Fuel pressure fluctuating

RussR

RussR

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Russ
1976 Piper PA-28-151, "Warrior", 150 hp O-320, carb'ed

Last couple of flights the fuel pressure gauge has been varying. This is something I don't usually stare at, so maybe it's done it before, but as far as I remember it has pretty much always stayed right on the "5" psi mark.

Here's the symptoms (with the electric pump off):
Climb - pressure drops to about 3-4
Level flight - pressure stays about 4
Descent - pressure builds to about 5
Takeoff run without the electric pump on (for testing) - drops to about 3

In all cases, turning the electric fuel pump on brings it right back to 5 psi. Note that the "green" range on the gauge is .5 to 8.0 psi, so I'm still well within that.

I have checked some pictures of past flights, and it seems to always be at 5.

The engine otherwise is running perfectly.

Only change that I can tell from prior to a few flights ago is that I've started running mogas now that I'm somewhere you can get it without ethanol. But I did that for years before with no problems that I recall. And I'm keeping it outside on the ramp (in the heat) for now. Vapor pressure?

Any idea what to look at? Fuel pump? Problem in the lines? I just moved here so I haven't found a mechanic yet, but I guess I'll have to soon!

I did read that this is a common problem with O-320s in RVs. Not sure if that's related.

Thanks!
 
#1 check fuel vents.

I have a high wing with a carbed o320 and two fuel pumps, pressure was wacky when a tank vent became plugged.
 
As Charlie says, it sounds normal. More throttle uses more fuel and the delivery pressure will fall as flow increases.

Dan
 
Well, maybe it is normal then, I just never noticed it before. As I said, typically when I look down it's been at 5, I think.

Would a failing fuel pump show any symptoms like this, or do they just work until they fail and pressure drops to zero?

I will check my vents and lines and such tonight.
 
Do not assume it's normal. Just because the plane can fly on .5 PSI and you have more doesn't mean a sudden drop of fuel pressure from what it used to be is not an indication that something isn't amiss.
 
RussR said:
Well, maybe it is normal then, I just never noticed it before. As I said, typically when I look down it's been at 5, I think.

Would a failing fuel pump show any symptoms like this, or do they just work until they fail and pressure drops to zero?

I will check my vents and lines and such tonight.
Click to expand...

It will vary depending on engine demand. If you see it very low in flight and it usually runs higher, there is a good chance there is a venting problem. If one tank is pluggled you'll see a very low pressure when feeding from that tank and normal pressure when you switch to a different one.

If you're seeing ~3 on all tanks, there is no problem.

My pressure runs ~7 at idle and ~3 in flight.
 
Pipers also have to lift the fuel, air leaks on the suction side of the fuel system can make things go wonky too.

But it doesn't sound to me like that's an issue.
 
There should be a vent line from the fuel pump that comes out the bottom of the cowl somewhere. If you see gas or engine oil coming from the line it means one of the two diaphragms has failed in the pump. Just one other failure mode to be aware of.


Vent is the 90 degree blue elbow on the far left and there is no fitting screwed into the vent in the pump on the right

 
Bug nest in the fuel pump vent!

Cleaned it out and the pressure seems to be behaving much more normally.

Thanks everybody!
 
:thumbsup:

Glad it was simple. I replaced two fuel pumps last year, not fun. Safety wiring the bolts just sucks. Getting the LONG bolts started isn't any better.

 
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