REFLECTOR: Odd fuel pressure Problem

[Scott Derrick]

A question for the fuel system guru's and those flying with lycoming 
injected engines.  I have a IO360 with a RSA-5 servo,  standard Lycoming 
fuel pump and a high pressure electric Aux pump, Vision Micro Systems 
Engine Monitor.

Long story.

Last week I got a low fuel pressure warning, first in 350 hours of flying 
this engine.  I was climbing through 8,000 ft full power.  Thats 1500 ft 
above field elevation.  I looked at the Vision and saw 10 PSI on the fuel 
pressure readout.

This pressure is taken from the servo block, on the fuel pump side, not the 
pressure to the spider.  The low redline for the RSA-5 is 14 PSI.

All other indications were normal,  fuel flow, rpm, egt's etc..  Engine 
sounded normal, power didn't change.  I turned on the aux pump and the 
pressure went up to 30 PSI which is lower than what I usually see with the 
aux pump. I started to turn back to the airport.  Turned off the aux and 
the pressure dropped to 24 PSI, in the normal range that I see.  Normally 
or what I have been seeing for the past 3 years, in cruise is a fluctuating 
pressure from 21 to 28, its steady at 26 PSI on the ground or at low rpms.

I decided to continue the climb as this was a test flight for Catto, to get 
ktas speeds and  power settings.  I was going to make a triangular flight 
path above the airport at 12,000 and 17,000 ft.  The pressure started to 
fluctuate like it usually does but then it dropped to 12 PSI.  I just let 
it run like that and the plane ran fine.  It would drop to between 10 and 
12 PSI and then climb back to around 20 PSI. It would repeat this a cycle 
between every 3  to 10 minutes.  The engine performed fine, no other 
indications of a problem.  After finishing the speed runs I descended back 
to the airport, and never got another low fuel warning.

I de cowled and visually inspected the engine. Nothing looked wrong.  Fuel 
filter was clean. No leaks.  I decided to replace the fuel pump. It only 
had 440 hours on it but..  5 hours previous I had the sump out and teh fuel 
system open to the air.  Possibly something got in and damaged the pump?  I 
certainly rate the R&R of the sump as a red flag.  I've lost track of how 
many times, I was the cause of a problem.

Got new pump, installed, flew the airplane for 30 minutes, no low pressure.

Now the bad news..  Trip to Tahoe was planned for Turkey Day, two days 
after test flight.

6am at the airport, loaded up the plane, got in with wife, took off and 
headed west at 8,500, smooth as glass, sun rising behind us,  beautiful.. 
This is why we fly our own planes...  Over Gallup,  20 minutes out, Low 
fuel Pressure Alert!! Damn!!!  There the pressure is at 11 PSI on the 
Vision readout.  I turned around and headed back home.   The pressure 
wandered up and own between 11 and 20 PSI like the old pump.  I guess 
throwing money at the plane didn't work this time.

I'm mad at my plane, wife is mad at GA.  Not going to make Thanksgiving at 
out daughter's/granddaughter's house this year.

We unload at the airport. I leave the plane sitting in front of the hanger, 
I'll be back after it warms up a bit.

I checked all the fuel fittings from tanks to serve. None were really 
loose.  I did tighten a few that were not as tight as I would normally 
tighten then.  I checked the vent lines, all clear.  I checked and cleaned 
the electrical connections at the pressure sensor and the Vision brain box. 
I checked the fuel screen in the servo, it was pristine clean.

I went for another test flight, left back seat out so I could see the fuel 
filter, it has a glass cover over the filter and you can see the fuel 
flowing through it.

Took off and climbed to 8500 towards Gallup like earlier in the morning 
when I got the Alerts.  Set everything the same, 75 LOP,  TAS at about 170 
knots.  Fuel pressure wandered between 22 and 28. Got over Gallup, turned 
around climbed to 13,500 towards Albuquerque.  Flew to Albuquerque, no low 
pressure warning, pressure remained above 21 PSI, flew back towards Grants 
at 12,500 in a leisurely  path, site seeing the pueblos and Mt. Tayler 
.  Everything normal, or as normal as it had ever been.

What was the problem, or had been the problem?  I hate not finding a 
smoking gun..   Was it a loose fitting allowing air into the lines, was it 
an poor connection on the sensor?  I read in the Vision manual after 
getting back home, that the sensor has a vent hole that must be kept clear, 
I didn't see it when I was fooling with the wires, did I accidently knock 
off something that blocked it?   Is the problem still lurking?

Now the decision, do we fly to Tahoe tomorrow, not knowing for sure?  We 
decided no.  I'm going to fly the plane a few times and we will make the 
trip a week late.

Any ideas?  Anybody see this before?

Scott

"Those who sacrifice freedom to get security, deserve neither."
- Benjamin Franklin