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  #11  
Old 08-15-2018, 06:42 PM
Tom @ N269CP Tom @ N269CP is offline
 
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Location: Green Cove Springs, FL
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Quote:
Originally Posted by YellowJacket RV9 View Post
Myself and several others have had the same problem, caused by a failing mechanical fuel pump. Don't ask me how. But if you confirm the gauge's reading, I don't see much besides a bad pump that would cause high pressure before the carb inlet.

Chris
Interesting.

Was the high pressure instantly cured for everyone by replacing the mechanical pump?

I've found a VAF thread from 2015 discussing high pressure in the fuel line, but most of the "victims" were running on mogas..possibly winter mogas after the weather started warming up causing suspected vapor lock. My situation is different. In my case, I'm based in Durango, CO, am running on 100LL, the high pressure presented immediately upon engine start at about 8am with ambient temperature in the 50's. I then taxied a mile or so down to the FBO and back to top off my tanks hoping the pressure would normalize...it did not. I then did a run-up to 2000 rpm all of which failed to reduce fuel pressure below 8-9 psi.

I've been concerned about the mechanical pump as its never been replaced since the plane was built, and oil drops are accumulating at the bottom of the FP casing bolts. Rubber diaphragms only last for so long.

It sounds like I might need to get another mechanical fuel pump on order. Hoping the new FP gauge is the cure.

Cheers!

Tom

Last edited by Tom @ N269CP : 08-15-2018 at 07:45 PM.
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  #12  
Old 08-15-2018, 06:51 PM
Tom @ N269CP Tom @ N269CP is offline
 
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Originally Posted by kaa View Post
My mechanical gauge behaves kind of the same way. It also sometimes shows high readings after the engine has been stopped for a while. I wonder if there is vapor somewhere in the system that heats up? I'm not sure this is a good explanation.

Also, what do these gauges actually measure? Differential pressure w.r.t. atmosphere?
Yes. "Gauge" pressure is exactly that...differential pressure w.r.t. local atmospheric pressure.

Or, in other words, "gauge" pressure equals "absolute" pressure minus local atmospheric pressure.

Kind regards,

Tom
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  #13  
Old 08-15-2018, 08:09 PM
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DanH DanH is offline
 
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The "why" is the fun part.

I suspect what we have here is a plugged pulsator diaphragm air passage. I'll put some photos together and explain.
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  #14  
Old 08-15-2018, 08:13 PM
YellowJacket RV9 YellowJacket RV9 is offline
 
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Location: Clearwater, FL / KZPH
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Quote:
Originally Posted by Tom @ N269CP View Post
Interesting.

Was the high pressure instantly cured for everyone by replacing the mechanical pump?

I've found a VAF thread from 2015 discussing high pressure in the fuel line, but most of the "victims" were running on mogas..possibly winter mogas after the weather started warming up causing suspected vapor lock. My situation is different. In my case, I'm based in Durango, CO, am running on 100LL, the high pressure presented immediately upon engine start at about 8am with ambient temperature in the 50's. I then taxied a mile or so down to the FBO and back to top off my tanks hoping the pressure would normalize...it did not. I then did a run-up to 2000 rpm all of which failed to reduce fuel pressure below 8-9 psi.

I've been concerned about the mechanical pump as its never been replaced since the plane was built, and oil drops are accumulating at the bottom of the FP casing bolts. Rubber diaphragms only last for so long.

It sounds like I might need to get another mechanical fuel pump on order. Hoping the new FP gauge is the cure.

Cheers!

Tom
My problem was instantly solved by a new pump, and I believe others' have been as well. I do not run mogas, and have never had vapor lock issues. In my case, the high fuel pressure may have also contributed to needing a carb rebuild, as well. It was running rich and leaking raw fuel into the airbox. The new fuel pump was a decent chunk of change, but installation is fairly simple, if you use some of the tips found here in other threads.

Chris
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  #15  
Old 08-15-2018, 10:03 PM
Tom @ N269CP Tom @ N269CP is offline
 
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Quote:
Originally Posted by DanH View Post
The "why" is the fun part.

I suspect what we have here is a plugged pulsator diaphragm air passage. I'll put some photos together and explain.
I look forward to your analysis, Dan.

Kind regards,

Tom
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  #16  
Old 08-15-2018, 10:09 PM
Tom @ N269CP Tom @ N269CP is offline
 
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Quote:
Originally Posted by YellowJacket RV9 View Post
My problem was instantly solved by a new pump, and I believe others' have been as well. I do not run mogas, and have never had vapor lock issues. In my case, the high fuel pressure may have also contributed to needing a carb rebuild, as well. It was running rich and leaking raw fuel into the airbox. The new fuel pump was a decent chunk of change, but installation is fairly simple, if you use some of the tips found here in other threads.

Chris
Thank you, Chris.

I've read some of the posts about the "string trick" as I suspected a new pump was in the not too distant future. I will start researching exactly what fuel pump I would need...want to make sure all the ports are in the exact same location so I don't have to modify any hoses. I'm glad I'm replacing it immediately upon noticing the high pressure before flying it again...hopefully avoiding any damage to the carb.

