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High Fuel Pressure

Tom @ N269CP

Well Known Member
My RV-8 is powered by an O-360-A1A with Marvel Schebler carburetor. The fuel pressure gauge is hard piped to the pressure tap on the inlet of the carb. I've been flying it regularly for the past couple of months.

Fuel pressure normally varies between about 3-6 psig during flight on the mechanical pump alone.

When I fired it up yesterday morning to go flying the fuel pressure was reading about 9 psi on the mechanical pump alone at about 1000 rpm. It read about 5-6 psi before starting on the electric booster pump alone. As fouling/stiction is a concern, my A&P checked the inlet strainer to the carb which was clean. We drained the carb bowl and flowed fuel through it to flush any debris.

After reassembly, I fired it up again this morning and the fuel pressure was still indicating 8+ psi at 1000 rpm with the mechanical pump alone. The pressure held steady at 8 psi after shutdown and even after cycling the mixture lever. We bled the panel gauge and the pressure immediately dropped to less than 1 psi. So the 8-9 psi pressure appears to be real.

I'd be grateful for any ideas and suggestions as to what the cause of this anomaly may be.

Thank you.

Tom
 
Or the bourdon tube in your gauge is getting weak and is indicating higher than real. You can check that by plumbing in a different gauge and comparing the readings.
 
Or the bourdon tube in your gauge is getting weak and is indicating higher than real. You can check that by plumbing in a different gauge and comparing the readings.

That's a good thought, Greg. I'm hoping not to have to pull the carb for o/h and want to rule out everything else first. I'll order up a new FP gauge just in case.

The plane is about 16 y/o, so I've been gradually replacing various accessory items which have given up the ghost. I don't believe the carb has ever been off. The mechanical fuel pump is dripping a tiny bit of oil at the lower bolt heads, so that's also on my "soon to replace" list. Love the new PMag I recently installed to replace the original right mag.

All the best,

Tom
 
Tom, your carb cannot cause an increase in fuel pressure.

Nor can flow restriction raise fuel pressure. Cleaning the inlet screen is never a bad idea, but a plugged screen would have nothing to do with an indicated pressure increase.

Pressure is set by a large pump spring pushing against a diaphragm. Line pressure downstream of the pump is essentially the same at no flow or at WOT fuel flow. The difference between a low pressure pump and a high pressure pump is the size of the spring.

A cam drives a pushrod which pushes against a rocker arm. The other end of the rocker arm pulls upward on a pull rod, which raises the fuel diaphragm, compressing the pump spring. When the cam rotates further and relaxes the pushrod/rocker/pull rod, the spring pushes the diaphragm downward, applying pressure to the fuel in the pump chamber. Note that the spring cannot suddenly become stronger.

There are two check valves, one each at the entrance and exit of the pump chamber. One allows fuel into the chamber, but not out. One allows fuel out, but not back in.

Check your gauge.
 
Tom, your carb cannot cause an increase in fuel pressure.

Nor can flow restriction raise fuel pressure. Cleaning the inlet screen is never a bad idea, but a plugged screen would have nothing to do with an indicated pressure increase.

Pressure is set by a large pump spring pushing against a diaphragm. Line pressure downstream of the pump is essentially the same at no flow or at WOT fuel flow. The difference between a low pressure pump and a high pressure pump is the size of the spring.

A cam drives a pushrod which pushes against a rocker arm. The other end of the rocker arm pulls upward on a pull rod, which raises the fuel diaphragm, compressing the pump spring. When the cam rotates further and relaxes the pushrod/rocker/pull rod, the spring pushes the diaphragm downward, applying pressure to the fuel in the pump chamber. Note that the spring cannot suddenly become stronger.

There are two check valves, one each at the entrance and exit of the pump chamber. One allows fuel into the chamber, but not out. One allows fuel out, but not back in.

Check your gauge.

Thanks, Dan. That sounds pretty definitive.

