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Fuel pressure anomaly

Bill Boyd

Well Known Member
Carb'ed O-320 E2D with standard RV-6A fuel system (electric boost pump in series with engine-driven pump) and GRT EIS with newly-replaced (due to age) pressure senders (about 20 hours on them).

I normally see fuel pressures of about 5-6psi with the Facet alone and no change with the engine running. The last two flights I have had a high fuel pressure alarm (I think I set the limit at 8) and have seen pressure readings as high as 9.3 psi on the ground. In cruise it now settles around 7.3-7.5, but on landing rollout and taxi, back into the red at 9+ psi. Turning off the boost pump in cruise or on the ground has no effect on the readings I'm seeing.

Not sure if the limits are set too conservatively on the gauge, but it's the CHANGE in behavior rather than the absolute value that concerns me. The needle and float are obviously preforming their job; there is no sign of richness or rough running in any flight regime. I'm just not sure why the higher readings have appeared.

My first two thoughts are a partial obstruction in the outflow from the engine-driven pump to the carb inlet screen, or (more likely IMO) a pressure transducer starting to fail prematurely.

On the risk side, I don't want to see fuel flooding the carb float bowl, nor a perforation of the mechanical pump diaphragm. Neither would be a good day.

Thoughts?
 
Definately worth investigating further. Most carb's can only hold about 8 PSI before the needle/seat is over run and the bowl overfill's, causing an overly rich condition.

Can't help in troubleshooting, beyond confirming that mechanical fuel pumps can fail with over pressure symptoms. I doubt a downstream obstruction is an issue. The fuel pump should be able to hold the designed pressure even with 0 flow on the outlet side. You can confirm this by watching your fuel pressure as you pull the knob to ICO on shut down. An internal obstruction of the pump's pressure relief circuit would cause this, but it is internal to the pump.

Larry
 
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Hey Bill---
What was the pressure BEFORE the senders were changed, or has anything else been done? I'd start like we always tell guys, get a KNOWN good gauge and check the pressure in all phases. If the readings match what you have on the EFIS, then the electronics are accurate. If not, then one or the other is wrong, and usually its the senders. Sender grounds---be sure they are properly grounded---seems to be an issue.

Tom
 
Tom, I changed the senders when I upgraded from a GRT EIS 2000 to a 4000 for full 4-channel readout. The oil pressure was starting to read >99 at times, so I replaced both fuel and oil transducers being they were 20+ years old and with the rep that the VDO units have anyways... No anomalous readings on the new setup until just the last 30 minutes of flight time over two short flights.
 
IMHO the easiest way to know for certain is to plumb a proven mechanical pressure gauge to compare with your other instrumentation.

You can change a lot of components out, like fuel pump and fuel senders, but that is an expensive way to start out,
unless the fuel pump is long overdue for replacement.
 
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One word: Grounds

Like I said...

Should expand a bit.
What does pressure read with engine off and boost pump on?
With engine running and boost off?
If both readings are high, it's a major clue that you have a measuring issue and not a pressure issue. Neither pump should be able to make more than design pressure, since the pressure is made by a spring; not the actuator.
 
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If you changed any of the ground wiring for the new sensors you might be seeing the effect of ground loops. They show up when there is any difference in ground potential between the sensor and the indicator. If so, make sure your sensors and indicators are grounded to the same point.
 
Good advice, all. I appreciate it.

I will get a reading on the Facet pump alone tomorrow, then on engine driven pump alone, and report back. Grounds are direct to the forest of tabs on both sides of the firewall that share a common brass through-bolt. Fast-ons could still present an issue.

I wasn't sure how the engine driven mechanical pump inner workings operated, nor what the failure modes looked like besides a diaphragm leak out the overflow tube or a failure to produce rated pressure.

For that matter, I'm not sure what the red line ought to be for carbureted fuel pressure. I made a guess when I set the upper alarm limit. :eek:
 
Like I said...

Should expand a bit.
What does pressure read with engine off and boost pump on?
With engine running and boost off?
If both readings are high, it's a major clue that you have a measuring issue and not a pressure issue. Neither pump should be able to make more than design pressure, since the pressure is made by a spring; not the actuator.

I have personal experience with an AC style pump delivering over designed pressure and flooding the carb. I could literally see the fuel pouring out of the carb vent. New pump solved the problem after rebuilding the carb twice to no avail. Auto engine with no FP gauge. Lyc uses a fuel pump design taken from the auto industry.

