What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

low fuel pressure

Lyle RV8

Member
Here?s a fuel pressure problem that seems a bit different than those documented in the site archives.

I have a new O-360-A2A (carbureted) engine on my RV-8, first flight was 10 days ago and now have just under 6 hours. In that time the fuel pressure has dropped to near zero 3 times. Details are:

? Each time the fuel pressure dropped to near zero the airplane was in level flight at ~75% power (2,450 RPM, 10.5 GPH with a Catto prop)

? Upon the fuel pressure dropping to 1 PSI the alarm sounded, I turned on the boost pump and the fuel pressure immediately returned to the normal range of ~ 5psi. At no time did the engine falter.

? After about one minute I turned off the boost pump, and the fuel pressure remained normal for the remainder of the flight using only the mechanical fuel pump. This included one flight where I circled over the airport for over 30 minutes to see if it would happen again, it did not.

? Environmental conditions were not stressful, OAT was between 60 and 70 degrees at 5,000 ft MSL.

I did manage to track the event one time. The fuel pressure fell in a linear fashion taking approximately 20 seconds to drop from the normal ~ 5 PSI to 1 PSI where upon I turned on the boost pump.

The mechanical fuel pump is a new Tempest AA15472, I did install a cooling shroud. The fuel sensor gauge is standard GRT LPS-02 low pressure sensor. After the second event I re-purged the fuel line to the sensor, it had no effect as the issue reoccurred the following flight.

Not sure where to go. It appears the fuel pressure sensor is working correctly since it reads normally when the boost pump is on. It also seems odd for a mechanical fuel pump to essentially fail completely and then run normally afterwards. I am hesitant to replace parts without a good reason to explain the events.

I have a message into the engine manufacturer but thought maybe someone in this group may have seen something similar. For now the airplane is grounded.
 
Lyle I just went through those same fuel pressure archives the other day as my O-360 powered RV-8 was experiencing the same thing you described with my pressure dropping 1.0-1.5 with the boost pump off. The Lycoming manual states the normal fuel pressure range of 0.5 to 3.0 psi. I came to the conclusion it was pretty normal in the RV group for the pressure to wander around in the lower pressure range and that the sender was probably not that accurate in the lower scale. Engine ran normally at all fuel pressure readings.
 
Last edited:
I had a similar issue and ended up replacing the fuel pump last year. I was told there may have been a slight leak in the diaphragm of the pump (not confirmed) and the problem went away. I have replaced the carb with FI recently and am selling the AF-15472 <text deleted>

Links to the big auction site are not allowed per VAF rules; S. Buchanan
 
Last edited by a moderator:
Same as stated!

...I have had the exact fuel pressure changes, and was concerned so i changed the mechanical pump. The new pump acts and responds exactly like the one i took off, with no changes. It appears this is normal, and it wasn't a problem in the first place. I also changed the Vans fuel pressure gauge and sender when chasing this non existent gremlin. Thanks, Allan:eek::D
 
What kind of display is your Fuel Pressure on? Dynon? Manual round gage?

I have both setup and was having the same problem on an RV9A purchased in October. Builder/previous owner installed manual gage for comparison and they were both doing the same. - but the manual gage would not get nearly as low as the Dynon. Dynon would go 0.5 while manual pressure was 2.
 
What kind of display is your Fuel Pressure on? Dynon? Manual round gage?

I have both setup and was having the same problem on an RV9A purchased in October. Builder/previous owner installed manual gage for comparison and they were both doing the same. - but the manual gage would not get nearly as low as the Dynon. Dynon would go 0.5 while manual pressure was 2.

...Mine was the Vans manual gauge!..:D
 
Just adding to the Lycoming fuel pump reliability statistics. Once upon a time after a three and a half hours of flight (international) the pump vent left a long thick trail of oil on the bottom of the fuselage. Pressure never fluctuated. The new pump was sourced locally and replaced. TT was well beyond recommended TBO. :)
 
...I have had the exact fuel pressure changes, and was concerned so i changed the mechanical pump. The new pump acts and responds exactly like the one i took off, with no changes. It appears this is normal, and it wasn't a problem in the first place. I also changed the Vans fuel pressure gauge and sender when chasing this non existent gremlin. Thanks, Allan:eek::D

Same here. First I changed the pressure transducer - no change.

Then I changed the engine driven fuel pump for a new one. No change.

Put a cooling shroud around the mech fuel pump. No change.

I think that when the pressure gets down to around 0.5 or so the system (including my 2 generations old EFIS) does not report that well.

One thing you can try:

Next XC see of the EGT's change appreciably between getting a reading and not getting a reading.
 
