Selection of a Racor Diesel Filter

[JHZR2]

My new to me 1993 Mercedes 300SD project car has signs of some rust in the fuel filler neck and cap. The fuel passes the prefilter clear, but I’d like to be sure that I’m filtering the fuel better.

Current design is:
Tank->prefilter->lift pump->Injection Pump->main filter->injector returns->tank return

I’m not going to remove the main filter. What Id like to do is add a Racor (or similar) fine filter after the prefilter, before the lift pump.

Figure 150hp at 14psi boost WOT. Not sure what the lift pump flows beyond consumption, but I’d assume 3-5x. I think I’d like to use a 2 or 5 micron filter.

What’s the best choice and right size to choose? There’s not a ton of space, so I don’t really want to mount some giant thing.

Recommendations?

 

[AP9]

2 micron is very fine filtration, overkill even for many modern common-rail diesels. Of course that's not a problem in and of itself, but consider pressure drop especially as the filter gets loaded up and restricted. That could be quite frequent especially if you're looking for a more compact filter. On the plus side you should then be able to run super long intervals on the main engine filter.

I'd say you'd get more of a benefit from the additional water separation capability (which is in general more effective under suction than positive pressure), which should also help slow down any further rust growth. I'm not too familiar with these Mercedes-Benz mechanical injection diesels though I'd think a 10 micron water coalescing filter as a suction-side prefilter would be more than sufficient.

 

[Alex_V]

I've worked on about 50 W123 and 115/124/126 diesels and some rust in the filler neck is very common. If the clear filter isn't filling up with any flecks of stuff there's no reason to think it's rusty down in the tank. The lift pumps are extremely durable and the factory spin-on filter is 10 microns already. I've seen many, many miles put on cars that even did pass some gunk into the clear filter and there was no accelerated wear on the injection system to be observed.