Fuel System Filtration
Fuel System Filtration by Paul Yaw @ Injector Dynamics
Just like an oil filter protects your engine from harmful particles of ‘trash’ that will cause damage to engine
components, your fuel filter protects your injectors.
We have seen a lot of injectors come back for inspection with a deviation in flow that is the result of
improper filtration. I don’t think anyone intends to build a fuel system that’s not right, it’s just not
understood in full by many. We want to change that!
Fuel filters are rated in microns. A micron is a unit of measurement, equivalent to .000039 inches. (39
Millionths of an inch.) The symbol for micron is µ, though you don’t see it too often in fuel filter ads.
To put this into perspective, a human hair is about 100 microns in diameter. Anything less than about 30-40 microns
can no longer be seen with the naked eye. A ’10 micron’ filter will block contaminants larger than 10
microns, and let those smaller than 10 microns pass.
Here’s Bosch’s take on fuel injector filtration.
“Resistance to Fuel Contamination – Quality of the Medium. Dirt particles and contaminants in fuel
represent a potential danger to the fuel injectors. They are to be avoided in order to preserve the correct
function of the injectors. The dirt content in the fuel system must therefore be minimized via a suitable
filter. Recommended filter quality: nominal rating 5µ, minimum 82% capture efficiency according to ISO/TR
19438; dirt particles >35µ are not permissible. The basket filter in the injector serves only to catch
residual particles. Nonconformance of the recommended filter quality can cause damage to and failure of
the components”
So Bosch recommends a 5µ filter and says anything over 35µ can cause injector damage. Think about that,
things you can’t see with your naked eye are big enough to cause damage to a fuel injector. Not only is there
the concern of the contamination clogging the internal filter enough to cause a reduction in flow, but the
contamination that makes its way into the injector scores the bore, affecting critical tolerances.
Seems simple enough, so why are so many people misinformed? I hear pretty regularly that people are told
by fuel filter manufacturers that their 10µ filters are not ethanol compatible, so if they’re going to run
ethanol, they need to run a 40µ or bigger stainless element. While this is true, a stainless element is much
better for use with ethanol, the manufacturer has just made a recommendation that will likely cause
injector problems down the road. People don’t like to hear this after the damage, trust me!
The filter before your pump serves mainly to protect the fuel pump. Depending on the pump, usually a 30-
100µ filter is used here. The filter after the pump serves to protect the injectors and as noted, needs to be
much finer. Being that the filter is protecting the injectors, anything after the filer is technically not
filtered. This isn’t a big concern in OEM fuel systems which mostly have hard lines, but when you’re plumbing
your race car with rubber hose, do yourself a favor and put the fuel filter right up by the rail inlet. This will
leave the least amount of unfiltered fuel system and protect your injectors.
A 12µ filter is generally sufficient and they are readily available, with paper elements. If you’re running
ethanol I suggest digging a little and finding a 12µ or similar ethanol compatible element. The 6 and 12 micron
microglass, ethanol compatible fuel filters, have done well in our testing and are recommended.
Also, stay away from small fuel filters, especially those tiny ones with a single round screen disk. Those
shouldn’t even be allowed on the market. The least bit of contamination and they’re clogged enough to be a
major restriction in the system. The more surface area your filter has, the longer the service interval will
be. A pleated filter cut apart and stretched out would probably surprise you with how much surface area of
filtration there is!
