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Fly with a bad fuel pump?


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I would ask for ferry permit, that way if they reject it, you can take that to airport manager.
If they approve, find a ferry pilot.
You got 7 lives left by my count, so if you do it yourself I would plot the course over roads, early Sunday morning to avoid traffic, and circle the airport to get altitude before departing.

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I would look for an air leak between the electric pump and the inlet to the mechanical pump. The mechanical pump makes a low pressure on its inlet side which can suck in air through the tiniest leak. With the electric pump on there is a positive pressure at the inlet to the mechanical pump. 

 

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1 hour ago, N201MKTurbo said:

When you said the engine was gasping, what was the fuel pressure?

I second this question.  It should be quite easy to see if the engine is fuel starved; if that is confirmed, then move to the why.  Engine driven pumps do fail, but it is not very common.

 

11 hours ago, jetdriven said:

Maybe your originating issue wasnt the fuel servo, maybe it was mechanical fuel pump failure.  Did you turn on the electric fuel pump as part of your engine failure flow?

 

Definitely part of the "flow" (points for easy pun).  In fact I can think of nothing in the trouble shooting process that should come ahead of electric fuel pump...perhaps verifying fullest tank. 

Edited by Shadrach
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Chris, not me.  The boost pump was rebuilt less than 2 years ago, if I remember correctly.  On this forum it has not been recommended to use the dukes pump any more than necessary. This flight is asking for it to be necessary. 

There is a filter at the boost pump. It was cleaned at the last annual.

Also, is the FS450 transducer allowing fuel to go thru, or restricting the fuel?

Engine pump was never touched in the years that I had the plane.  May be old.

Be safe, what is your but worth?

Ron

 

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I wouldn't assume it's the mechanical pump just yet, as has been alluded to above.   If engaging the electric pump didn't fix it before, it's unlikely it's what's broken now.   The most likely issues will be around what just got changed, i.e., the servo installation, lines etc.

 

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16 hours ago, Bartman said:

You may hav had a bad fuel servo, but knowing this in my WAG is I think the reason you landed on the Navy base was because of a failed fuel pump, not the servo. 

It was a combination of both (plus the gasket in the throttle body?), I think, though how exactly that all transpired I have no idea. Even when I put the electric pump on, the engine was as dead as a Norwegian Blue. It wasn't until I pulled the mixture back aggressively that it restarted. IDK, could the stress of trying to force fuel through a blocked throttle body / bad servo have failed a marginal mechanical pump? (The plane runs with the mechanical pump, but barely; only partial power, and without the electric fuel pump it's getting about 2 psi.)

 

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14 hours ago, jetdriven said:

Maybe your originating issue wasnt the fuel servo, maybe it was mechanical fuel pump failure.  Did you turn on the electric fuel pump as part of your engine failure flow?

 

After pitching for best glide and picking the least sucky spot to put down, yes, first thing I did was fuel pump on, switch tanks, mixture rich.

It didn't come back to life until I pulled the mixture back.

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3 hours ago, Shadrach said:

Definitely part of the "flow" (points for easy pun).  In fact I can think of nothing in the trouble shooting process that should come ahead of electric fuel pump...perhaps verifying fullest tank. 

Yeah, that was the first thing I tried, along with switching tanks. (Both tanks were about the same, about 20 gallons per side. I'd filled to tabs at KRAL and flown to KCMA, 30 minutes on each tank.)

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It sounds to me like you may have multiple problems. The piece of rubber likely caused your in flight problem. Your current fuel pressure problem may or may not have anything to do with the in flight problem. It may have been introduced during the maintenance afterwards.

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2 hours ago, N201MKTurbo said:

It sounds to me like you may have multiple problems. The piece of rubber likely caused your in flight problem. Your current fuel pressure problem may or may not have anything to do with the in flight problem. It may have been introduced during the maintenance afterwards.

What's weird is, the current problem (partial power) was evident in flight, too. Chronology:

Flying along, engine running great, no hint of an issue.

Instant total power loss.

Retard the throttle (per the owner's manual) ahead of a potential restart.

Electric pump on - no change. 

Switch tanks - no change.

Pull the mixture back.

Engine roars back to life, but as configured, only makes about 2200 rpm. (Prop control was full forward, mixture about 1/2 way back, throttle about 1/2 way back).

Don't touch anything (including boost pump, which remained on) until setting up to land. Throttle idle, prop full forward, engine running like you'd expect with the throttle at idle.

Come to a stop, use the throttle to taxi off the runway.

Leave it sit overnight.

The next afternoon, run through a normal cold start procedure, inherited from @N803RM: Master on, throttle cracked (~1000 rpm), mixture rich, boost pump for ~5 seconds, wait a second or two (mixture still full rich, fuel pressure dropping linearly as it always does), engage starter (mixture still full rich).

Engine starts easily (2-3 rotations). Bring the mixture back leaning for ground ops. I got the RPMs up to about 1500 for taxi purposes, but didn't want to do much more than that until an A&P had looked at it.

So it was running fine with the mixture full rich on the ground, even though it took pulling the mixture back for the engine to restart (at 3500' MSL) in flight. It was running fine without the boost pump on the ground (albeit at a low power setting).

Even with the boost pump on in flight, though, I wasn't able to get more than 2200 rpm - but again, once I had a working(ish) engine configuration, I didn't touch a thing until I could glide to a runway. But still.

think I had both issues (low fuel pressure; blockage at or around the servo) in flight, simultaneously.

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I know you just did it, but I would be pulling the finger screen on the fuel servo for one more looksie.   I get where the mechanic is coming from.  The whole fuel system needs to be gone through to figure all this out.....   Sucks to do it on the ramp, but given the whole system needs to be gone through, do you really want to be in the air with a fuel system that needs to be gone through.

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The servo senses airflow from the difference between the dynamic pressure (Pitot) on the four sense tubes and the low pressure in the throat of the servo.

The rubber acts like a choke in a carburetor causing an extremely low pressure at the low pressure sense port. The servo would sense this as a HUGE airflow and compensate with a HUGE fuel flow.

Your engine quite from being too rich. Pulling the mixture knob leaned the mixture until it could run again.

It only made 2200 RPM because there was a piece of rubber in the airway restricting the airflow.

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Wait.   Did the new fuel servo get put on or did we just find the rubber piece and troubleshoot from there?   If it is the old fuel servo, pull the finger screen and some of those impact tubes in the old servo could be damaged.

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9 minutes ago, Yetti said:

Wait.   Did the new fuel servo get put on or did we just find the rubber piece and troubleshoot from there?   If it is the old fuel servo, pull the finger screen and some of those impact tubes in the old servo could be damaged.

New fuel servo is installed. The mechanical pump (?) issue showed up when he did a run-up after installation.

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Okay, here's what it's doing at the moment (rubber piece removed, new fuel servo installed):

"The mechanical fuel pump pressure is one to two psi up until you call for full power, at which point the fuel pressure drops to unmeasurable and the engine will only make about 2100 rpm. With the boost pump on the engine makes full power."

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The fuel pressure is supposed to be 25-30 PSI for the Lycoming Fuel Injection. Electric boost or mechanical. Sound like the PRIMARY mechanical fuel pump is not happy.

Edited by Yetti
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