Fuel Injection Fuel Pressure

Coincidentally, two RVs here in ABQ have been experiencing a similar issue and we're not sure why. For the first 150-250 hours, the fuel pressure was pretty constant at about 28psi. Now, the fuel pressure bobbles around in cruise between 23 and 28 psi. In my plane, it is not uncommon to see 18-20 psi at some point during climb out, while burning about 12-14 gph. It's even gone as low as 15 psi for a few minutes at a time. Boost pump brings it up to about 20 psi.

Both are RV-7s with Lycoming injected engines, AFP fuel pumps, no gascoltor, and plumbed as per Van's plans. The FF sender is just forward of the fuel pump. Field elevation is 5800 ft.

We spoke to our respective engine builders. One says it should stay steady at 28 psi, the other says it can jump around and operate as low as 14 psi before being concerned. Both are credible sources and reputable engine builders. We're curious what the forum has to say. Is this a leading indicator of mechanical fuel pump failure or normal operation? Thanks in advance.

There is a possibility of another phenomenon at play here. I know for a fact it affected me. As one gets more familiar with one's plane, more time is spent observing engine parameters. This is a possible explanation.

I have FI also, and if I'm cruising leaned, the pressure will be around 24 (engine pump). However, if I change power and/or mixture to cause an abrupt increase in fuel flow, the pressure will drop into the teens for perhaps 5 seconds. I would think any steady state pressure below 20 to be a little suspicious, and perhaps indicative of some problem either with the pressure or the instrumentation. There is also the well documented (in these forums) issue with non-vented transducers showing low pressures for a while after a long climb.

Hi Marc:

I have a similar setup to yours, but with a Superior XP-360 and PA Silverhawk EX fuel injection. I'm noticing the fluctuating fuel pressures as well (23-28PSI). I followed the Van's FWF plans on this stuff. I have the fuel pressure line coming out of the outlet side of the engine-driven fuel pump. I also have the restrictor fitting in there. I'm using the pressure transducers I received from Dynon.



I'm not sure what it means. I have 23 hours on my plane now. I noticed it after a few flights. Hitting the boost pump will bring the FP up to about 28-29psi and it will hold steady for just a second, then start fluctuating again.

I'm suspicious of vapor getting inside the fuel pressure transducer line. If vapor gets in the line, it will rise to the transducer. I'm not sure if the vapor would actually affect the transducer's output or not -- but I'm sure someone will chime in.

Fuel pressure varies

IO-360 with Precision (Bendix RSA) Silverhawk fuel servo and Van's FP gauge.

I have noticed this as well. It had me concerned originally, but it is fairly consistent. Normally I see about 27 PSI when I start my flight, after a climb to altitude I will often see it drop to 24 PSI during the climb and en route. I can switch on the electronic boost pump and it will jump back up to 27 PSI and when I turn it off it generally slowly settles back down to 24 PSI. If I switch tanks (always with boost on to charge the lines as a precaution), sometimes it will stay at 27 or so for a while and then drop down later to 24 again. Never seen it go into the teens though. that would certainly get my attention.

I have all lines firesleeved EXCEPT my transducer line. I do have a restrictor fitting mounted right to the T-fitting on my engine driven pump. I suspect a non-vented transducer (Vans - non-vented? anybody know?) or the lack of firesleeving on that transducer line that sees basically no flow at all and doesn't really have a way to purge itself.

I found this source - FAA documentation on the IO-360:

http://rgl.faa.gov/Regulatory_and_G...1b405c2d5097a8525670e0052caa6/$FILE/E16ea.pdf

Measurements at inlet to injector (in other words - after engine driven pump):

Idle: 12PSI
Min: 20PSI
Max: 45 PSI

My 2c. Glad you guys brought this up. Was curious what other folks were seeing.

-Rob

Marc, I have a TMX-IO 360 with the Silverhawk, but I'm not flying yet so I can't comment, but try posting on the Lycoming Group at Yahoogroups.com, perhaps Mahlon has an opinion.

Larry

Same thing.

