors except for two of them (#3 and #8). The HP pump made the pressure but instead of delivering to the combustion chambers, six (of the eight) injectors dumped the fuel to the return-fuel system, condemning the injectors as being mechanically faulty. A specification for return-fuel pressure wasn’t needed. The process of elimination determined where the fault lay.

Waiting to exhale
Now, it is almost unheard of, for me to make a claim like the above without having concrete-proof of a fix. Unfortunately, the customer chose not to invest the money in the suggested replacement of the all eight injectors. There is still some take-away from this case-study. The story never changes, just the characters…have a thorough understanding of components/system’s functionality, understand the basic-fundamentals and the limitations of the tools/test you implement and there are very few tough-to-find faults you won’t make quick work of.