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Fuel pressure drop

High_Flyer

Well Known Member
Hi,

I have a RV-7 with a Aerosport IO-375-M1S engine.
Lately I noticed that my fuel pressure drops erratically when above about 8-9000 Ft. I saw as low as 10 Psi. When I switch the boost pump on fuel press goes back to normal, boost pump off fuel press drops again erratically.
This happens much frequently on hot days...
Here is a graph of the fuel press agains Pressure alt:
FPRESS_PALT.jpg


The positive peaks are when switching the boost pump on.

I suspect vapor lock, but I'd like more opinion.
What could be done to avoid this?

Thanks,
Alain.
 
Alain---as with most of these issues, its generally an instrumentation/sender/wiring connection issue. You are taking fuel pressure and converting it to an electronic signal so the EFIS can read it. As we ask most of our clients that ask us about this, we ask them How do they KNOW its a real pressure issue, or instrumentation? We have them plumb in a mechanical pressure gauge along with the electrical sender to read BOTH gauges at the same time.
If they read the same, then obviously there is a real delivery issue. If the mechanical gauge reads normal pressure, then suspect the sender or the connections. It may take some extra time and effort to verify, but sure beats banging your head against the wall when you spend a bunch of $$ on a new pump or other components and find that you have the same problem.
Test---dont guess.

Tom
 
Hi,

I have a RV-7 with a Aerosport IO-375-M1S engine.
Lately I noticed that my fuel pressure drops erratically when above about 8-9000 Ft. I saw as low as 10 Psi. When I switch the boost pump on fuel press goes back to normal, boost pump off fuel press drops again erratically.
This happens much frequently on hot days...
Here is a graph of the fuel press agains Pressure alt:
FPRESS_PALT.jpg


The positive peaks are when switching the boost pump on.

I suspect vapor lock, but I'd like more opinion.
What could be done to avoid this?

Thanks,
Alain.

i have been flying my RV-8 for almost 9 years and I got same indications as yours, pretty sure is vapor lock. Anyway I stoped turning on the electrical pump every time the pressure drops, specially because I have enough altitude when this happens, but I always have an eye over fuel pressure during cruise:D
 
Thanks for the replies.

Alain---as with most of these issues, its generally an instrumentation/sender/wiring connection issue. You are taking fuel pressure and converting it to an electronic signal so the EFIS can read it. As we ask most of our clients that ask us about this, we ask them How do they KNOW its a real pressure issue, or instrumentation? We have them plumb in a mechanical pressure gauge along with the electrical sender to read BOTH gauges at the same time.
If they read the same, then obviously there is a real delivery issue. If the mechanical gauge reads normal pressure, then suspect the sender or the connections. It may take some extra time and effort to verify, but sure beats banging your head against the wall when you spend a bunch of $$ on a new pump or other components and find that you have the same problem.
Test---dont guess.

Tom

I don't think it's only an indication/sensor problem because on the most extreme fuel pressure drops, which is not in the graph I posted here, the engine RPM drops slightly as well, and the EGT/CHT also changes (less fuel flow), and as soon as I siwtch the boost pump ON everything, including the indication, gets back to normal.

As with AirbusPilot another RV-8 owner here in France told me he has the same problem above 5000 Ft, although much worse than me with his engine almost quitting when the fuel pressure drops... Boost pump ON and all back to normal.

I'll keep monitoring and troubleshooting. Maybe if gets worse I'll do the mechanical pressure gage test.

Thanks
 
Fuel System Restrictions

Thanks for the replies.



I don't think it's only an indication/sensor problem because on the most extreme fuel pressure drops, which is not in the graph I posted here, the engine RPM drops slightly as well, and the EGT/CHT also changes (less fuel flow), and as soon as I siwtch the boost pump ON everything, including the indication, gets back to normal.

As with AirbusPilot another RV-8 owner here in France told me he has the same problem above 5000 Ft, although much worse than me with his engine almost quitting when the fuel pressure drops... Boost pump ON and all back to normal.

I'll keep monitoring and troubleshooting. Maybe if gets worse I'll do the mechanical pressure gage test.

Thanks

There are a number of things that could contribute to this behavior. If you can answer several questions below, it might help identify the root cause and possible solutions.

1. What kind of fuel are you using?
2. Do you have a fuel flow meter in your system, and if so where is it located in relation to the mechanical and electric fuel pumps?
3. Are your fuel lines insulated and protected from exhaust manifold heat?
4. Do you have a cooling shroud on your mechanical fuel pump?
5. Have you checked for evidence of very small leaks upstream of the mechanical fuel pump (these can draw in small amounts of air when the engine is running)?

Skylor
RV-8
 
Check you fuel filters - if you have one partially plugged up, this behavior will get worse.
 
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