I've realized that I've fundamentally miss-stated things in the middle paragraph above. This is the risk of commenting without having hands on the parts as I write. I found a description I wrote some time ago that reminded me of what is happening.Thats kind of what I was thinking. This suggests that by the time metal starts making it out of the pump that things are probably pretty bad inside. When you look at the flow pathway's in the pump, there really isn't a lot of circulation through the pump itself. One hole dumps fuel into the LP cavity. Another hole basically delivers fuel out to the Fuel Metering valve.
The predominant recirc takes place directly with the incoming fuel pushing past the overflow valve. I believe this pump is basically a dead end for swarf that goes round and round until there is lots of it. I'm sure there's a lot of fuel movement within the LP body, but nothing I can really see that would tend to move it out of the body.
The only other recirculation paths involve passage through the CR rail and either back through the Pressure Regulator or through injector leak off. I want to believe that any contamination making it to the CR rail would pose a major threat to the injection equipment. My conclusion is that very little contamination gets out of the pump via the HP side.
The fuel flow enters the LP cavity of the pump through one hole which communicates with the inlet hose barb. Fuel exits the through another hole in the LP casing. This exit hole communicates with an exit pocket. A drilled passage from this pocket supplies the Fuel Metering Valve through an internal drilled passage. This exit pocket also communicates with the Overflow Valve. This valve sets the supply pressure to the Fuel Metering Valve by bleeding off excess pressure to the return system. This passage through the Overflow Valve is the primary recirculation path. Bosch places a screen on the inlet to the Overflow valve (to protect it from debris). This can be seen in the picture below:So what's peoples thought of a nice long high flow pull at say 3-4K for a few miles to flush out the HPFP? more harm than good?
So basically any large wear particles remain trapped in the pump by the screen on the inlet of the Overflow Valve. So to answer your question. The amount of debris that can recirculate back to the fuel tank is a function of the size of the debris and the condition of the filter screen on the inlet to your Overflow Valve. I recognize this explanation is clear as mud - but thats where we are right now scrounging around in the mud for answers. Sometimes all you can do is restate the obvious and hope insights follow.
The fact that outflow through the Overflow Valve has to pass through a screen is probably why I found wear material inside the HPFP. But I really didn't find much of any on the screen it's self. So even this basic idea that wear material becomes trapped in the pump has limited credibility based on parts I've looked at.
We do know from 2micron's measurements that the entire fuel tank contents likely pass through the HPFP is ~ 20 minutes time. Let's hope that trash that makes it in has at least as good a chance to make it out. The fuel filter does the bulk of work keeping fuel tank trash out of the inside of the HPFP.
The open question is how wear materials created within the pump make their way out.
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