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fuel enrichment problem?


t56jeff
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we are having a fuel enrichment problem.....note this is on an FCF aircraft so we can't pass until we get this fix.....it has us baffled....

enrichment works fine....until running engines for aprox 20-40 mins

it is inop upon restart.and is consistant with the engine heating up......we have changed relay box, both LH and RH distro harnesses, enrichment valve, pressure switch, and last but not least speed switch.(pmesparky).....also we have "wrang out" all the wiring associated and used the "gray box" (electrial component test set) to aid in a portion of this...any help would be much appreciated...other than dynomite in the engine....

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Looking at a schematic, the enrichment valve fuel gets ported back into the fuel control, so I suppose that is a possible cause, but first I would change the ignition relay and another 50psi switch for good measure in case the first new one was bad in the same way. I have seen ignition relays fail partially before, and from what you said, you haven't changed it yet.

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  • 2 weeks later...

OK we need to hear from you...

If you start with enrichment, STAY on the ground, shutdown the engine and restart with enrichment does it work?

You have stated it is consistant with heated engines... so take out the flight varible and try it staying on the ground....

Another thought would/could/should be wing flex, engine torque flex anything that could "move" a wire just enough to open it and cause your enrichment to be inoperative.....

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Another thought: if the starts are being done on the ground, is this engine being started

using APU/GTC bleed air, or engine bleed air? And are the tank boost pumps being used,

more specifically aux and/or external tanks?

Edited by pjvr99
... afterthought .....
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"Another thought: if the starts are being done on the ground, is this engine being started

using APU/GTC bleed air, or engine bleed air?"

May need to clairify this as I can not make a connection between air source and enrichment other than a slower or faster rise to 16%.

In flight there is no air source other than the propeller rotating in the airstream.

"And are the tank boost pumps being used, more specifically aux and/or external tanks?"

Again the source of fuel should not change the enrichment as the pressure used is downstream of the fuel pump assy. There is a connection with fuel source and the secondary fuel pump pressure light illuminating.

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Natops, you hit the nail on the head. If the engine is being started with +-80psi instead

of 30 to 35psi the spin up time is often markedly shorter. In turn, 50psi fuel pressure

to cut off enrichment is also reached quicker. It is thus possible that the window of

opportunity to see anything on the gauges is very small. A hot engine starts and spins

up somewhat faster than a cold engine.

Main tank boost pumps operate at 18 - 24psi, while aux and external at 30 - 40 psi. Basically

this is already fuel pressure in the manifold, meaning that the time available to see some-

thing before 50psi is reached, is reduced.

I know, I'm looking for something highly improbable, but sometimes it's just those little things

that trip us up. I honestly do hope it's a bad cockpit switch or broken wire ....... something

regular and simple

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I understand what you are saying but at 16% the fuel pumps are turning at the same RPM no matter the time frame to get there.

As for the fuel pressure, input pressure say 40PSI will be far less than the fuel pump output pressure and the fuel manifold pressure will be zero until 16%. The system is just waiting on 16% to allow fuel flow. The 50 PSI will be obtained at fairly the same time/rate....

I too hope it is simple and we get the outcome posted...SOON

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  • 4 weeks later...

Try unhooking the 50 PSI switch cannon plug. If you get enrichment with a hot engine, worse thing you'll have to do is stop start. At least you'll know if it's excessive fuel pressure at the enrichment kill switch or a bad signal from said switch. Had one once that the first start of the day would give you an enrichment start with good de-enrichment without enrichment selected. Every start after that was fine. Did all the steps you did with the enrichment valve/harness's, grey box etc.., finally R2'd the fuel control and never came back. Not exactly the same problem you're having though.

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  • 1 year later...

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