Fuel Pressure and engine driven fuel pumps

[Bob_Belville]

I wonder if my EDM 930 is sending me a warning about the fuel pump or some related issue. The fuel pressure indication is fluctuating with greater amplitude and frequency than it was on a flight from last year that I pulled up to compare. The extremes are about 15 to 25 psi though most of the time the range is 20-24. 

[jetdriven]

Bob, my fuel pressure at low altitude was OK but above 5-7K, and especially at 9500', the fuel pressure would wander from 13 PSI (lower red line) to around 20-22. At 12,000 it would definately read low, and shoving the mixture to full rich from cruise at that altitude would cause a big drop. Replacing the fuel pump fixed this issue.

 

We also pulled apart and resealed the fuel selector a year previous. It seemed to help but not like the fuel pump replacement.

[Bob_Belville]

Byron, thanks. My average FP is okay but the "gauge" fluctuates constantly. It did not used to do that. The green line is FP, the scale is the middle right.

[Guest]

Open the fuel line at the gauge and at the fuel servo, drain or blow the line clear and re connect the lines. Many times the line fills with fuel and the fuel pressure gauge reads the pulses from the pump. Filling the line with some air cushions the pulses.

Clarence

[Bob_Belville]

Clarence, sounds like a great idea, the "gauge" would be the transducer mounted on the firewall. The second pic is in the middle of the rubber line running from the throttle body to the transducer, do you suppose that is a snubber?

 

[Guest]

Bob,

In the original configuration the fuel line was live from the servo finger screen to the gauge and the restricted fitting was the elbow in the servo. It is possible that there is a restriction in the fitting shown. I would still try clearing the line.

Clarence

[M016576]

Check all your screens in the system, too, to make sure they are clear. That includes the gascolator screen

[Bob_Belville]

I've gone through the data from about 40 flights. This behavior in not new,  flights soon after the 930 was installed in Nov. 2012 have at least portions of the graph with fluctuations, though recent flight are much worse and while there were times prior 3 months ago when the graph was steady, everything more recent is fluctuating. This includes during take off and initial climb when the boost pump is on which would seem to make it a sensor issue more than 

 

I'm headed to the airport to get the line from to the transducer checked out...

[carusoam]

Does the FP compensate for altitude?

In this case, maybe, not compensate properly for altitude?

I'm not that familiar with the FPs in my plane...

Best regards,

-a-

[Bob_Belville]

Anthony, the indicated Fuel Pressure does not seem to vary  much with altitude, at least up to 10,000.  

 

We disconnected the fuel line going to the transducer (gauge) on but ends and found no fuel at all in that line. We primed it with fuel using the boost pump and reconnected but I did not have time to fly so don't know if that (air in the line) was what was going on. I will report result. After studying graphs from many flights going back to the 930 installation I'm pretty sure there's nothing wrong with the pump. 

[romair]

Hmm....I have almost the same problem except that my fuel flow also fluctuates, especially when close to peak EGT. It can fluctuate by up to 0.5gph. Fuel pressure fluctuates between 20 and 24 just like yours. I changed the fuel pump, the hoses and recently overhauled the fuel injection system. No change so I'm a bit puzzled. Waiting to see what you find out...I guess I need to start looking at the schematic of the system and see what else it can be.

[Bob_Belville]

Hmm....I have almost the same problem except that my fuel flow also fluctuates, especially when close to peak EGT. It can fluctuate by up to 0.5gph. Fuel pressure fluctuates between 20 and 24 just like yours. I changed the fuel pump, the hoses and recently overhauled the fuel injection system. No change so I'm a bit puzzled. Waiting to see what you find out...I guess I need to start looking at the schematic of the system and see what else it can be.

FWIW, my FF is pretty stable. You might see a little co-relationship in the attached. It looks like to me that the FP was stable at high power in climb in the graph of flt 23 when the EDM was brand new. I hope to fly today and will report.

[romair]

I just sent my graphs to savvyanalysis. Will see if they can come up with something

[Bob_Belville]

Anthony, the indicated Fuel Pressure does not seem to vary  much with altitude, at least up to 10,000.  

 

We disconnected the fuel line going to the transducer (gauge) on but ends and found no fuel at all in that line. We primed it with fuel using the boost pump and reconnected but I did not have time to fly so don't know if that (air in the line) was what was going on. I will report result. After studying graphs from many flights going back to the 930 installation I'm pretty sure there's nothing wrong with the pump. 

Update: I purge the fuel line from the throttle body to the fuel pressure transducer of air. I flew today, 2 short sightseeing flights, longest 17 minutes, fuel pressure stayed pretty steady! I'm headed to MA later in the week and will know better when I climb and run a couple of hours in steady state conditions but I am optimistic. Stay tuned. 

[carusoam]

There must have been a procedure for that...?