** New Lycoming fuel pump p/n LW-15472 and p/n 60096 gasket now on order

Kind regards,

Tom

Last edited by Tom @ N269CP : 08-16-2018 at 11:54 AM.
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  #17  
Old 08-16-2018, 05:39 PM
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DanH DanH is offline
 
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Tom, want to conduct a little post-mortem after replacement? Chris, still have your old pump?

Remove the bottom cover, and see if the chamber marked "IN" has oil in it:

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  #18  
Old 08-16-2018, 11:06 PM
Tom @ N269CP Tom @ N269CP is offline
 
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Quote:
Originally Posted by DanH View Post
Tom, want to conduct a little post-mortem after replacement? Chris, still have your old pump?

Remove the bottom cover, and see if the chamber marked "IN" has oil in it:

I'd be glad to, Dan. I hope to swap it out next week.

I would not be surprised if there's oil in that chamber as all the lower bolt heads have droplets of oil accumulated on them, which has likely migrated down through the bolthole clearances in the diaphragm/spacer stack. There are no other oil leaks on my engine, and I've got an AntiSplat oil mist separator, so the engine compartment is otherwise oil-free apart from those fuel pump bolt heads. My pump has about 1400 tach hours on it.
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Kind regards,

Tom
Machinery Engineer
==================================
RV-8 N269CP
O-360-A1A w/Hartzell CS prop on 100LL
Slick-IC+PMag ignitions
Steam gauges, EI UBG-16 & FP-5, Garmin Aera 660
TruTrak autopilot (GPS coupled)
Infinity grip w/Matronics trim speed control
Reiff engine preheater
AntiSplat oil mist separator
MH O2 system

Location: Durango, CO (KDRO)
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  #19  
Old 08-17-2018, 07:31 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by Tom @ N269CP View Post
I would not be surprised if there's oil in that chamber as all the lower bolt heads have droplets of oil accumulated on them, which has likely migrated down through the bolthole clearances in the diaphragm/spacer stack.
The red line in the previous photo and below marks the vent passage for that chamber. The vent passage leads up to the space between the oil diaphragm and the fuel diaphragm, via this little hole in the vent ring....



....which leads to the atmosphere via the standard external vent/telltale fitting.



Note that oil or fuel leaking into the vent ring space can be pumped down the vent passage and fill the cavity in the bottom cover, which would probably prevent the pulsator diaphragm from working as intended.

The same would be true if the vent passage to the pulsator cavity was plugged with debris. The holes in the ring and the bottom cover are very small. Please look for blockage too.



BTW, don't feel bad about replacing a 1400 hr pump. This one was a Lycoming brand with about 10 years and a bit less than 800 hours. Both the fuel and oil diaphragms were on the road to sho'nuff failure.

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  #20  
Old 08-17-2018, 04:54 PM
Tom @ N269CP Tom @ N269CP is offline
 
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Quote:
Originally Posted by DanH View Post
The red line in the previous photo and below marks the vent passage for that chamber. The vent passage leads up to the space between the oil diaphragm and the fuel diaphragm, via this little hole in the vent ring....

Note that oil or fuel leaking into the vent ring space can be pumped down the vent passage and fill the cavity in the bottom cover, which would probably prevent the pulsator diaphragm from working as intended.

The same would be true if the vent passage to the pulsator cavity was plugged with debris. The holes in the ring and the bottom cover are very small. Please look for blockage too.

BTW, don't feel bad about replacing a 1400 hr pump. This one was a Lycoming brand with about 10 years and a bit less than 800 hours. Both the fuel and oil diaphragms were on the road to sho'nuff failure.
The "fail to high pressure" failure mechanism is still a puzzle. For some reason the pump is putting out more flow than required by the carburetor (assuming carb is fine). As output pressure is controlled by the spring force (F = k * x), it's possible the pump output strokes may be bottoming at a shorter compressed spring length. So it's possible we're looking for a failure mode that results in higher output flow while also not allowing the spring to fully extend during the output stroke. That said, the spring extension length should vary inversely and automatically with the backpressure/restriction applied by the carb. So basically, the pump would have to be putting out more flow than design at reduced pump stroke length (higher spring force).

If there was a flap-like tear in the lower diaphragm that acted like a check valve, and if the volume between the upper and lower diaphragms was not constant and got smaller during the downstroke, the upper diaphragm might effectively be pumping in parallel with the lower diaphragm. The telltale hole would also have to be plugged or at least partially plugged....there's no sign of leakage there. Inlet flow to the interdiaphragm volume might be entering via that vent port from the pulsator, which may also be leaking. Could be some fluidic effects limiting backflow to the pulsator. Seems a complicated explanation, but such things could happen.

In retrospect, I should have changed out all the engine accessories when I bought the plane given it had over 1000 hours on it. It beats this piecemeal process I've been going through.
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Kind regards,

Tom
Machinery Engineer
==================================
RV-8 N269CP
O-360-A1A w/Hartzell CS prop on 100LL
Slick-IC+PMag ignitions
Steam gauges, EI UBG-16 & FP-5, Garmin Aera 660
TruTrak autopilot (GPS coupled)
Infinity grip w/Matronics trim speed control
Reiff engine preheater
AntiSplat oil mist separator
MH O2 system

Location: Durango, CO (KDRO)

Last edited by Tom @ N269CP : 08-17-2018 at 05:16 PM.
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