PA-28 Fuel Pressure

[Victorian]

Has anyone experienced an issue with Warrior fuel pressure dropping to a low, but still well in the green, level for a few mins and then recovering to normal without apparent cause? This low pressure is instantly rectified by the electric pump and occurs in any stage of flight including taxying. Fitting a new mechanical pump doesn't fix it. Niether does changing tanks in flight.

I appreciate that many sites say "It's normal, live with it" but it's not normal in this aircraft. If it's caused by a restriction someplace, why would the fuel pressure settle at a steady, lower, level regardless of power setting and recover of it's own accord? Obviously the fuel lines, gascolator etc. have been checked for any contamination, which there isn't.

Any ideas welcome.

[Dave Gittins]

Never known such a problem .. but my eyes aren't usually glued to the fuel pressure gauge.

Is this a matter of idle curiousity or are you suffering real puckering of the sphincter ?

With such a safety critical part of the kit I'd be inclined to ask the maintenance organisation for a view.

No doubt there'll be lots of thoughts along in a minute

[Victorian]

It was a joke. Deleted.

[mad_jock]

I suspect its sucking air in from somewhere.

The transducers come off a T piece and then go up to the sensor so any air goes up and has no way to escape and compressibility gives false readings.

An engineer will just put the fuel pump on and slacken off the transducer couple until fuel comes out and that's the air gone and then tighten it up.

If it clears for a week or two then starts again you have air getting in somewhere.

Hopefully a spanner will come along soon and confirm or deny my theory.

[PA28181]

From my time in engineering, the first rule always was check the simple things first. You appear to have changed the mech fuel pump and not cured it, the fuel lines seem ok as you have changed tanks and not cured it. It may well be the fuel pressure sensor that is at fault, as Sherlock says, once you have eliminated etc,etc,etc.

You say there is no problem when the elec pump is on, does this not show there isn't any restrictions in the fuel supply lines?? Without looking at the drawings, how far away from the pump/s is the pressure sensor?

POST crossed with previous, didn't realise there is a possibility of air getting in to the pressurised fuel system, never heard of that problem before and I looked after two PA28140's for many years.

[Victorian]

Thanks for the replies. The whole setup is on the engine side of the firewall. There's no transducer or sensor, just a t-piece right at the carburettor which feeds a Bourdon type gauge. The gauge behaves impeccably, rising when the boost pump is on to the correct value, etc. We also know that when the gauge indicates zero, the engine stops.

[PA28181]

Well, as the gauge is the only means of checking the pressure maybe the gauge is becoming U/S?

As for a "correct" value is this not dependent on ambient temperatures & pressures. Probably why there is quite a large "Green" area that is almost at the bottom end of the gauge?

[Big Pistons Forever]

First off it is good to see a pilot paying attention to what the engine gauges are reading. Too many pilots just blunder along never looking at any engine gauge except for maybe, briefly, at the runup.

The first thing I do when a mechanical gauge is doing something weird is tap the face of the instrument. There have been numerous instances where I have seen a good rap makes the gauge stand up and pay attention :)

Assuming this is not the problem then I would look at the fuel filter to see if there was any crud in it and finally you might have to change out the fuel sender assembly and see if the problem persists.

[mad_jock]

try cracking the nut that joins the gauge to the line with the fuel pump on and see if there is any air up it.

[cockney steve]

wild guess here....assuming the pump is camshaft-operated.........worn camshaft lobe...by swapping the pump, you've eliminated operating-lever, spring, diaphragm, valves......so that leaves the operating part of the engine and the actual pipework......there is an outside chance the fueltank is not venting properly (partial blockage) but i'd expect the tank to "oildrum" * when the boost pump is running as well.

*the "boinging" noise as the tank sides flex in, due to fuel being sucked out and insufficient air getting in to replace it.

[Dave Wilson]

The first thing I do when a mechanical gauge is doing something weird is tap the face of the instrument. There have been numerous instances where I have seen a good rap makes the gauge stand up and pay attention

Cessna fuel gauges usually respond well to percussive rectification. I'm one of those oddballs that includes the engine dials in my scan and I can't say I've ever noticed any fuel pressure weirdness. I don't know what you are flying but I fly the 161 version. I would always suspect a dial problem first although obviously having said that if it was fuel, oil pressure or temp then get on the ground whether you think it's a dial or not and sort it out with your feet on the grass.

[Victorian]

First off it is good to see a pilot paying attention to what the engine gauges are reading. . Thanks for the kind remark BPF. I was surprised I'd noticed as well, but perhaps I'm more tuned in than usual having also had an oil pressure failure (in a different aircraft) this year as well.

It is a -161. It's had at least one fuel pump failure in the time I've owned it, that was an obvious one way street and otherwise the pressure has been rock solid until now. I suspect the issue might be more widespread than generally recognised, based on the number of forum threads that describe similar symptoms and then write it off as 'normal behaviour'.

[flarepilot]

percusive rectification...that's a good one
long ago I learned not to hit the face, but near the face on the panel, in that way you don't get grease from your hand/fingers on the glass and over eager folks don't break the glass/plastic.

and why don't you write to piper directly and ask them? they might know

[Dave Wilson]

percusive rectification...that's a good one

It's an old RAF term meaning 'If it's stuck whack it with a hammer'..:)

[A and C]

As stated above this could be the start of a mechanical fuel pump failure, it is almost always the rubber diaphragm that fails.