The AC pump is a constant displacement style and capable of delivering around 40+ GPH. When demand is less than this, the pressure rises (pressure and flow are inversely related) and it must overflow the excess to maintain the designe pressure (4-5 PSI), just like the lycoming oil pressure relief circuit. If that internal relief diaphragm/spring fails or sticks, you get an over pressure scenario.

The AC relief internally overflows from the output chamber to the input chamber.

Larry
 
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For that matter, I'm not sure what the red line ought to be for carbureted fuel pressure. I made a guess when I set the upper alarm limit. :eek:

Look at the lycoming Operating manual. If I recall correctly, it is 7.5 PSI. As I mentioned, most carbs can't handle much over 8 PSI or so.
 
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Larry,

Interesting data point. All the info I've ever read about the AC style engine driven pumps says that the spring sets pressure and the lever just 'cocks' the spring.
 
Larry,

Interesting data point. All the info I've ever read about the AC style engine driven pumps says that the spring sets pressure and the lever just 'cocks' the spring.

You may be correct, though I thought there was a relief diaphragm to regulate pressure. I am certain about the pressure tapering off as volume increases. That came directly from tempest. Either way, I have personally experienced high pressure on this style of pump, so can assure you that it is a possible faillure scenario. I did not perform an autopsy, as Dan did.

I don't see how the main spring can control pressure across a range from 0 to 45 GPH, but I suppose it's possible. Maybe Dan can post a link to his thread with the teardown pics. That would confirm it.

Larry
 
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I think it's an instrumentation issue

I did some checking today, and powdered up the EIS before boost pump engaged / engine start to take a baseline reading. (Normally never do this because the boost pump is part of my start routine and the avionics come only after the cranking is done or they will reboot). Turns out the resting fuel pressure reads 5.1 psi with no pumps running, then slowly wanders a bit down to 2.7, back to 5.0. Enough of that - crank to start and the idle fuel pressure is 8.1 (alarm flashing at that point) wandering to 7.9 and back. In flight this evening it was a pretty steady 8.

I'm confident this is some kind of biasing issue in the transducer/display system and not a true overpressure. Could be a fast-on on the forest of tabs ground; could be the bias voltage supplied by the GRT EIS-4000. An error in setup table values is also possible. I'll need to un-cowl and poke around, wiggle things, maybe hook up a mechanical gauge.
 
Fuel pressure

RV6A 0-360. My fuel pressure indication is all over the place from 5 to 18 PSI on every flight. Fuel flow also fluxuates at the same time. Engine continues to run perfectly. GRT EFIS, Only stays on for a couple of seconds and goes away. I replaced the VDO 800 hours ago and kept trying to get a good ground. I?ve learned to live with it, but I would like a good fix. Any ideas would be appreciated.
 
Plumb yourself a Mech fuel pressure guage.

Either it is a false indication, or it is a real indication from a fuel pump that is getting tired with age.

Best to know for certain before you have no power when you need it most.

If it is a tired pump, summer heat will help it show off pressure droops from a few added vapour bubbles.
Perhaps a new pump is less trouble than plumbing a mechanical line & gauge.

A hose clamp and ground wire is easy, but your indications (5-18) are very consistent with a tired mech pump.
When VDO transducers fail they often display wildly on the high side 50 -60 psi fluctuations.
RV6A 0-360. My fuel pressure indication is all over the place from 5 to 18 PSI on every flight. Fuel flow also fluxuates at the same time. Engine continues to run perfectly. GRT EFIS, Only stays on for a couple of seconds and goes away. I replaced the VDO 800 hours ago and kept trying to get a good ground. I’ve learned to live with it, but I would like a good fix. Any ideas would be appreciated.

My bad.
I see you have an O 360(with a carb) not an IO 360... if so, then 5-18 is on the high side and could be either the transducer or the pump.
try a ground wire hose clamped to the transducer body. or else a mechanical guage teed in and compare the two side by side.
 
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The AC pump is a constant displacement style and capable of delivering around 40+ GPH. When demand is less than this, the pressure rises (pressure and flow are inversely related) and it must overflow the excess to maintain the designe pressure (4-5 PSI), just like the lycoming oil pressure relief circuit. If that internal relief diaphragm/spring fails or sticks, you get an over pressure scenario.