I was thinking possibly an issue with the carb float valve. The 5-6 psi on the electric booster pump alone was pretty normal. So to have a significantly higher pressure (9 psi) on the mechanical pump alone seemed anomalous. If the gauge were bad, I'd have expected an anomalously high pressure (2-3 psi higher) on the electric boost pump alone as well. Also, the gauge pressure dropped from 8+ psi to less than 1 psi upon breaking the fitting at the gauge. After stopping the engine by chopping the mixture, which should drain the float chamber, I'd have expected cycling the mixture control, to full rich and back a couple of times, to bleed the pressure to the gauge. But I may be missing something on the internal operation of the carb. Fuel flows during idle and runup to 2000 rpm were normal.

A new mechanical FP gauge is winging it's way to me, so will swap the old gauge out in a few days and will report back.

Kind regards,

Tom
 
I?ve had 3 VDO fuel pressure senders fail. All failures resulted in an abnormally high fuel pressure reading.
 
Thanks, Dan. That sounds pretty definitive.

I was thinking possibly an issue with the carb float valve. The 5-6 psi on the electric booster pump alone was pretty normal. So to have a significantly higher pressure (9 psi) on the mechanical pump alone seemed anomalous. If the gauge were bad, I'd have expected an anomalously high pressure (2-3 psi higher) on the electric boost pump alone as well.

A reasonable deduction. I have heard other reports of increased pressure with the low pressure pump. The previous principles of operation are factual, but I won't discount the possibility of some phenomenon not realized.

Also, the gauge pressure dropped from 8+ psi to less than 1 psi upon breaking the fitting at the gauge. After stopping the engine by chopping the mixture, which should drain the float chamber...

Mixture control regulates flow from the float bowl, not its fill level. Fill level is controlled by the float valve. Mixture cutoff stops flow, bowl fills, float needle closes and traps fuel in line, with pressure applied by the pump spring/diaphragm.

Cycling the mixture with the engine not running doesn't flow any fuel from the bowl because there is no pressure drop in the venturi. Bowl stays full, float needle closed. Pumping the throttle would eventually drain the system via the accelerator pump, but that would take a long time.

BTW, if I understand correctly, you have a mechanical gauge, not electrical? Jeremy is correct about the VDO senders. Mine is reading 145 psi right now ;)
 
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A reasonable deduction. I have heard other reports of increased pressure with the low pressure pump. The previous principles of operation are factual, but I won't discount the possibility of some phenomenon not realized.



Mixture control regulates flow from the float bowl, not its fill level. Fill level is controlled by the float valve. Mixture cutoff stops flow, bowl fills, float needle closes and traps fuel in line, with pressure applied by the pump spring/diaphragm.

Cycling the mixture with the engine not running doesn't flow any fuel from the bowl because there is no pressure drop in the venturi. Bowl stays full, float needle closed. Pumping the throttle would eventually drain the system via the accelerator pump, but that would take a long time.

BTW, if I understand correctly, you have a mechanical gauge, not electrical? Jeremy is correct about the VDO senders. Mine is reading 145 psi right now ;)

Ah, okay...my mechanic suggested the mixture valve was located upstream of the float chamber. So this tends to point me back towards the gauge...or possible restriction at the float valve. I really need to study up on the carb internals.

Yes, I have a mechanical FP gauge...the replacement should be here tomorrow.

Cheers!

Tom
 
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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
 
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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?
 
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
 
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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
 
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.
 
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
 
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
 
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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:

Vent.jpg
 
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:

Vent.jpg

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.
 
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....

Vent%20Ring.jpg


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

Vent%20Ring%202.jpg


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.

Base%20and%20Pulsator.jpg


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.

Damaged%20Fuel%20Diaphragm.jpg
 
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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The "fail to high pressure" failure mechanism is still a puzzle.

Maybe not. Humor me; check that pulsator chamber and vent passage. If it is as I suspect, I'll explain.

For some reason the pump is putting out more flow than required by the carburetor (assuming carb is fine).