The tell tail drain should be inspected for leaking fuel, I suspect that if it is leaking this is the way that air is getting into the system when the engine is not running.

[flarepilot]

I know of one case in which the gascolator drain was touching the cowling in such a way that when the cowling vibrated, it opened the gascolator reducing fuel flow

so on preflight, check the gascolator valve (near nose where you drain fuel)

good one dave wilson.

they gave the mercury astronauts a small hammer just before launch and if something did not work they could give it a whack. they all made it.

so I give you a round of body percussion.

(clapping)

[stevef]

If it came into the hangar, I'd be inclined to carry out a fuel flow check through all tank selections before anything else. That would prove the plumbing. Approximately an hour's labour.

[N5555W]

Yep, my buddy was flying our Cherokee along, hot day in Arizona but high up. He wasn't paying attention to the fuel gauge, and the engine shut off. Luckily he was high up. Turned on the electric booster pump, and the engine started right back up. We'd had the fuel pump replaced about 15 flight hours earlier, because we were getting no fuel pressure from it. Everything seemed to work great until this new incident. Lately we've had problems when we shut the mechanical fuel pump off. Sometimes the pressure drops, sometimes it doesn't. Checked the tank vents. They're all clear (you can blow into the bottom vent and get air out the top with no resistance). But lately this problems seems to be related to altitude. If you fly lower than about 4000', no problem. But up above that, consistently a problem. This really has got us stumped. Anybody got any ideas why this would be related to altitude when the vents seem to work okay?

[MrAverage]

How can you "shut the mechanical pump off" ?

[ChickenHouse]

Thanks for the replies. The whole setup is on the engine side of the firewall. There's no transducer or sensor, just a t-piece right at the carburettor which feeds a Bourdon type gauge. The gauge behaves impeccably, rising when the boost pump is on to the correct value, etc. We also know that when the gauge indicates zero, the engine stops.
Speculating
1. gauge is slowly going to die?
2. pump worn, when is it lifetime due?
3. leaking fuel or sucking air?
Setup on the engine side of the firewall only means you have to mount an action cam to forward firewall for in flight inspection ...

[Victorian]

Going back to 2013:

The problem went away after changing both (perfectly serviceable) electric and mechanical pumps. It was never explained, and has not recurred in 1000 hours flying since. My opinion is that the PA 28 fuel system is fundamentally flawed from design onwards and susceptible to minor variations in component behaviour.

My further opinion is that there are many engine failure incidents in PA28’s where a conveniently undetectable cause (carb icing) is attributed when the actual cause is never identified. I’ve never seen the slightest suggestion of carb ice in best part of 2000 Hrs of PA28-161.

I should say say that my loss of pressure never went anywhere the 0.5 psi minimum and certainly didn’t cause any sphincter puckering. But it is something you notice during a long sea crossing!

[Ebbie 2003]

I saw this in my Archer II late last year, pressure on the mechanical pump dropped to about half its normal position. Applied the electru0ic pump and it went to normal, after a minute ir so, electric pump off and it stayed in the normal position - never happened since. Was at 6,500 half way direct Barbados to Canefield, Dominica, 2,200 rpm, 14C OAT, full tanks at Barbados, so likely maybe 40 gallons at the point I saw the "problem".

[jmmoric]

Yeah, happens in our PA28 from time to time as well, rarely though.

I see it as one of the oddities of a mechanical clock... so as long as it stays in the green, and engine is not loosing power or cylinder temperature is not changing.... I'm happy.

I'm one of the scanning guys.... even do it in my car now.... damn training.... ruined the relaxation of driving.

[B2N2]

Being an electric gauge and a $2 one at that I’d be suspicious of a bad ground or a bad wire somewhere.
Especially after MX or an engine swap or rebuild or gauge replacement.
Easy to nick a 40 year old wire somewhere.

[Victorian]

Being an electric gauge .

If it was an electric gauge I'd be with you 100%. But it isn't, it's mechanical. And it's continued working OK for 8 yrs and 1000 Hrs to date.

[B2N2]

If it was an electric gauge I'd be with you 100%. But it isn't, it's mechanical. And it's continued working OK for 8 yrs and 1000 Hrs to date.

Its one of those where the fuel line actually runs into the cockpit through the firewall?

:=

( Can’t remember the specifics of the PA28)

[Pilot DAR]

I had a progressively lowering fuel pressure on my O-360. After a lot of troubleshooting, it turned out to be a failing sender. The senders are not intended to be mounted hard to anything that vibrates (like and engine), they wear. They should be remoted to a non/less vibrating mounting with a flex line. I found when I disassembled my failing sender, the wiper which runs on the rheostat wire coil had worn so much that it had a much large effective contact width, and it shorted out a number of the rheostat wires, causing a reading which was false, and changing as the wiper wore more. It would still make an electrical connection, and send the signal for a pressure, but it was inconsistent and wrong.

Try changing out the sender. If you can mount it remotely, that will help preserve it.