Fuel leaks w/ boost pump on - out sniffer valve

RVTOY · #1 11-04-2012, 10:58 PM

IO 360 horizonal injected. With engine not running! If you let boost pump stay on for just a few seconds then fuel will begin to come out of the sniffer valve. New to injection system operation. ?????

scsmith · #2 11-05-2012, 01:51 AM

Just to be sure, are you really sure its the sniffer valve, and not the overflow/drain for the mechanical fuel pump? On the surface, that might seem like an annoying question, but they probably both have drain lines leading down to some place to dump any fuel that comes out, so it may be easy to confuse them.

So, if by chance you mistook the mechanical fuel pump drain for the sniffer valve drain, then the symptom you describe probably indicates a failed mechanical fuel pump.

If, on the other hand, it really is the sniffer valve, then what that means is that fuel is being pumped into the engine through the injectors, and is draining into the intake plenum (in the sump), and then out the sniffer valve drain. So, the next question would be: will it do this if the fuel injection servo is in "idle cut-off"? If so, I think you have a bad fuel servo that needs service. It is allowing fuel to pass through it when in idle cut-off. If you only get fuel out the sniffer valve with mixture rich, then that means you are priming the engine with the boost pump, to the point of flooding it and it drains out into the sump and then out the sniffer valve. While I would not call that "normal", I think a properly functioning system will do that if you leave the boost pump on long enough with the mixture rich.

Now that I think of it, there may be a third possibility. It may be possible for some leak to form in the fuel injection servo that would allow fuel to go directly into the sump WITHOUT going through the injectors. I'm not familiar enough with the servo to know if that is possible or not.

Others feel free to correct me if what I said here is wrong.

AltonD · #3 11-05-2012, 04:30 AM

Mixture: idle cutoff. Other wise, expect to flood the engineand fuel come out the "Sniffle" valve.

There are 1000 ways to skin a cat, look up how to start a fuel injected engine. the trick is to prime it enought to start, but not enough to flood.

RVTOY · #4 11-05-2012, 07:28 PM

This is new M1B with new eveything from Vans with 68 hours time. I picked RV 7A up from an estate sale (did himself in over a divorce) and have vey little info on many items. Reworked almost everything to finishing project and fix sqauks. Ready to paint. Okay then, mixture full lean and thottle closed, run boost pump for one to two seconds creates a little puddle, leave it on longer and the puddle grows. Cowling off and fuel comes out sniffer. Checked that there is no interference with control cables and their respective stops. Guess I may need to look up how to repair servo for dummies. Scatch built Kitfox 7 Both in T-hanger with Kitfox wings folded.

scsmith · #5 11-05-2012, 07:34 PM

OK, based on what you said, mixture lean/idle cut-off and it is against the stops so it is not a mixture cable adjustment issue, then I would say you have a faulty fuel injection servo.

Especially if it shows up in one-two seconds. No way it is going to the injectors, draining out through an open intake valve into the sump, and then accumulating enough to drip out in just a few seconds.

So, somehow, you have an internal leak that is spraying fuel into the throttle body/intake manifold/sump quickly and accumulating enough fuel to drip out, in just a few seconds.

Hopefully goes without saying, but Don't fly it until its fixed.

I would call Don at Airflow performance and ask him if you should pull it and send it in, or if someone local would have the expertise to service it.

N427EF · #6 11-05-2012, 07:50 PM

You don't say what kind of fuel injection you have.
AFP servo for example does not completely cut off fuel flow in the idle cut off position. Fuel will leak down into the filter box when the boost pump is on and drip out the sniffle valve or whatever means there is for fuel to escape the filter
box.
Not sure what other systems will do but Don will know about all of them.

DarylT · #7 11-06-2012, 09:30 AM

If its Precision FI then the process is:

Crack throttle to 1/4
Mixture full open
Fuel pump on for 5 seconds
Close mixture to ICO (idle cutoff)
Engage starter.

When the engine fires feed in the mixture slowly. This takes a bit of practice. I finally have it down now. If you feed to to fast the engine sometimes dies.

Once you get the hang its easy. Its very consistent also.