My RV-8 with an Aero Sport O-375 does the same thing every flight or two, right from the first taxi test. The fuel pressure will at some point show a drop to 0.5 or 0 then will either recover on it's own or if not with the boost pump. It can happen on the ground at low RPM or in the air. Once it has recovered it will stay up for the rest of the flight, possibly several hours or perhaps do another drop some time later.

To see what would happen, I once let it continue low in a very aggressive climb at full throttle and got an engine stumble so it seems like the pressure is actually low when this happens rather than a sensing issue. It has never failed to instantly recover with the boost pump...

I have a cooling shroud on the mechanical pump but long lines (fire sleeved) and a gascolator (with blast tube) firewall forward so some fuel warming is possible...

Got me...? Bubble in the line upstream of the mechanical pump? Seems fairly common... Would be nice to know...
 
Last edited:
Few things at play here;

1. resolution of the instrument/ sender
2. Carby has a valve/float so flow can be on/off at a fast interval, so pressure rises and falls, your fuel flow is averaged a little otherwise it would rise and fall too.
3. You have winter fuel? Mogas maybe?

What you are seeing is not unusual.

:)
 
So, question,....over in PAM (Siverhawk / RSA fuel injection) info on troubleshooting

There is mention of possible leakage of air,...even though there is NO. FUEL LEAKAGE,...and use of a short piece of clear tubing to take a look with engine running 😳

What caught my eye with respect to OP is the mention that if booster pump bring pressure back up,...it could be an air leak between booster and main pump...

With that,...will sit back and listen to what folks that know about the original subject have to say.
 
Fuel Pressure Head Effects

Few things at play here;

1. resolution of the instrument/ sender
2. Carby has a valve/float so flow can be on/off at a fast interval, so pressure rises and falls, your fuel flow is averaged a little otherwise it would rise and fall too.
3. You have winter fuel? Mogas maybe?

What you are seeing is not unusual.

:)

One thing that I haven?t seen mentioned in all these threads about low fuel pressure is the potential fluid-head pressure effects on the pressure reading. If, for example, you have your fuel pressure transducer mounted high on the fire-wall AND the line to the sender has filled with fuel, you will essentially be seeing a difference in head pressure as measured by the sender from that of the actual fuel pump outlet which is below the sender. So, for example, if the pressure sender is 12 inches above the fuel pump outlet, the sender will measure pressure ~ 0.3 psi below the actual outlet pressure of the pump. This is pretty significant if the pump is only puting out the minimum pressure of 0.5 psi.

Skylor
 
One thing that I haven?t seen mentioned in all these threads about low fuel pressure is the potential fluid-head pressure effects on the pressure reading. If, for example, you have your fuel pressure transducer mounted high on the fire-wall AND the line to the sender has filled with fuel, you will essentially be seeing a difference in head pressure as measured by the sender from that of the actual fuel pump outlet which is below the sender. So, for example, if the pressure sender is 12 inches above the fuel pump outlet, the sender will measure pressure ~ 0.3 psi below the actual outlet pressure of the pump. This is pretty significant if the pump is only puting out the minimum pressure of 0.5 psi.

Skylor

I was wondering the same thing because my transducer is located above the fuel pump. I just hadn't made the calculation.
 
I had a similar problem with a carbureted O-360. The fuel pressure would fluctuate intermittently and trigger a low fuel pressure warning. I never experienced a stumble in the engine due to the low fuel pressure. As a precaution, I changed the fuel pump, then changed the fuel pressure sensor supplied by GRT. My GRT EIS provides the low fuel pressure warning. Neither seemed to help. Then a good friend and fellow formation pilot said that I should upgrade the fuel pressure sensor. GRT offers a higher quality sensor (for a higher quality price!). I installed one of those and got some assistance from GRT to properly set it up with the EIS. End of problem.
 
Thank you for the input. After thoroughly inspecting both the fuel system and fuel pressure sensor I have been flying my RV-8 again. In 2 of the past 5 flights the fuel pressure reading fell to zero again (actually -0.5 psi). Rather than turning on the boost pump I let it be and flew circles over the airport for 15 minutes. While the fuel pressure readings varied from -0.5 psi and about 2 psi, fuel flow and engine operation were unaffected, so clearly there was fuel pressure and the readings were inaccurate. I will be calling GRT this week per Terry's suggestion.
 
Fuel pressure

1000 hrs. O360 fuel pressure reading varies from .5 to 17 a lot, GRT EFIS. Never a stumble in the engine in any configuration. I use the electric pump for TO and landing. Cheap sending unit replaced 600 hrs. Ago, no change.
 