Let's not confuse "flow" with "pressure". This kind of pump can generate rated pressure with no flow. Within the normal gph envelope, pressure should drop very little as flow rate increases.

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.

The spring is compressed when the pump lever lifts a pull rod during the intake stroke. The compression dimension (x) is fixed by the mechanics of the cam, pushrod, pivot, and lever. The spring is compressed the same amount with every every rotation of the pump drive cam. The physical relationships can't change, so more spring compression than normal is not possible.
 
Maybe not. Humor me; check that pulsator chamber and vent passage. If it is as I suspect, I'll explain.



Let's not confuse "flow" with "pressure". This kind of pump can generate rated pressure with no flow. Within the normal gph envelope, pressure should drop very little as flow rate increases.



The spring is compressed when the pump lever lifts a pull rod during the intake stroke. The compression dimension (x) is fixed by the mechanics of the cam, pushrod, pivot, and lever. The spring is compressed the same amount with every every rotation of the pump drive cam. The physical relationships can't change, so more spring compression than normal is not possible.

Understood.

Let's say the pump normally delivers 2 gph at 3 psi outlet pressure at idle....this is where it normally operated . However, now it delivers 2 gph at 9 psi at idle. As discharge pressure is generated purely by spring force, this means the spring is "now" more compressed at the bottom of its downstroke....ie. for some reason its downstroke must now be less than normal to deliver 2 gph at 9 psi. As the carburetor is only taking 2 gph, this suggests the "effective" pump displacement per inch of downstroke is now higher than it should be for some reason. This is consistent with reports by others of flooded carburetors and running rich. I always lean to peak rpm at idle and have a flowmeter.

I'm arranging to have the pump replaced on Tuesday. I'll inspect the internals once it's out and report back. I could also give Lycoming a call on Monday to see what their experience is with failure to high pressure.

FWIW, I'm a process machinery engineer. We'll figure this out. :)
 
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Understood.

Let's say the pump normally delivers 2 gph at 3 psi outlet pressure at idle....this is where it normally operated . However, now it delivers 2 gph at 9 psi at idle. As discharge pressure is generated purely by spring force, this means the spring is "now" more compressed at the bottom of its downstroke....ie. for some reason its downstroke must now be less than normal to deliver 2 gph at 9 psi. As the carburetor is only taking 2 gph, this suggests the "effective" pump displacement per inch of downstroke is now higher than it should be for some reason. This is consistent with reports by others of flooded carburetors and running rich. I always lean to peak rpm at idle and have a flowmeter.

I'm arranging to have the pump replaced on Tuesday. I'll inspect the internals once it's out and report back. I could also give Lycoming a call on Monday to see what their experience is with failure to high pressure.

FWIW, I'm a process machinery engineer. We'll figure this out. :)

These pumps have an inverse relationship between flow and pressure potential. At low GPH rates it can produces internal pressures greater than rated pressure and at max flow, they usually produce just under the rated pressure. However, there is an internal mechanism to reduce output pressure to a max of rated pressure, plus or minus a tolerance. I don't know the details of the mechanism used to regulate pressure, but presume it is related to diaphragms and/or springs. Whatever that mechanism is, it is compromised/failed in your pump allowing higher pressures at lower flow rates and why you see the highest pressure as idle. This is the same mechanism that keeps pressures at rated even with 0 flow.

Larry
 
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However, there is an internal mechanism to reduce output pressure to a max of rated pressure, plus or minus a tolerance.

No such mechanism Larry. As Tom noted in engineer-speak, output pressure is controlled by the spring force, (F = k*x).

David Carr posted an excellent pump tour some years ago, and I saved it all as a Word doc. FWIW, I also have a high pressure pump (for constant flow fuel injection) dismantled on my bench right now.