AFP, SilverHawk, Aerosport IO360, 24-28 fluctuations without boost pump. Seems to be normal. I noticed that several "collector" devices have been made to even out the fluctations, but I dont see it as a problem so I have not worried about it.

Same Thing

I have Aerosport Power IO-360, AFP boost pump, do not know off-hand my engine driven pump. The fuel pressure usually starts around 28 and gradually drops to around 22. Never gets below. My fuel pressure hose comes off the Bendix RSA (Precision) fuel servo downstream of the filter screen. I have firesleeve on everything.

Thanks guys. I'll post on the Lycoming group and see what they say. I'm not too concerned about the fluctuations in the mid 20s. It's mostly when it drops to 20 or lower - not usually in cruise but during that later parts of climbout. The engine runs fine, though.

If it drops to 15 the boost pump will bring it up to about 19, lower than I expected.

BTW, one of us has the AFP and the other a Silverhawk EX FI system.

Lycoming Group

What's this Lycoming Group you mention? Would like to lurk there, as well.

What is your idle fuel pressure?

I have about 6 hours on my Silverhawk injected IO-360 now. My fuel pressure with the boost pump off and the engine idling will drop down into the 13-15 psi range. Is that normal? In-flight the pressure stays up in the mid 20's.

Is your pressure gauge reading pressure relative to ambient or differential pressure of the fuel pressure to the manifold pressure. Since injector flow varies with the square-root of a pressure change, a 2% pressure change will cause fuel flow to change by 1%. Many systems control fuel pressure relative to the manifold pressure so that if MAP drops by 6", about 3 psi, the fuel pressure indicated on a differential gauge will show no pressure change, but a gauge vented to ambient will show the change!

For the first 140hrs or so fuel pressure was steady at 28psi then it started 'dropping' to 23psi for about a second then back to 28psi, this repeated every 5-10 seconds. With the boost pump on the pressure was steady at 30psi. After scratching my head for a while I replaced the pressure transducer (Dynon supplied) and the problem went away. Being the sort of person I am I took the transducer to bits and the tracks were worn, I'm guessing at the 28psi reading. It looks like when the boost pump was on it pushed the slider to an unworn part of the track and I got a steady reading.

My fuel pressure runs between 24-26 PSI but mostly around 25 psi on idle or cruise and with the auxiliary fuel pump on, the pressure will go to around 30 PSI but then after a few second it will start coming down to the normal 25-26 psi.

Keep in mind that you have a pressure relive valve at your auxiliary pump setup that is suppose to open up if the pressure build up higher then normal. Some times the o-ring may get stock and keep the valve from operating as it should. The o-ring may be affected by the blend of the fuel which can vary from station to station.

Airflow Performance pump pressure too high?

RV7 QB IO360 180 hp.

I am using the Airflow Performance 7/7A F.I. PUMP INSTAL kit on my RV7.
With the AFP pump on engine not running I get 50psi downstream (after) the mechanical pump and 25 psi upstream (before) the mechanical pump. I used two different new mechanical pressure gauges and get the same reading on both. The AFP manual says the pressure relieve valve is set at 26 psi so the AFP pressure relief valve is serviceable.
Can anyone tell me why I am getting a 25 psi increase going through the mechanical pump?
Is this normal?

It looks like enough people are having similar behavior with their fuel pressure to conclude that all are not having a problem.

I have the Silver Hawk injection servo with the pressure line coming off the ourflow from the mechanical engine pump. I also have the cooling shroud on the pump.

After starting the pressure can flutuate a lot during the first couple of minutes but typically does not go below 15 psi. With a cold or hot start I am putting the electric pump on after it starts for a couple of minutes. When I turn it off it settles at around 28 psi and doesn't vary much from there.
I have 65 hours on a new engine.

A few hours ago I decided to bleed the line going from the mechanical pump to the Dynon pressure transducer on the manifold supplied by Vans for fuel and oil pressure. I never did it before but should have. After doing this I found the pressure stabilized more rapidly. If there is air in this line it is the pressure of this air you are reading at the transducer and not fuel which if the line is free of air should give a more instantaneous reading.