I have seen a similar drop in fuel pressure, but mine has been limited to the ground run after the first start of the day. I would start the engine, warm up to an oil temp of 100 degrees but get a low fuel pressure warning with pressure at .8 psi. during the warm up. I turned on the electric pump and the pressure went to about 5 psi (normal) instantly. After takeoff the pressure stayed at 4.7 psi the entire flight. I am using a Garmin G3X with Kavlico sensors. I do not suspect an indication problem. My pump has about 125 hours since new. It is a Tempest pump. Here is a link to a great YouTube video by Tempest on the operation of the diaphragm pump. https://www.youtube.com/watch?v=PYKoLUsTJU4. I have not been able to find a cause of the drop in pressure yet. The last 4 flights, the problem has not shown up again.
 
I?ve got the same issue, totally random can stay like that for a whole flight, boost pump comes on and the pressure goes up on the gauge, changed out the gauge same thing,

Airplane runs fine considered changing the mechanical pump until I saw this thread and now reliaze it may just be the way it is going to be. Lol
 
Here?s a fuel pressure problem that seems a bit different than those documented in the site archives.

I have a new O-360-A2A (carbureted) engine on my RV-8, first flight was 10 days ago and now have just under 6 hours. In that time the fuel pressure has dropped to near zero 3 times. Details are:

? Each time the fuel pressure dropped to near zero the airplane was in level flight at ~75% power (2,450 RPM, 10.5 GPH with a Catto prop)

? Upon the fuel pressure dropping to 1 PSI the alarm sounded, I turned on the boost pump and the fuel pressure immediately returned to the normal range of ~ 5psi. At no time did the engine falter.

? After about one minute I turned off the boost pump, and the fuel pressure remained normal for the remainder of the flight using only the mechanical fuel pump. This included one flight where I circled over the airport for over 30 minutes to see if it would happen again, it did not.

? Environmental conditions were not stressful, OAT was between 60 and 70 degrees at 5,000 ft MSL.

I did manage to track the event one time. The fuel pressure fell in a linear fashion taking approximately 20 seconds to drop from the normal ~ 5 PSI to 1 PSI where upon I turned on the boost pump.

The mechanical fuel pump is a new Tempest AA15472, I did install a cooling shroud. The fuel sensor gauge is standard GRT LPS-02 low pressure sensor. After the second event I re-purged the fuel line to the sensor, it had no effect as the issue reoccurred the following flight.

Not sure where to go. It appears the fuel pressure sensor is working correctly since it reads normally when the boost pump is on. It also seems odd for a mechanical fuel pump to essentially fail completely and then run normally afterwards. I am hesitant to replace parts without a good reason to explain the events.

I have a message into the engine manufacturer but thought maybe someone in this group may have seen something similar. For now the airplane is grounded.

LYLE:

I hope you've solved your issue by now. If not, here's my recent experience.

(Note: My engine is an O-360-A1A, pressure transducer is from Van's kit and mounted on the firewall)

I started seeing the same low fuel pressure (as in almost zero pressure) issue as you a couple of months ago. Interestingly I had just finished the condition inspection so I wondered if I had disconnected something, but after checking all items I couldn't find any obvious discrepancies.

Like you, the fuel pressure read almost zero on takeoff, and would drop to 1 psi at times during flight and in steep turns. If I turned the electric fuel pump on, the pressure went back to the normal (green) range. With or without the fuel pump on, the engine kept chugging along as usual so it did not appear to be an engine driven or boost pump issue.

After discussing this with a fellow RV-8er, I decided to replace the fuel pressure transducer as it would appear to be the most likely weak link. After 13 years of flying, these parts tend to wear out. In the case of Van's transducer, I believe there's a diaphragm inside (probably a stiff paperlike material or similar) that is continually being stressed while measuring the pressure. The physical displacement o the diaphragm is probably to the same spot 95% of the time the engine is running, so it gets worn. The transducer may give a better (higher) reading during the infrequent times the electric fuel pump is turned on due to flexing of the diaphragm to a different position that is less worn out. I'm certainly no expert on these things but the concept at least makes sense to me.

So I ordered a new transducer from Van ($44 plus shipping). Installation took about ten minutes. So far the change seems to have answered the issue. Everything back to normal (see photos and captions).

I hope the helps or may help someone in the future. It is certainly comforting to see a pressure reading in the green again.

Chris


Old fuel pressure transducer mounted on firewall


New Vans transducer



New transducer mounted below EI oil pressure transducer.


After replacement: level flight in cruise, no electric fuel pump on. The pressure does vary at times with aircraft maneuvering but seemed to stay inside the green arc or just slightly below it at all times. With fuel boost pump on it would read at top of the green arc.
 
Back
Top