Here is a link to the Word doc:

https://www.danhorton.net/VAF/High Fuel Pressure/Fuel Pump Dismantled.docx

Illustration below from the Lycoming overhaul manual, 4-cyl pump cam, pushrod, and pump lever. Lycoming calls the cam an "eccentric", which is closer to the truth, as it is round with an off-center axis, rather than the familiar egg-shape lobe with the axis concentric with a base circle. The eccentric turns at one-half engine RPM.

In this illustration, the eccentric is shown rotated with its lowest point at the pushrod, which allows the fuel diaphragm to reach the limit of its output stroke, if there is no output restriction, and enough time to empty the pump chamber. However, when the engine is running, the diaphragm never moves that far.

See the Word doc. The lever lifts the diaphragm pull rod to compress the pump spring, but cannot push the diaphragm. When the eccentric rotates off its high point, removing force from the pushrod and lever, the diaphragm moves downward into the pump chamber by spring pressure alone. The lever, however, does indeed move through the entire range you see in the illustration. There is a separate light spring stacked on top of the pull rod's lift cap which keeps the the slack out of the lever/pushrod/ecccentric contact faces while the end of the diaphragm's pull rod is floating above the end of the lever.

See the return spring under the lever in the illustration? IIRC, that's the old pump style with a laminated arm. It's not present on late model pumps. The newer design incorporated the light spring you'll see in the Word doc.

Cam%20and%20Pushrod.jpg
 
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Previously I wrote:

In this illustration, the eccentric is shown rotated with its lowest point at the pushrod, which allows the fuel diaphragm to reach the limit of its output stroke, if there is no output restriction, and enough time to empty the pump chamber. However, when the engine is running, the diaphragm never moves that far.

Let's do the math and see how far it actually moves. Check me.

Assume 15 gph for a 180 hp engine at 2700 RPM, best power mixture. That's 0.25 gallons per minute, or 2 pints. The pump eccentric rotates at half speed, so the linkage pulls the diaphragm upward 1350 times per minute. Thus, the quantity per pump cycle is 2/1350, or 0.0014814 pints per stroke, which converts to 0.042775 cubic inches.

Because it is a diaphragm with a bellows at its perimeter, its motion can be described as cylindrical displacement. The effective diameter of the fuel diaphragm is about 1.75", so radius is 0.875". The volume of a cylinder is

volume = radius squared * pi * length

Here we want to determine the length (i.e. diaphragm stroke) needed to displace 0.042775 cubic inches, so the equation becomes

length = volume / (radius squared * pi), or

length = 0.042775 / (0.875^2 * 3.14), or

length = diaphragm stroke = 0.01779

The diaphragm moves about 0.018" per stroke...less than half a millimeter, at best power and 2700. (Same pump on a hot rod 540 at 30 gph would be twice the stroke...a whopping 0.032")

Consider the implications for spring force, F = k * x. I'll get a measurement to be exact, but for now, assume the compressed spring has a length of 1.75". During the pumping stroke, it relaxes by 0.018", a very small percentage of its length, so F varies by only a tiny bit. That's why the pump is almost constant pressure, regardless of flow, idle to redline.
 
We replaced the mechanical fuel pump this afternoon. During the subsequent test run, indicated fuel pressure at 1070 rpm was 4 psi. With the boost pump running in tandem, it was 6 psi. At 1700 rpm, the fuel pressure was also well within range (did not make note of the pressure...it was getting late). The high pressure (>8 psi) anomaly appears to have been resolved. I would also note that the pressures were rock steady. Whereas before pump replacement, the fuel pressure tended to be somewhat erratic. I did not replace the fuel pressure gauge today, and will keep the new gauge as a spare.

I partially disassembled the old mechanical pump. I cannot fully disassemble it as I lack the tools necessary to detach the plunger rod & upper/lower diaphragms and spacer ring from the actuator lever & spring housing. But I did manage to remove the lower cover, pulsator diaphragm and valve housing.

There were puddles of fuel under the pulsator diaphragm in the recesses on both the suction and discharge side of the lower cover. The pulsator diaphragm appeared to be in reasonably good condition, no tears, but the inlet side of the diaphragm was somewhat stiffer than the discharge side. No sign of fouling in the small vent hole in the lower cover. The throughbolts fastening the pulsator cover/diaphragm to the valve body were barely more than finger tight, which seems abnormal.

