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Author Topic: Oil and Fuel Pump/Pressure Relationship  (Read 1279 times)

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FH Flyer

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Oil and Fuel Pump/Pressure Relationship
« on: August 28, 2016, 12:05:48 PM »
I have a 1979 PA-32 Cherokee Six with a Lycoming IO-540.  On a few occasions, the fuel pressure has dropped dramatically when advancing to full throttle on the takeoff roll. In all cases the booster pump is on, and it has happened with various levels of fuel in the tanks and with either tank selected.  In talking with an overhaul shop, my mechanic thinks the engine-driven fuel pump is on its way to failure.

I downloaded the data from our JPI engine monitor and was intrigued to see (at times) a strong inverse relationship between oil pressure and fuel pressure.  I am imagining the fuel pump temporarily seizing, causing a spike in oil pressure.  Does this make sense to anyone else?  A second question is if the engine-driven pump does seize, would the booster pump be ineffective?

https://www.dropbox.com/s/xxk5jup6u4xs3yo/Oil%20and%20Fuel%20Pressure%20%202.jpg?dl=0
https://www.dropbox.com/s/s14b3g13yvkpx6n/Oil%20and%20Fuel%20Pressure%2025Aug16.jpg?dl=0

Thanks for any thoughts / insights!
Travis

Klaus

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Re: Oil and Fuel Pump/Pressure Relationship
« Reply #1 on: August 28, 2016, 01:12:09 PM »
  The most important gauge for fuel on your panel is the "Manifold/Fuel Flow" gauge. What is your fuel flow on takeoff?
  Does the IO-540 have a gear driven or diaphragm type pump?

  The fuel pressure does go down when fuel flow is increased and the oil pressure will go down after with RPM is reduced and/or temperature increases. If there is any slight chance that the engine driven fuel pump is not turning it will break the quill shaft in a split second and stop operating. If the engine has a diaphragm fuel pump it is slightly possible for it to preform poorly at high RPM (slight).
Klaus Marx
Juneau, AK (PAJN) & East Wenatchee, WA (KEAT)

Cabbage

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Re: Oil and Fuel Pump/Pressure Relationship
« Reply #2 on: August 28, 2016, 02:31:57 PM »
Not a mechanic, but this is how I would look at in logically.

Fuel Pressure:  Fluid dynamics says that the energy in a fluid exists in one of three states; velocity, pressure, or elevation.  Within an engine there is effectively no elevation change.  As you increase power, more fuel is required which means a higher flow rate.  Since the size of the tubing hasn't changed, the velocity must have increased.  That being the case, something else has to give.  In this case, that would be pressure.  So, assuming the fuel pump is only capable of a fixed amount of energy, as the power goes up, the pressure should come down.  How much, I don't know.

Oil pressure:  Since pump speed is related to RPM, logic tells me that pressure would be related to RPM.  So if RPM remains constant, but power changes, I would not expect much change in oil pressure (unless there is also a change in oil temperature).  However, if the power increase you refer to is accomplished by increasing RPM (like running the power from idle to max for takeoff), I would expect to see a higher oil pressure.

In other words, the relationship between your fuel and oil pressure may be coincidental, not necessarily cause and effect.

FH Flyer

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Re: Oil and Fuel Pump/Pressure Relationship
« Reply #3 on: August 28, 2016, 03:14:21 PM »
Hi Klaus;
The pump is engine driven as far as I know. 
Looking through the engine monitor data, fuel flow goes up to 30gph on the takeoff roll pretty consistently.  In most cases, fuel pressure dips from 26 psi to around 17, which is still in the green range, before increasing again. 

On Thursday when I aborted takeoff, fuel pressure went from 26 to 11psi, which is yellow/bordering on red range.  Fuel flow looks like it was headed to 30 gph before I chopped the throttle.

Thanks,
Travis

Klaus

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Re: Oil and Fuel Pump/Pressure Relationship
« Reply #4 on: August 28, 2016, 04:00:24 PM »
  The type of engine driven fuel pump is important to this situation. IO-540-K1G5 is the Diaphragm type fuel pump originally made by AC Delco Remy. The IO-540-K1A5 is a geared vain type fuel pump commonly made by Lear Romec.

   The type of pump will determine if you should make another flight or not. Both types of pumps have airworthiness Directives against them. The A.D.'s are subject to serial numbers. Sometimes during manufacture or overhaul the parts don't get installed right or have a defect. The geared vain type pump can cause catastrophic engine failure even with the electric fuel pump on. The "Plunger"/diaphragm type usually leaks and gets worn out causing low pressure but the electric fuel pump will push fuel past it.

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/fadd1eab9c294a8286257e90007376b0/$FILE/1E4_Rev_25.pdf


Geared Vain Type Fuel Pump


Diaphragm "Plunger" type
« Last Edit: August 28, 2016, 06:13:32 PM by Klaus »
Klaus Marx
Juneau, AK (PAJN) & East Wenatchee, WA (KEAT)

FH Flyer

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Re: Oil and Fuel Pump/Pressure Relationship
« Reply #5 on: August 28, 2016, 05:31:07 PM »
Thanks for the additional info.  Oops- I meant to say gear-driven, not engine driven  ???

 According to my POH I have a IO-540-K1G5.  At this point I am leaning towards replacing that according to my mechanic's recommendation and seeing if that fixes the issue.

CubFlying.com

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Re: Oil and Fuel Pump/Pressure Relationship
« Reply #6 on: August 28, 2016, 05:56:02 PM »
Thanks for the additional info.  Oops- I meant to say gear-driven, not engine driven  ???

 According to my POH I have a IO-540-K1G5.  At this point I am leaning towards replacing that according to my mechanic's recommendation and seeing if that fixes the issue.

Thanks Klaus for sharing your knowledge with all of us...Sounds like that would be $380 well spent.

Daryl

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Klaus

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Re: Oil and Fuel Pump/Pressure Relationship
« Reply #7 on: August 28, 2016, 06:18:22 PM »
  I just caught my mistake and fixed it also:
 from the TCDS 1E4 page 9 of 12;
Quote
- K1G5  Same as - K1A5 except equipped with a diaphragm type fuel pump and drive.


http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/fadd1eab9c294a8286257e90007376b0/$FILE/1E4_Rev_25.pdf
Klaus Marx
Juneau, AK (PAJN) & East Wenatchee, WA (KEAT)

FH Flyer

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Re: Oil and Fuel Pump/Pressure Relationship
« Reply #8 on: August 28, 2016, 08:26:23 PM »
Thanks Klaus - I saw that in the link you sent and was a little confused.  I appreciate you correcting/clarifying!  I'll post results here after replacing the pump. 

FH Flyer

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Re: Oil and Fuel Pump/Pressure Relationship
« Reply #9 on: November 10, 2016, 10:58:03 AM »
Klaus - it looks like your diagnosis was spot-on!  Shortly after this exchange my mechanic installed a new pump and the fuel pressure problem seems to be fixed. 

Thanks very much for your help.
Travis