Engine Fuel Pump Failure

[DanH]

I have a new Tempest pump on order right now. The original Lycoming pump began squeezing oil out along the topmost diaphragm clamp line. I easily tightened the screws a 1/4 turn or more, but when inspecting a few hours later I realized it is also weeping oil from the relief tube. The upper oil diaphragm seems to be failing.

[jonweisw]

All interesting replies and thank you all for the interest!

True that 14psi is considered 'normal' according to the lycoming manual; alternatively, I would argue that you don't really know exactly what the pressure is because the transducer is not calibrated. What I did observe is that there was a transient change in the pressure that was alarming. This change was rectified by boost pump and ultimately by replacement. I don't buy that 'simple mechanical pumps don't fail'. That seems like wishful thinking to me.

Yet, I also agree that given that I didn't pinpoint a physical fault in the pump by disassembling it after removing it, I cannot say with 100% certainty that the pump is the culprit. However, given the choice of replacing a transducer (cheaper, non-critical) versus a pump (more expensive, critical), I am going to replace the critical component, especially because they aren't THAT expensive and the full failure of it could be problematic from a safety point of view (unlike a transducer).

If faced with the same situation tomorrow, I would do the same.

[RV7A Flyer]

Originally Posted by jonweisw

All interesting replies and thank you all for the interest!

True that 14psi is considered 'normal' according to the lycoming manual; alternatively, I would argue that you don't really know exactly what the pressure is because the transducer is not calibrated. What I did observe is that there was a transient change in the pressure that was alarming. This change was rectified by boost pump and ultimately by replacement. I don't buy that 'simple mechanical pumps don't fail'. That seems like wishful thinking to me.

I don't think anybody said that. I think they said that it's *unlikely*.

Yet, I also agree that given that I didn't pinpoint a physical fault in the pump by disassembling it after removing it, I cannot say with 100% certainty that the pump is the culprit. However, given the choice of replacing a transducer (cheaper, non-critical) versus a pump (more expensive, critical), I am going to replace the critical component, especially because they aren't THAT expensive and the full failure of it could be problematic from a safety point of view (unlike a transducer).

Now we get to the interesting part...you don't *know* what is happening to cause the intermittent fluctuation, and you are removing and replacing critical components. I will direct your attention to the well-known Waddington Effect.

I don't even think that I'd start by replacing the transducer unless I knew it was failing or always wrong.

Bottom line...you replaced a part that you didn't know was bad, with a new part that might not fix the problem, because you don't know what the actual problem is in the first place. To top it off, you've messed with a critical system, thus poking the ghost of Waddington who demonstrated that most failures occur in the hours after significant maintenance.

If faced with the same situation tomorrow, I would do the same.

Well, you might be...if the pump wasn't the problem.

[jonweisw]

But it did fix the problem!

I am surprised by the resistance here. Sorry.... Im not going to let something that is a critical component and is acting up just go ahead and fail if I can help it. Waddington effect or not. This seems like common sense to me.

I will try to fish out the old pump and take it apart to see if this can shed any more light on this. I was certainly NOT going to disassemble the pump and NOT replace it.

Unlikely or not, what was causing the pressure fluctuation was the old pump. Fortunately it had no consequence. The new pump is functioning as a new pump should.

[RV7A Flyer]

Originally Posted by jonweisw

But it did fix the problem!
...
Unlikely or not, what was causing the pressure fluctuation was the old pump.

I thought you said

I didn't pinpoint a physical fault in the pump by disassembling it after removing it, I cannot say with 100% certainty that the pump is the culprit.

[jonweisw]

Yes, your honor.

[dwrichey]

Since we're discussing fuel pressure, I would like to ask another question. I have a Lycoming IO-360, with a Dynon Skyview and Kavlico sensor. Since very first engine start, displayed fuel pressures fluctuate between low 20's, to medium-to-high 20's, while in cruise. I tapped the fuel pressure sensor line into the main fuel line at the fitting coming out of the mechanical fuel pump. The tap fitting has a restricted orifice (unfortunitly I can't recall the diameter, likely down around #55 drill size). Later, I added another restriction at the fitting entering the Kavlico, which helped, but did not solve the fluctuations.
Any suggestions? Maybe go even smaller orifice diameters? Any suggestion on how small I should go?
Thanks for any input/suggestions.

[YellowJacket RV9]

Originally Posted by dwrichey

Since we're discussing fuel pressure, I would like to ask another question. I have a Lycoming IO-360, with a Dynon Skyview and Kavlico sensor. Since very first engine start, displayed fuel pressures fluctuate between low 20's, to medium-to-high 20's, while in cruise. I tapped the fuel pressure sensor line into the main fuel line at the fitting coming out of the mechanical fuel pump. The tap fitting has a restricted orifice (unfortunitly I can't recall the diameter, likely down around #55 drill size). Later, I added another restriction at the fitting entering the Kavlico, which helped, but did not solve the fluctuations.
Any suggestions? Maybe go even smaller orifice diameters? Any suggestion on how small I should go?
Thanks for any input/suggestions.

If you have any air in the line, that can cause fluctuations. Have you bled the line to the sender recently, or before first start even?

Chris

[9GT]

Years ago when we got the Cozy MKIV flying with a brand new Superior IO-360 engine, the fuel pressure started acting up in the early stages of Phase 1 to the point the test pilot declared an emergency on climb out. The engine still ran but would not develop full power and the return to field and landing was uneventful. After thorough discussions with Superior and data transfer to them, they felt that the mechanical fuel pump was bad and sent out a new one under warranty. Problem solved. FWIW.

[Walt]

Originally Posted by jonweisw

But it did fix the problem!

I am surprised by the resistance here. Sorry.... Im not going to let something that is a critical component and is acting up just go ahead and fail if I can help it. Waddington effect or not. This seems like common sense to me.

Unlikely or not, what was causing the pressure fluctuation was the old pump. Fortunately it had no consequence. The new pump is functioning as a new pump should.

Just for the record I think you did the right thing, some folks prefer getting a** deep into the theory behind every little problem. Like you said, it was not that big a deal to replace the pump and it was the most likely cause of your problem, good for you, pro-active mantenance IMO.

Maintenance must be done regardless of the "waddington effect", perhaps some folks like to use that as a reason not to do stuff