From what I could see of the upper/lower pump diaphragms, they appeared to be in reasonably good condition as well...better than yours, Dan. No outward signs of wear...further disassembly is necessary for a complete disposition. But there were a couple of strands (~1" long) of soft fibrous material that fell out of the space between the two pump diaphragms. Some oil also appeared to spill out of this space, but I cannot rule out overspill from the upper actuator housing.

Not sure if/when I'll be able to completely disassemble the plunger/diaphragm/spring assembly as I only have limited hand tools.
 
I would also note that the pressures were rock steady. Whereas before pump replacement, the fuel pressure tended to be somewhat erratic.

Ok, I'm a believer. There really is something going on here. As before, the fun part is determining what and why.

You mentioned a 3-6 psi range in your first post, but now rock steady. That's interesting. Can you expand on "erratic"?

I partially disassembled the old mechanical pump. I cannot fully disassemble it as I lack the tools necessary to detach the plunger rod & upper/lower diaphragms and spacer ring from the actuator lever & spring housing.

It's actually not hard to do. File off one end of the pump lever pivot pin, then drive it out of the case with a punch, after which the lever and bushing will slide out. With the lever's fork no longer hooked under the pull rod cap, the diaphragms, pull rod, and springs will all pop out of the case with a good tug. There is nothing holding them but a seal pressed into the case.

But I did manage to remove the lower cover, pulsator diaphragm and valve housing. There were puddles of fuel under the pulsator diaphragm in the recesses on both the suction and discharge side of the lower cover. The pulsator diaphragm appeared to be in reasonably good condition, no tears, but the inlet side of the diaphragm was somewhat stiffer than the discharge side. No sign of fouling in the small vent hole in the lower cover. The throughbolts fastening the pulsator cover/diaphragm to the valve body were barely more than finger tight, which seems abnormal.

You think the pulsator chambers were full before disassembly, or did you manage to keep the pump upright when removing the bottom bolts?

From what I could see of the upper/lower pump diaphragms, they appeared to be in reasonably good condition as well...better than yours, Dan. No outward signs of wear...further disassembly is necessary for a complete disposition. But there were a couple of strands (~1" long) of soft fibrous material that fell out of the space between the two pump diaphragms. Some oil also appeared to spill out of this space, but I cannot rule out overspill from the upper actuator housing.

Not sure if/when I'll be able to completely disassemble the plunger/diaphragm/spring assembly as I only have limited hand tools.

Fibrous material is also interesting. The vent to atmosphere is really tiny.

If you like, send it to me for disassembly.
 
Ok, I'm a believer. There really is something going on here. As before, the fun part is determining what and why.

You mentioned a 3-6 psi range in your first post, but now rock steady. That's interesting. Can you expand on "erratic"?

You think the pulsator chambers were full before disassembly, or did you manage to keep the pump upright when removing the bottom bolts?

Fibrous material is also interesting. The vent to atmosphere is really tiny.

If you like, send it to me for disassembly.

By "erratic", I never knew where the fuel pressure was going to settle after startup. Anecdotally, it could shoot to a higher level then come down, flicker up and down by a couple of psi over a period of several seconds between first start and takeoff. It wasn't steady or predictable. But with the boost pump-only it was steady. It was like that since I bought the plane last August. I didn't pay much attention to it as fuel pressure was always within Lycoming's wide allowable pressure range. But when it went and stayed above that range, that caught my attention.

When I removed the pulsator cover the diaphragm was stuck to it around the entire circumference. The fuel puddles were evident when I peeled the pulsator diaphragm off the cover. The pockets were roughly 30-50% full.

