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.

[Steve W]

It was EFII that gave me that response. Stuff happens. Loose flush caps out in the elements, a hurried preflight, several ounces or more or water in the tanks and what happens to the noise on the front of the plane? I understand small amounts can be forced through the system with a boost pump but where is the anti ass pucker factor here? Yes a hangared plane will probably never see a drop of water in the fuel system throughout its lifetime but……

[jaredyates]

I'm not an expert on this, but you are still draining your tank sumps, which will at least let you know about a major fuel cap problem. Carbs are especially vulnerable to water contamination due to a surface tension problem, where small jet passages are blocked at low gravity-fed pressures. Having the higher system pressure available, fuel injection systems can stumble and relight vs complete stoppage with the carb.

[alaskabearhawk]

Even though EFII doesn't have a gascolator in their installation diagram, I have the one from the old Bearhawk Aircraft store installed in mine upstream from the FF=2 filter with the selector valve in-between the two. I just look at it as another low-point drain and I have yet to see any water or debris in my fuel. On my first condition inspection I cut apart the $40 filter to satisfy my curiosity. It was clean as a whistle.

Prefilter.png 20250418_192158.jpg 20250418_192256.jpg 20250418_192211.jpg

[Steve W]

That’s kinda what I was getting at, whether the two filters were restrictive in any way. How was your flow test? I wonder what the micron rating is of Bob’s gascolator? The screen looked very fine when I had it apart.

[alaskabearhawk]

We estimated it to be around 50GPH with the pump on, which is pretty close to the pump capacity spec of 155LPH. That comes out to be just over 40GPH. We only had a 5 gal can and the fuel came out like it was water out of a garden hose, so we overestimated it using what we had. It's still impressive. I forgot what the flow rate was with the pump off. I'll have to check my logs, but it was also substantial.

The mesh Bob specified is pretty coarse, 30x30. Google translates that to about 595 microns.

https://bearhawk.tips/923

[Battson]

I would always want a gascolator for a low point fuel drain and to collect any big stuff, which I can drain and see - before the filter clogs and I lose fuel pressure.
What's the downside apart from a little extra weight?

My system has a selector, then gascolator, then pre-filter, then fuel transducer, then the EFII pump, then the firewall etc, and finally the finger strainer inside the fuel control unit.

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.

That may be true, but you'll still get momentary losses of power if the engine eats water in the fuel. It's quite startling

On my first condition inspection I cut apart the $40 filter to satisfy my curiosity. It was clean as a whistle.

I cut mine and it was kinda dirty, first time around. I assume dust and debris in the lines from a build in a dusty environment.
I change the filter periodically and cut it open, its always been clean since.

[svyolo]

When I was referencing EFII I was talking about their EFI product, not their boost pump. I should have specified. For what it is worth the part number of the pump is probably the same. 155 lph capacity.

[Battson]

Interestingly, when I bought just the pump, I think the instructions said the same - no gascolator, just the pre-filter. Possibly this is a sale-pitch type of benefit. We went through a risk assessment and determined that the gascolator was a good idea.