However, I still remain suspicious of some kind of air leak in the plumbing that cold cause erratic readings at startup -- in other words, sufficient air leaking that allows the fuel to drain back to the tank from the lines to the fuel selector and then to the filter which in my case is vertical on the gear tower. This might add a fuel bubbles in the system that are eliminated when the electric pump is put on for a couple of minutes. Also supporting this is the very, very slight fuel smell I get and while there are no drips I have found a little staining on the lines below the fuel selector. Something to be tweaked at the first condition inspection.

The bottom line is that I think pressures as low as 15psi at low power settings and then in the mid 20's are normal. Since I am at sea level I haven't paid a lot of attention to the effect of altitude and while I've flow up to 10,000 ft havent paid a lot of attention to the reading while there. I see further back in the posts Marc Ausman who lives is much higher up in New Mexico. Someone else noted that ambient pressure effects the reading so that might be a factor. On my other plane with a gravity feed system to an engine driven pump I loose a couple psi of fuel pressure above 5000ft on the guage.

Some instalations require a ground wire if the transducer is located at the end of the pressure line and tied up in some fashion. This set up, while providing the least fatigue on the transducer threaded connector presents a difficult grounding solution. Sometimes a tab clamped to the transducer. A marginal ground in this or any other mount senario can cause the pressure to indicate low.

I would suspect the electric transducer as being flaky.

Other than that, a friend had problems with pressure recently with a new system and found the big filter clogged with lots of junk, cleaned it and pressure returned to normal.

My system runs at 26-28 with the boost pump on and 21-22 on the mechanical engine pump. Pressures are rock steady. Altitude does not seem to be a factor. Two days ago I recorded some numbers at 9500' MSL (DA 12,220') and fuel pressure was 21 psi running on the mechanical pump.

The pressure regulator is a ball and spring deal, kind of bullet proof, and the pumps are generally very reliable, except ethanol could ruin the mechanical pump.

The weak link in the system is the pressure transducer and/or how your particular indicating system is dealing with the volt signal from the device. The system here is Grand Rapids EIS 4000 and it has been working quite well across the board.

I have seen similar things in my plane. The pressure has dropped several times just after refueling, on hot days, at the top of climb to 7-9000ft. I have a separate sensor for the "idiot" light and it flashes on as well. This makes me think that it is not the transducer. I now keep the boost pump on until the top of climb plus a minute or two and leave the pump on after changing tanks for several minutes. I suspect it is vapor related. I plan on installing a fuel pump cooling shroud and more insulation on the fuel line from the firewall to the selector valve.

Van's fuel pressure gauge
Bendix Silver Hawk injection
Horizontal induction
Firesleeved lines
100LL

High Fuel Pressure

Came across this thread while looking for guidance on high fuel pressure I'm seeing on my RV-8, IO-360-A1B6D. I've had it about a year, and the GRT EFIS and EIS most often are reading way over the 45 psi limit in Lyc manuals, often 50-60. Engine runs fine, it seems, but does take a lot of leaning to bring FF in cruise to book reference fuel flows, and it seems to need a lot of leaning on ground. As I say, operation seems ok, but concerned. I'm wondering if pressure sensor is in wrong place in system? Any ideas would be appreciated.

Jamie said:

I'm suspicious of vapor getting inside the fuel pressure transducer line. If vapor gets in the line, it will rise to the transducer. I'm not sure if the vapor would actually affect the transducer's output or not -- but I'm sure someone will chime in.

Click to expand...

Pressure's pressure. I could see where a vapor pocket might act as a "spring" in an oscillating pressure fluctuation, but it shouldn't affect the steady state number much.

What kind of transducer? The VDOs are s**t. Replaced mine with Kavlicos, no issues, rock steady.

