REFLECTOR: Odd fuel pressure Problem

[John Dibble]

I have a carburated franlkin, so this may or may not help.  I have had fluctuating
fuel pressure on my RMI monitor.  In my case the normal pressure is 7 psig and I have
confirmed this was a pressure gauge.  When the ground is bad, the pressure can
fluctuate from 10-25 psig.  Since your pressure is low, perhaps it is the other wire
that is faulty, but that is just a guess as I'm not an expert on how the electronics
work.  I would suggest running new wires to the sensor and see if things change.

John

Scott Derrick wrote:

> 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
>
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