Gascolator-Prefilter-Boost Pump

[Steve W]

Just received this reply to a question whether a prefilter was necessary in a boost pump set up for FI with a gascolator already upstream:
“Mechanical or electronic fuel injection have no place where water droplets can collect and do not need a gascolator“
Really??
Please educate the old one here.

[Bcone1381]

I'm curious Who gave that answer? My research shows a C-172SP has a Lycoming IO-360 engine with a gasocolator. (red arrow)
Screenshot 2025-10-12 at 4.23.31 AM.png



I have no knowledge of EFI. I installed mechanical FI in my Patrol in accordance with Bob's fuel system that is described in his "Patrol Book". It is specific for Carburators but still jives with the Cessna C-172SP diagram. But due my uncertainty (at the time I built it) over what an industry best practice/standard was, I reached out to Kyle at Airflow Performance. AFP are Mechanical FI fabricators and overhaulers of Bendix FI. They built their business around Experimental Builders like us. They are my information and supplier source for anything GA mechanical FI.

Kyle approved of me installing Bob's Gascolator. but maybe slightly favored their filter over Bob's Gascolator. If I recall he did not want both installed. AFP's pump is sensitive to foreign object damage and so their filter may be designed to better protect it. I also believe with 75% confidence that a mechanical FI system can swallow a tablespoon of water without serious repercussions if the water is not frozen and that filter must be tolerant of water. (metal screen, not paper filter)

Living in Michigan. I think about freezing water at the fuel system low point so I like the gascolator to catch it. The Bendix FI and all others I know of have a final filter at the servo. it gets inspected every annual.

I've not yet sumped out any water from my gascolator. Maybe 3 drops total from my two tanks in 100 hours.

[svyolo]

SDS EFI, and I am 95% sure EFII, don't recommend a gascolator. SDS recommends against it. Their filters are much finer than a gascolator. But the gascolator serves as a low point drain to get water out of the system. SDS's answer is that EFI will just eat small amounts of water.

None of that pertains to Bendix style FI. Airflow PF boost pumps have a electric pump that is an automotive EFI pump.

As far as prefilters, very modern cars have a coarse prefilter built in to the in tank fuel pump. Less modern cars, before in tank pumps, aka 80's to 90's, mostly just had a filter after the pump.