E-17B said:

Came across this thread while looking for guidance on high fuel pressure I'm seeing on my RV-8, IO-360-A1B6D. I've had it about a year, and the GRT EFIS and EIS most often are reading way over the 45 psi limit in Lyc manuals, often 50-60. Engine runs fine, it seems, but does take a lot of leaning to bring FF in cruise to book reference fuel flows, and it seems to need a lot of leaning on ground. As I say, operation seems ok, but concerned. I'm wondering if pressure sensor is in wrong place in system? Any ideas would be appreciated.

Click to expand...

Bill, offhand I can think of no mechanical problem that would result in high fuel pressure with the electric pump off.

A pressure transducer problem is possible, but usually they fail low and erratic.

Sounds like you purchased your RV-8. Be aware that the GRT EIS has no dedicated fuel pressure input. The fuel pressure transducer is connected to one of four AUX (general use) inputs, then the system is calibrated by entering "scale factor" and "offset" values. You may wish to check your settings, and perhaps the resistor value. Full information in the EIS user manual, and here:

http://www.grtavionics.com/documents/EIS/VDO_LowP.pdf

http://www.grtavionics.com/documents/EIS/VDO 80 and 150 Pressure Sender Aux Input.pdf

You did not mention which fuel injection system you have, but be aware that 50~60 psi, if you actually have it, is not a big deal for both AFP and Bendix/Precision systems. The dual diaphragm regulator works based on relative pressure across a main jet pressure drop, not input pressure. Either system will tolerate 80~90 psi...actually up to the limit of the internal seals. The manufacturer's legal limit is in the manuals. I'm not saying you should skip checking or correcting, just that it ain't gonna fall out of the sky while you do.

My .02 worth is to verify with a known accurate mechanical guage, and compare the pressure readings. MANY pressure fluxuations have been sensor issues, and not and actual pressure loss.
Tom

TS Flightlines said:

My .02 worth is to verify with a known accurate mechanical guage, and compare the pressure readings. MANY pressure fluxuations have been sensor issues, and not and actual pressure loss.
Tom

Click to expand...

I had to do this right at the last, just before starting phase 1. It helped me discover I had mismatched resistor values and VDO sensors from the GRT sheets Dan provided links for.

Also had a VDO sensor fail (the numbers went whacko) as others have already said.

Thanks! I knew I would get good info here. I also couldn't see how the real pressure could be that high. Doing fuel inj set up on Continentals at the moment, will rig Gage's afterward to verify the indications. I'm ordering a bunch of UMA steam gages to install this winter, I think I'll add FP to the rest of the order. Not a glass fan in light a/c, especially.

fuel pressure fluctuations

hi there,
am having problems with the fuel pressure indications of my IO-360-M1B. Am using the Dynon EMS120 with he Kavlico 50 sensor. Fuel pressure I take from the port at the Silverhawk injector. Fuel pressure fluctuates between 10 and 34 psi during flight. Have exchanged the pressure sensor as I thought the sensor might be defect, but the problem remains with the new sensor. The engine does not show any symptoms, runs smooth at any of the indicated pressure at high power at take off and low power cruise. Does anyone have an idea what the problem might be ? Could it be vapor lock in the fuel pressure line connected to the sensor ?

best regards,

Josef / RV8-A

problem solved...

hi everyone,
thanks a lot for the numerous replies and advice regarding my problem
with the fluctuating fuel pressure on my YIO-360-M1B engine. Actually I
have solved the problem now. The cause for the fluctuating fuel pressure
indication was VAPOR LOCK in the fuel pressure hose. I have connected
the fuel pressure hose to the silverhawk injector which is located at
the front lower end of the engine. The fuel pressure hose I have routed
underneath cylinder 1 and cylinder 3 back to the firewall where the
pressure sensor is located. It seems that the fuel in the pressure hose
got heated up during longer flights at higher levels - thus causing
vapor locks in the hose and thus causing the erratic readings of the
fuel pressure. I have put an additional heat protection sleeve on the
fuel pressure hose now and re-routed it as far away as possible from the
hot areas near the exhaust pipes of cylinder 1 and 3. Problem solved, no
more erratic fuel pressure readings. Now I have one perfectly good
Kavlico 50 fuel pressure sensor as spare...

best regards,

Josef