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A PID-o-full diagnosis on a 2008 Ford F-250

Tuesday, October 1, 2019 - 06:00
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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?

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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.

Figure D

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.

Figure E

 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.

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