API fuel filter blocking

[African Bear]

Hi All,
On my Bearhawk I have an IO390 with API fuel injection system. Lately I am having problems with the pencil filter that feeds fuel into the metering body. It blocks after about 12 to 15 hrs. So far I've always caught the problem by monitoring EGT rates of change , fuel flow and RPM. My tanks are clean, I am meticulous when fueling using proper filter funnels. Gascolator shows no significant debris. I've cleaned in a bowl of fuel and see no dirt coming out of the pencil filter. I've cleaned it the last few times in an ultrasonic bath and see no debris. I've even kept the washings in a glass jar to see if anything settles and still nothing. After cleaning there is great flow and one can almost breath through the filter. My AMO guy and myself think that the filter element is failing in itself. Any ideas from anyone or experiences?
Best Andrew

[Battson]

So you can't tell what is blocking it, and when you clean it - you find nothing, i.e. no dirt, larger debris, swarf / metal particulates, rubbery particles, or cobwebs....?

Just to clarify, do you have an upstream filter before the "finger strainer" AKA "pencil filter"? I guess it would be before your boost pump.

I understand fuel filtration for an injected engine should look something like this, in reducing order of screen size:

1. Tank inlet strainers
2. Gascolator
3. Fuel filter, before boost pump
4. Servo finger strainer

If you can't tell what's blocking it, how are you successfully unblocking it? Are you sure the filter is the root of the problem, or is the act of changing it somehow addressing a different problem (e.g. partial blockage in the flexible fuel line)?

[African Bear]

Battson Thanks for your comments. As you see, the plot thickens with no logical answer so far! My response in red

So you can't tell what is blocking it, and when you clean it - you find nothing, i.e. no dirt, larger debris, swarf / metal particulates, rubbery particles, or cobwebs....?
That is correct.

Just to clarify, do you have an upstream filter before the "finger strainer" AKA "pencil filter"? I guess it would be before your boost pump.
There is inly the screen in the gascolator, and that shows no debris and also no debris in the bowl of the gascolator. The fuel flow at this point is huge on free discharge.

I understand fuel filtration for an injected engine should look something like this, in reducing order of screen size:

1. Tank inlet strainers..None
2. Gascolator..Fine stainess screen that shows no dirt.
3. Fuel filter, before boost pump. None
4. Servo finger strainer Gets blocked with "invisible" debris

If you can't tell what's blocking it, how are you successfully unblocking it? After I remove the pencil filter when I suspect a blocking, I can barely blow through it. If it has fuel in it I can only get it to drip out when blowing by mouth. To clean it I reverse blow it with compressed air into a white tissue and see no debris. That gets me going again. When at home base I remove the pencil filter and clean it in an ultrasonic bath and I find no particles. After cleaning and blowing back and forward with Avgas and carb cleaner etc I can blow through it easily and can actually breath through it. Engine performance comes back to normal, fuel flows, EGT responds to mixture control movements etc. Are you sure the filter is the root of the problem, or is the act of changing it somehow addressing a different problem (e.g. partial blockage in the flexible fuel line)? As sure as I can be for the above reasons. I've also disconnected the supply pie to the metering body and pumped 20 litres directly into the can using the aux pump and there is a large fuel flow and no change of flow during that time.

Not a lot makes sense , hence an aero engineer that helps me and myself feel that the filter may be failing but not sure. It is a very sturdy washable filter, it looks like a wound ,extremely fine perforated plate. ( API approves ultrasonic cleaning for this filter)

[AZBearhawk272]

Look for clear chemical agents that dissolve or are soluable in gasoline.
styrofoam, PVA mold release, polyester resins, degradable plastic, think funnels, transfer cans, nylon lined hoses.
you are not lot looking for particulates.

Evaporate off a couple liters of fuel drained from toy plane in a clear glass container. Place a metal spoon in the container during evaporation.

if after evaporation if you have any white / translucent residue on spoon or in the container you have found the
Transfer solvent / carrier now to find the source…….

Hope this helps.

[African Bear]

Great idea, I'll do that straight away and post the results. Thank you AZ

[arborite]

Look for clear chemical agents that dissolve or are soluable in gasoline.
styrofoam, PVA mold release, polyester resins, degradable plastic, think funnels, transfer cans, nylon lined hoses.
you are not lot looking for particulates.

Evaporate off a couple liters of fuel drained from toy plane in a clear glass container. Place a metal spoon in the container during evaporation.

if after evaporation if you have any white / translucent residue on spoon or in the container you have found the
Transfer solvent / carrier now to find the source…….

Hope this helps.

Agree on this direction. I have seen similar blockage issues on fine mesh filtration systems from surfactants in cleaning products. are you using the 40 micron or the 74 micron filter?

[African Bear]

Not sure. It's pretty fine . I will contact API and ask what filtration size it is. There are no numbers on it.

[Bcone1381]

Could it be Fuel Lube caught by the screen?

[African Bear]

Definitely not fuel lube as I don't use it. I feel AZBearhawk's comment is pointing in the right direction

[Nev]

Could it be Fuel Lube caught by the screen?

Definitely worth checking this out - EZ lube is used to lube the fuel cap thread mechanism to prevent galling. Kev ( AZBearhawk272) might be able to chime in on this theory, but if some of the fuel lube found its way through the system it could possibly congeal in the wrong places. To be clear, this is only my theory, not a data point.

However I did have several occasions where the my own finger filter in the fuel injection unit was partially blocked. Upon cleaning, I found very minute metallic looking particles that were difficult to see with the naked eye. We hypothesized that it was coming from the electric fuel pump running in.

[Ryan Koeneke]

so still use EZ lube just be sure to keep it on the outside of the filler neck (if i'm understanding correctly?)

[jaredyates]

My understanding is that our current best practice is to not use fuel lube on the fuel cap o-rings, but do use it on the central threads.

[Nev]

Thanks Jared, correct. I've re-worded the post and removed part of it so as not to inadvertently send the wrong or confusing message.

Ryan, EZ lube on the threads only. Nothing anywhere near the O rings. They should fit easily with gentle pressure.

[African Bear]

Nev I'm going to do a lot of cleaning this week and will let you know. it's not fuel lube as I don't use it, unless it was used before I bought the plane and has only just come home to roost. As above, blowing the filter out into a white tissue shows nothing (to the naked eye). I have a feeling that AZBearhawk272 may be onto something in that something is being transferred by a solvent (avgas)

[kestrel]

Consider blowing out the filter onto a black cloth instead of white.

[African Bear]

kestrel Good point ...maybe it is just a black and white problem

[Battson]

Do you get much water in the fuel drains? The other thing I would suggest is a fuel tank internal inspection or "wipe test".

If the so-called "fuel bug" has gotten in there, which may be a rare possibility in carefully handed AVGAS but hey - it's a diverse environment out there - you could be getting fungal or bacterial material building up in the fine filter screen. I think it's normally characterised by a dark coloured build up on the inside of the tank or in the filter. If it's bad you will see dark coloured sludge-like build up in the filter.

Possibly not the most likely cause, but hey - at this stage we're just guessing

If you are running a fuel injection system, you may want to consider putting another filter in the fuel line between the gascolator and the servo's pencil filter. That pencil filter is very fine and blocks fast, whereas a larger filter will give a slower onset with some more warning - before engine stoppage. It would also offer protection to your fuel boost pump.