I always try to be careful when disassembling failed machinery as solids and other debris can be strong evidence pointing to root cause. Many years ago when I was with Shell, we had a Kobe pitot pump that was failing main shaft bearings repeatedly after it was shutdown and restarted. The mechanics would disassemble the pump and clean it up before the reliability engineers would examine it. The pump was located downstream of a sand filter in a tertiary oil recovery process. I got involved and instructed the mechanics not to clean up anything after the next failure. After the next failure we found a considerable amount of sand inside the internal rotating casing. The pitot pump was basically an excellent centrifuge which removed carried over sand distributing it evenly around the ID of the rotating casing. But when shutdown the sand would slough off to the bottom of the casing creating a massive rotor unbalance which overloaded the bearings. So, I was careful when disassembling this pump. :)

It's probably best that I ship the fuel pump to you. All I have is a small collection of hand tools now. I sold off my well-equipped workshop with Shizuoka CNC knee mill a few years ago (sniff, sniff). If you can PM me your address, I'll get it on its way ASAP.
 
Tom, with the pump diaphragm assembly out of the pump body, this one seems obvious.

Both high and low pressure pumps have a seal on the pull rod shaft. Those seals fail by tearing out the center portion. This is your low pressure pump:

IMG_20180901_125452192_HDR.jpg


Slightly different view. Between the two, you can see how the center rips out:

IMG_20180901_125455969_HDR.jpg


Here, when the spring is compressed to a degree similar to an almost fully raised position, the inner core of the damaged seal catches on the edge of the remainder of the seal still in the upper spring seat. When it does, any additional raising of the pull rod must compress both the pump spring and the seal parts. There is a distinct momentary increase in force required to compress the pump spring, until the seal parts pop past each other...the extra spring force we were looking for.

IMG_20180901_125610584.jpg


Can I prove it? Nope, not without a spare accessory case, pushrod, and pump eccentric. However, I can send it back to you. You play with it and see if you're not left with the same conclusion.

The fuel and oil diaphragms were intact in all respects. All the vent passages were open, including the tiny restrictor in the main vent to atmosphere. There was minor evidence of oil between the diaphragms, "minor" described as much of the surfaces being oil-free. Given an intact oil diaphragm, the little oil present had to have gotten there via bolt holes due to low clamp force. I suspect the fuel in the pulsator cover chambers got there via the vent passage down the side of the case, sneaking between the underside of the fuel diaphragm and the pump chamber flange, again due to low clamp force, the bottom bolts being not much more than finger tight in your description.
 
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Tom, with the pump diaphragm assembly out of the pump body, this one seems obvious.

Both high and low pressure pumps have a seal on the pull rod shaft. Those seals fail by tearing out the center portion. This is your low pressure pump:

IMG_20180901_125452192_HDR.jpg


Slightly different view. Between the two, you can see how the center rips out:

IMG_20180901_125455969_HDR.jpg


Here, when the spring is compressed to a degree similar to an almost fully raised position, the inner core of the damaged seal catches on the edge of the remainder of the seal still in the upper spring seat. When it does, any additional raising of the pull rod must compress both the pump spring and the seal parts. There is a distinct momentary increase in force required to compress the pump spring, until the seal parts pop past each other...the extra spring force we were looking for.

IMG_20180901_125610584.jpg


Can I prove it? Nope, not without a spare accessory case, pushrod, and pump eccentric. However, I can send it back to you. You play with it and see if you're not left with the same conclusion.

The fuel and oil diaphragms were intact in all respects. All the vent passages were open, including the tiny restrictor in the main vent to atmosphere. There was minor evidence of oil between the diaphragms, "minor" described as much of the surfaces being oil-free. Given an intact oil diaphragm, the little oil present had to have gotten there via bolt holes due to low clamp force. I suspect the fuel in the pulsator cover chambers got there via the vent passage down the side of the case, sneaking between the underside of the fuel diaphragm and the pump chamber flange, again due to low clamp force, the bottom bolts being not much more than finger tight in your description.

