FI problems...need advice

sf3543 · #1 04-30-2006, 03:32 PM

I have an IO 360 with the Bendix horizontal FI, but I am not getting enough fuel flow to get 100% power. Here's the symptoms:
Uneven fuel from the injectores, using the bottle test. Did this several times and fuel rates from the injectors is different each time. We noticed bubbles coming out of the injectors when they got submerged in the fuel.
50 GPH at the servo intake. The large fuel line.
5 to 7 GPH coming out of the servo with bubbles in it when submerged in the fuel. The smaller fuel line that goes to the fuel flow divider.
I can only figure that there is a problem with the servo itself, but don't know enough about servos/FI systems to know for sure. My current plan is to call the supplier and ask if it needs to be replaced.
Any ideas or advice?
Thanks in advance.

BruceMe · #2 05-01-2006, 01:11 PM

-> I AM NOT THE EXPERT <-

That said, from my RC days, air in unpressurized section of the fuel lines usually came from a hairline crack. Too thin to leak fuel out significantly, but enough so air can get in. I'd check all the fitting and lines for blue die.

-> I AM NOT THE EXPERT <-

sf3543 · #3 05-01-2006, 03:12 PM

So far, I agree with your comments. The engine guys recommended I check every B nut to make sure they are tight, since they will not present as a leak. When it's sucking in air, it doesn't leak and may not be noticeable at rest.
Wish me luck!

tacchi88 · #4 05-01-2006, 05:54 PM

Fuel divider diaphram may be worn, check all fuel line connections. Check fuel strainer screen at fuel inlet. Be sure you have sufficient fuel pressure, and correct size injector nozzle assemblies for your application, and yes, it can possible the servo. Check the easy stuff first.

zav6a · #5 05-01-2006, 08:49 PM

Had a similar problem less the bubbles. The fine filter at the servo inlet was the culprit. As a matter of fact looked at it twice before trying to clean it. Could not see a thing but significant back pressure when I tried to blow though it. Did an initial clean over a white paper towel with contact cleaner and got a "lot" of very fine dark sediment. Running great ever since.

sf3543 · #6 05-01-2006, 10:00 PM

Tonight I checked every B nut in the system and every thing was tight. I even went to a straight hose from the firewall to the servo to isolate the engine driven pump and the fuel flow totalizer. Still get bubbles out of the feed to the divider, w/ about 5 GPH fuel flow. Another test was to cap off the line to the divider, throttle and mixture to full and then turn on the pump to see if there is fuel coming out of the seals in the servo. Everything was tight there.
Next steps..recheck the finger strainer at the servo inlet and pressurize the fuel lines to see if there are any leaks. Also, I plan to put a clear line at the servo inlet to verify that there are no bubbles in the fuel. I'll verify that the O-ring in the fuel selector valve isn't leaking, but I don't think so.
I guess it could still be a line leak, but I am leaning towards a servo problem.
Still looking for suggestions, but I'll keep you updated.

tacchi88 · #7 05-02-2006, 07:40 PM

You didn't mention fuel pressure. FIs should have at least 15psi. If not seeing that much pressure with both engine pump and/or boost pump, the system may be cavitating, hence bubles. Rarely do both pumps fail. If so, then a look at fuel pick ups and valve is in order. Good luck.
T88

sf3543 · #8 05-08-2006, 08:01 AM

Well, I finally got the Fuel Injection system working.
Turns out that while the engine was pickled, some of the oil got into the servo air section. During idle or sitting static, the pumps would only push out about 5 or 6 GPH of fuel to the injectors, from the servo. We verified this with the bottle test by removing the injectors from each cylinder and putting them in bottles to see how much fuel came out over a given period of time. (You get the picture!) But, you really need about a 16 GPH flow to get full power, so something was a miss.
Anyway, if you look at the servo from the front, intake side, you'll see 4 jets or intakes in the venturi. I was advised to squirt some Brakleen into these intakes to clean out any gunk that might be in there. (There is a regulator, activated by ram air, in there, which increases the fuel flow.) Sure enough, when I squirted it in the tubes, the fluid came out of the piddle valve all black. Apparently there was some oil in it, which kept the regulator from working.
At any rate, fuel flow went up where it belongs and I can now get full power.

Janekom · #9 05-29-2006, 07:20 AM

Just two days ago the current Aerobatic Champion (Glen Dell of South Africa) told me about the same problem. Only in his case the four small holes in the venture took in oil from his smoke system on his extra 300 and the same thing happened. It actually happened during a display with thousand looking and the engine started coughing seriously.They cleaned the 4 holes and bingo!

penguin · #10 05-29-2006, 08:30 AM

Guys, as a bit of background the tubes at the front of the injector body are for sensing the mass flow of air. The injector tries to schedule the correct amount of fuel based on air mass flow - and the tubes sticking forward are part of the mass flow sensing arrangement. The injector body regulates fuel flow by changing the output fuel pressure to the spider, so (I always thought that) the spider (and the injectors) will only see full pressure when the engine is running at full throttle.

Wow - to get smoke oil into the injector body the Extra 300 man must have been doing some wild stuff!

Pete