Rear engine fuel drains

[wslade2]

one other thing...you can crack the lines from the flow divider to the injectors with tank open and throttle/flow off. see if any dripping. If not, then that's not the source.

in terms of checking pressure, do you mean pressure output of the pump with engine running? I was able to make a rig to do this with items from amazon pretty cheap: the appropriate AN fitting, 30 PSI pressure gauge, valve and tee (to purge the line of air), fuel line from autozone. Hooked it up to line between pump and throttle control. We (A&P and I) had to do this to double check and adjust pressure on the reman fuel pump I recently installed (the old one was leaking).

By the way, pressure with engine off, mixture and throttle closed was "0" as I remember it-so very low even though there is trickle flow when you open things up. For kicks I tried some old physics just now (pressure of a liquid fuel gas column with vertical height from fuel tank to the flow divider-lots of assumptions so don't quote me) and the pressure calculates out pretty close to "0" at the fuel pump coming down from tank. Low enough to allow the flow divider to cut off the flow yet have big trickle with lines open.

Also have you checked your mixture control at the pump to be sure it is going to "full off" position?

If flow divider is allowing dribble out, check for grit/sediment in the system. It could keep the diaphragm from closing. I had a defective screen in the strainer, sediment made it to injectors with some altered engine performance. But if that sediment blocks the diaphragm from closing...drip drip

[JAG]

My vote is to check the primer as well - could be something under the seat of the primer and it is allowing fuel through even when locked. Undo a primer line at cylinder and see if it drips statically.
Jeff

[hharney]

Has anyone called Continental Motors? They are really good to talk to about their products, how they work and what to troubleshoot. Just a thought.......

[n86121]

It looks like we finally got it: Like most complex things, simple once understood.

Turned out to be just the check valve beneath the fuel pump, against the rear firewall.
Facing DOWN from the fuel pump into the airframe,
it connects excess vapor/fuel return from the pump back into manifold/fuel tanks.

Since the tanks are higher up, hydraulic pressure from the tanks above comes UP the check valve, which mounts vertically.

With the check valve disconnected above the check valve, it would stay dry.
With no fuel coming back UP from the manifold/tanks.
The check valve holds, because hydraulic pressure coming UP the valve from the fuel manifold/tanks would push the valve up/closed.

With check valve disconnected below the check valve ...it would also remain dry,
because nothing coming down from the fuel pump at idle cutoff.

But fill both sides with fuel, so pressure is balanced above and below the check valve,
and the small valve inside would seep just enough pressure back into the fuel pump to also get past the flow divider, then out to the cylinders, then out the drains.

...Now I just need to cleanup the goat I slaughtered trying to divine the answer.

[wslade2]

very interesting

not to overthink this or mess with success....the end point for flow stoppage is the cutoff diaphragm in the flow divider. flow will come from increased pressure at the diaphragm above the set cutoff level pushing it open (the previously referenced 1 PSI). Pressure exerted at the cutoff diaphragm in the flow divider should be impartial to where it comes from: backflow through a check valve or forward flow through the system. Either way I would be suspicious of increased pressure in the system while at rest.

eg-probably check fuel tank vents to be sure they are clear and tank is not pressurizing from a blocked vent

I cleared my scupper drains (about same size as fuel vent) this summer due to water collection at fuel caps leading to water in my tanks. 10 days later an insect had made a nest in one of them again discovered via water collecting in one out of my 4 scuppers after a rain storm. It would likewise be easy for a tank vent to get plugged.

[n86121]

Am beginning to think the OH flow divider not OH so well.....?

[n86121]

I popped the fuel primer line off. That's not it.

It would appear there are only two paths for fuel into cylinders when shut down:
1. Primer and /
2. sump/pump/throttle/flow divider shutoff.

I have the flow divider that came out. Will send it off for bench check.

[wslade2]

Yes. Are you absolutely sure it is a cylinder drain dripping?

[n86121]

"Once you have eliminated the most likely, the least likely becomes most likely"

(Paraphrasing Sherlock Holmes)

So,

1. NOT flow divider: $500

2. NOT fuel check return valve: $300.
Found check valve hose end badly gnarled, replaced valve and hose end fitting.

3. Labor thus far: $300

SUB TOTAL around $1,100

--

An AI on field suggested following the drip UP from the bottom.

So

1. Cracked open a cylinder drain to confirm .....drip drip drip
2. Cracked open injector line...dry
3. Cracked open turbo vent line...dry
4. Scratched head: If not flow divider, and not check valve..maybe fuel coming UP?

5. Cracked turbo pressure ref line on fuel pump... Fuel oozing UP from pump.

EUREKA !!!

There should NEVER be fuel on the 'air' side of the pump, so some internal seal dying.
So the drool coming UP the turbo pressure ref line into the throttle,
then back down into cylinders.

Another $1,200 OH and maybe another $300-400 labor to remove and replace.
(I have the mesh screen on rear cowl opening, must de-cowl to remove and replace pump).

So about $3k to solve the drip.

Wife thrown from horse: Medivac to trauma center. X rays. She's fine.
...$6,000 deductible under Obamacare now.

Another unexpected $10,000 THIS month, plus the normal overhead.

What's to worry!?

At least my fuel will stop dripping. Happiness.

[patrolpilot]

Oh boy, so sorry to hear about your wife.

I think all of us can feel your pain with the aircraft expenses.

[wslade2]

excellent to hear problem solved
so sorry about wife, hope she's better
if it makes you feel better....similarly had fuel pump leak issue and also filter drain issue with dripping boost pump to boot. I figured if seals wore out in fuel pump/filter/boost pump, other rubber in fuel system likely at end of life. So I replaced throttle control and fuel flow divider, various o-rings, fuel cap seal, all flexible fuel line hoses.
Got my fuel system education
All to say, don't feel bad about replacing what may not have directly related to drip. Those fuel system parts were likely near the end of their service life as are others you have not yet replaced.