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_Gu...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. :o

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.