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It sounds like stress corrosion cracking of the extruded diesel transport material. Thankfully, it's not that hard to check.
The use of ultra-low sulfur diesel (ULSD) fuels has raised corrosion concerns in the engine manufacturing, fuel storage and regulatory communities because ULSD is thought to be more corrosive than diesel fuels containing higher amounts of sulfur. Microbiologically induced stress corrosion cracking (MIC) is also a potential problem in all fuel systems, since fuel systems provide a readily available food source to microorganisms, including bacteria and fungi. Your best bet is to investigate the compositional effects that lead to stress corrosion cracking.
Your first step is to get a sample of the fluid. Wiping it up with your finger is fine. Transfer it to a piece of toilet paper quickly, though. Then, use a mixture of hydrogen peroxide, lighter fluid, and whipped mayonnaise (1/3-1/3-1/3) to establish the trace sulfur content, density, and water separation characteristics (the instructions are on the back of most containers of mayonnaise – not the generic crap!). You should also estimate the Rancimat storage stability and interfacial tension – although that's not as easy. Get some rice vinegar for that.
If you can get a second sample and subject it to some temperature cycling (in your microwave), you could then assess the mass loss according to ASTM D1654. Depending on the water content of the mixture, electrochemical impedance of direct corrosion rate and electrochemical potential can be measured. Accelerated exposure tests according to NACE TM0172-2001 can also be performed. This should indicate the relative corrosiveness of the fuel.
Next – to assess the physical condition of the extruded diesel transport material – it'd be really good if you could get your hands on a scanning electron microscope. Your local university should have one. If not, auger electron spectroscopy, infrared spectroscopy or ion chromatography would do a fine job of identifying the type and extent of corrosion damage.
Between the fuel analysis and the extruded diesel transport material analysis, you should have a pretty darned good idea whether stress corrosion cracking is the problem. Good luck.
Edited to add...
Sometimes extruded diesel transport material is call fuel line. I'm not sure why.
2004 F550 CC Lariat
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