The AC relief internally overflows from the output chamber to the input chamber. Larry

No.

Not constant displacement. Displacement is entirely based on output demand.

The cam, pushrod, and lever raise the fuel diaphragm, increasing the pump chamber volume and compressing the large diaphragm spring. Fuel flows in through a spring-loaded poppet valve.

When the fuel pump cam rotates past TDC, lever pressure is relaxed and diaphragm pressure is provided by the main spring. The size of the installed diaphragm spring determines rated output pressure. Low pressure pumps get a light spring, and FI pumps get a heavy spring.

There is no pressure relief bypass, nothing at all like a Lycoming oil pressure relief.

Whole thread on the subject of pressure slightly high in carb pumps here:

http://www.vansairforce.com/community/showthread.php?t=163196&highlight=diaphragm

Can't absolutely, positively finger the seal catch as the cause of slight pressure increase, but I would be happy to tear down Bill's pump to see if it exhibits the same issue.
 
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I have personal experience with an AC style pump delivering over designed pressure and flooding the carb. I could literally see the fuel pouring out of the carb vent. New pump solved the problem after rebuilding the carb twice to no avail. Auto engine with no FP gauge. Lyc uses a fuel pump design taken from the auto industry.

The first time I started my O-320, it would run for a few seconds then quit with fuel running out the carb. This continued even after checking the float needle to make sure it wasn't stuck.

The pump was shipped with the engine from an aircraft salvage company. Turns out the high pressure pump for an injected engine looks identical to the low-pressure pump on a carbed engine. It was only after careful examination of the part number that it became clear the wrong pump was on the engine.

Just a datapoint.
 
I just went through an almost identical situation with my O320-DIA powered RV4.

I was seeing abnormally high FP readings, much like you,, in the 7-8 range.
I am still using a vintage, WM1000 which has performed flawlessly over the last 20 years.

So like you, I started here, looking through the archives.

Now don't ask me why, but I immediately went to the most (IMHO) difficult fault scenario: fuel pump.

So out it went, replaced with a new pump from AS&S.

Fired up,
Guess what, problem was still there.
(But at least I now have a brand new pump with fresh rubber diaphragms!)

So after a brief email exchange with our VAF-resident VM1000 expert, Reggie Smith,
I was able to isolate the issue as a faulty transducer. Reggie happened to have a serviceable replacement and once installed, pressure was down to normal.

Expensive and time-consuming method of troubleshooting, but now I have a brand new pump. (Followed closely by a brand new carb, but unrelated to the FP issue)
 
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I had a very similar issue and it was a bad fuel pump. Engine ran fine, but fuel was pouring out of air box on shut down.

Transducer would be the easiest fix so definitely confirm that first, as well as the grounds and wiring. I left mine in place and connected it to a pressurized air source to confirm the reading.

Anyways, just be careful, it seems that these pumps can and do fail high and the results could be very bad.

Chris
 
Eis

Could it be that the Aux input scale factor and offset is programmed incorrectly?

The GRT EIs has 6 aux inputs. One of these is for your fuel pressure input. The offset # and scale factor has to be programmed for your particular fuel pressure sending unit to read out the correct data.

If you don’t have these numbers, check in with GRT service and they should be able to help.
 
You tube video

That video that Bill posted in #13 above is very educational. Will now be checking my fuel pump drain during preflight - and know what to look for and why. A potential lifesaver.
 
Agree, the video was very informative.
I learned something new today, I wasn't familiar with the function / importance of the drain on the fuel pump.

So it sounds like there is a generic recommendation for overhaul / replacement at 10 calendar years.

How many are actually doing that?

That makes something to add to the list for my next condition inspection.
 
Agree, the video was very informative.
I learned something new today, I wasn't familiar with the function / importance of the drain on the fuel pump.

So it sounds like there is a generic recommendation for overhaul / replacement at 10 calendar years.

How many are actually doing that?

That makes something to add to the list for my next condition inspection.

As I noted above, I pulled mine after 20 years and dissected the pump just because I was curious. (remember that my pump was not the cause of the high fuel pressure).

I was surprised to see that everything looked very serviceable. Rubber seemed intact with no cracks and no oil.

But I don't regret changing it out (although it was a major PITA!)
 
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