That much air would have difficulty getting out on its own.

Keep looking for a repeat performance... Having air leaking in would be another challenge.

I am still not a mechanic, so every one knows....

Best regards,

-a-

[Bob_Belville]

There must have been a procedure for that...?

That much air would have difficulty getting out on its own.

Keep looking for a repeat performance... Having air leaking in would be another challenge.

I am still not a mechanic, so every one knows....

Best regards,

-a-

It was pretty easy to purge, we disconnected both ends of the hose, the transducer is high on the firewal so we reconnected the bottom end, cracked the mixture and cycled the electric fuel pump until gas reached the top of the hose which we then tightened. I'd be interested in any informed advice on how to keep air from getting into the hose which of course goes nowhere. Any air at all will be at the top of the fluid column which is what the sensor sees.

[romair]

Thats interesting. Could you have a leak in the system that is sucking in air? That's what I am considering right now. I have a new fuel pump and also I overhauled the injection system, so I know it has to be uphill from the throttle body. Since it happens on both tanks, the furthermost point I need to inspect is the fuel selector.

[Bob_Belville]

Thats interesting. Could you have a leak in the system that is sucking in air? That's what I am considering right now. I have a new fuel pump and also I overhauled the injection system, so I know it has to be uphill from the throttle body. Since it happens on both tanks, the furthermost point I need to inspect is the fuel selector.

I don't understand where you think air might get in. If I had a leak in the fuel line I suppose fuel would leak out since the line is holding 22-25 psi pressure whenever he engine is running as indicated by the fuel pressure gauge sensing at the end of that line. I guess think it's possible the hose was installed without being purged when the engine removed last October. 

 

I  suppose someone here knows exactly the what, why,  and how of this system. Clarence suggested we get the fuel out of the fuel line, we did the opposite. (The engine builder had never heard of purposely having air in that line.)

[Bob_Belville]

Well, the fuel pressure is still erratic. After purging the fuel line to the sensor of air it seemed to be okay during a 15 minute test flight but this chart from Firday's trip to N57 looks as bad as ever. I do not believe the problem is the pump itself, I turned on the boost pump for a minute or two at cruise and the fibrillation continued. 

 

The "control" flight was shortly after the installation of the EDM.

 

[carusoam]

Did you See if the leg filled with air again?

Did we talk about if there is a smoothing function (averaging over time) of the JPI?

Best regards,

-a-

[Bob_Belville]

Did you See if the leg filled with air again?

Did we talk about if there is a smoothing function (averaging over time) of the JPI?

Best regards,

-a-

I will check on the line tomorrow. 

I have run the JPI on 1 sec recording and at default 6 sec recording. The pattern is about the same. As is the display itself in real time. 

 

I don't know of any dampening function. FWIW, we had a jumpy MAP reading as well. JPI provided a snubber valve that solved that problem. 

[carusoam]

Request snubber for FP???

MAP and FP can be pretty noisy functions. They are somewhat different than a continuous process

The FP cycles seem to be spread over seconds...

Compared to fuel injector/ intake cycles that are 4X20+ cycles per second...(pressure in waves departing the system)

Or the fuel pump sending pressure in in waves...

It is hard to believe that the system is capable of accidentally picking up these waves...

At 1 to 6 seconds per data collection it is randomly picking up peaks and valleys.

The snubber is a physical device that smooths over the waves near the sensor.

Electronic averaging essentially does the same thing without a tuned physical device.

What happens prior to engine start? Pressure comes up with the electric fuel pump and stays steady?

Calling JPI is usually challenging. You need to prepare your patience prior to dialing..

Check all the grounds...? (To eliminate possible electrical noise)

I believe the old mechanical gauges may have a snubber as part of their internal design.

The O1 doesn't merit an FP gauge. We only have FF. If FF goes to 0 then you know you have an FP problem....

As you may know, my experience comes from the chemical industry. I do not have any real aviation experience. I am not a mechanic.

Good luck,

-a-

[Bob_Belville]

Anthony, that's helpful. I will try to contact JPI, I have already found them to be spotty, helpful at times and not so much at other times. I got the MAP snubber from them through the installing avionics shop.

[Bob_Belville]

It appears snubbers are used for fuel pressure:

 

www.ebay.com/gds/Vision-Microsystems-Snubber-Fitting-What-does-it-do-/10000000177682802/g.html

[N601RX]

The reason that making sure the line has air in it instead of being completely full of fuel is that the air is compressible and the fuel is not. If the line is completely full of fuel. It is going to show every pulse of the engine pump. It there is a snubber at the servo and the line is full of air, then the snubber is going to limit the amount of fuel that move back and fourth through it each time the engine pulses. The air is going to compress a small amount each time and and smooth out the pressure pulses. The small amount of fuel that is allowed through the snubber during each pulse will only have a negligible affect on a line full of air.