Air in fuel lines.

deltamike197 · #1 11-06-2009, 08:53 PM

Hi all:

Been having a lot of trouble with a rough idle problem on my Fuel Injected 0-360 parallel valve engine. I've been going through several tests trying to locate the source of the problem but so far no joy. One suggestion that I have received concerns air getting into the fuel lines. It seems a little far fetched that air could get in through a fitting that is not leaking fuel but I'm running out of ideas so I'll go through and tighten all my fittings to see if that helps.

But, that leaves one other possibility and that's air getting in through the fuel selector valve. I'm using the Van's fuel selector that came with my RV-6 kit. It sat in its little brown bag on the shelf in my shop for several years before it got used but seems to working fine. It's not hard to move the selector and its not leaking, at least externally.

Anyone experience leaking of this valve or any issue with that might let air into the fuel lines through it? Any ideas on how I might test the valve to see if it's leaking air into the system?

I'm using the Van's high pressure pump setup that puts the pump ahead of the selector valve so there will be some suction at the valve. I'm running out of ideas and thinking about switching to a carb. Hate to do that but can't find anything wrong after working on it for the last year, maddening. Thanks.

Dean Psiropoulos
RV-6A N197DM

frankh · #2 11-06-2009, 10:24 PM

If you suspect air then drop the fitting off at the inlet to the boost pump..take a flex hose and fashion a connection to the inlet of the boost pump...drop the other end in a can of fuel and see if you still have the problem..Thats will eliminate it at least.

Rough idle normally happens after landing..I.e there is very low flow thru the lines..and a HOT engine is enough to boil the fuel and cause a lumpy idle..They pretty much all do it..Thats normal.

Do you have your idle mixture wound in too lean?..You should be able to adjust this easily..set it to be much richer and use the red knob to make it run smooth if its running too rich.

There are some things to go look at..i doubt you are sucking in air through the valve, but its easy to eliminate.

Frank

frankh · #3 11-06-2009, 10:27 PM

What about an induction leak?? That will do it too..you can test this with propane with the engine running direct propane (non burning) around all the joints..as soon as the engine runs smooth you have found the leak...Not sure I would do it with that big prop whirring around but it has been done.

Frank

Steve · #4 11-07-2009, 08:35 AM

Remove the bottom wing root fairings and tighten the vent tube and fuel line fittings at the wing root one more time. It happened to us on an IO-320.

erich weaver · #5 11-09-2009, 11:46 AM

Since both the mechanical pump and the electric (boost) pump suck (rather than pressurize) fuel between the wing tanks and the pumps it is entirely possible to have air leaking into the fuel lines but no fuel leaking out. For the same reason, when you first open the canopy before a flight, you can get a fuel smell, yet have no visible sign of fuel anywhere. You can check for air leaks by closing your fuel selector valve, sealing your tank vents, and LIGHTLY pressurizing your fuel tanks (cut the valve out from a bicycle inner tube along with a surrounding flange of rubber, then duct tape to tank fill hole and use a bicycle pump). Soapy foam over the fuel line joints will then show air leaks.

Been there.

erich

David-aviator · #6 11-09-2009, 01:25 PM

Dean,

Bubbles occur if there is any vaporization going on due to heat issues. That needs to be checked. Does the roughness go away if fuel flow is increased above idle. If it does, the bubbles probably are cause by heat and increased flow moves them through as fuel does not sit around long enough to be vaporized.

Are you running a return line to a fuel tank? If the return line is above tank fuel level that could be the source of air IF the pressure regulator has a bleed down feature. Some do and will permit air to be sucked from the tank.

I find it difficult to imagine gravity fed fuel not leaking through a fitting that would permit air to enter under suction pressure, but what do I know? It could be so. If that is the case, the engine would also run rough at all power settings as the bubbles would be present in ever greater numbers as fuel flow goes up and more air is sucked in.