Hi Dan,

Thank you so much for taking a more detailed look for the root cause of failure. That failed seal is certainly abnormal and your hypothesis regarding the resulting higher spring force sounds plausible. Installing a new pump certainly seems to have resolved the high FP issue.

Sorry for the late reply but have been out flying in this beautiful late summer weather, and now dealing with yet a new "squawk" to fix. :)

I don't need the old pump back, so feel free to trash it or use as a paperweight. ;)

Thank you again, Dan, for all your help.
 
I have about 1000 hours on my -7 behind an IO-360 and have been seeing intermittent high Fuel pressure as indicated by the SkyView. Yesterday's flight showed about 38 psi on run-up and after 30 minutes dropped back to the high end of the green. Pressure drops momentarily when I apply throttle and always drops after flying a while. After a fuel or lunch stop, while still warm, pressure still normal. Seems like the first run-up and take off for the day, I see high pressure. I have replaced the mech fuel pump and the VDO sensor (twice). I think I still have a bad sensor. Thoughts from the group?
 
I am still seeing intermittent high fuel pressure Skyview messages on my IO360 equipped -7. 2 new VDO sensors and new mechanical fuel pump make no change. Never see it after the first flight of the day. Thoughts?
 
Plumb in a flex line and a good quality mechanical fuel pressure gauge.

The only way know for certain what the actual pressures are.
 
I went through heck with erratic and high RPM and fuel pressure indications. I dealt with the RPM issues for over two years and the fuel pressure about 6 months. I have a Vision Micro Systems VM1000C. It uses a Hall effect sender on the magneto for the tach and a VDO sender for fp (fuel pressure).

These problems were very frustrating because I was doing all of the ?right? things to trouble shoot and as soon as I thought I fixed it, the problems surfaced again. I checked connections and did continuity checks multiple timeis. I sprayed contact cleaner. I pulled on pins and wires. I replaced connectors. I replaced the Hall effect on the magneto twice. And fortunately, a friend of mine had another VM1000C display that I swapped out. The problem persisted. With only 3 basic components (the sender, the display, and the wiring), I had replaced everything except for the wires themselves. I doubted replacing the wiring would fix it because of the many continuity checks I had done. Finally, I replaced the three wire harness to include the pins on the display end, the Molex connector on the sender end, and the wires in between. I?m happy to report that the problem with my tach is finally fixed. As soon I was convinced that my tach was fixed, I replaced the wiring for the fuel pressure. And voil?, my fuel pressure was fixed. I wish I would have known it was that easy. I had been through three VDO senders and various connectors. The frustrating part was that I had done many many continuity checks while moving and jiggling wires.

Good luck and let us know how it works out.
 
Just wanted to chime in that I also to experienced high fuel pressure. New to me rv6a carb o-360, fuel pressure was approaching 9 psi with the boost pump off. I changed the 20 year old pump and now it's 6 psi boost pump on or off and very stable reading. Seems these pumps can fail high
 
Fuel pressure

O-360 A1A carburetor. Fuel pressure indicates all over the place up to 20+ PSI. Engine runs great but warning high pressure comes on about very 5 minutes. Is it the VDO?
 
O-360 A1A carburetor. Fuel pressure indicates all over the place up to 20+ PSI. Engine runs great but warning high pressure comes on about very 5 minutes. Is it the VDO?

Sounds very likely either sender or ground. They don't tend to ground well by themselves, I used a hose clamp to attach a grounding wire straight to the firewall from the sensor. If you are sure ground is good a new sensor is a cheap fix. Could put inline gauge there if you wanted to confirm and watch it (carefully) during a ground run, also.

I have had a pump fail high, but it was consistent, not sporadic. You description sounds like a grounding or sensor problem - I'd guess grounding.

Chris
 
O-360 A1A carburetor. Fuel pressure indicates all over the place up to 20+ PSI. Engine runs great but warning high pressure comes on about very 5 minutes. Is it the VDO?

Yup - VDO;

Did y'all clean up the ground wire/lugs like we talked about?

Brian
 
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