Temporary Low fuel pressure fluctuations

[ArtVandelay]

On 1 leg of a 5000+ nm cross country trip, I had the fuel pressure fluctuating down to 14 which is below minimum pressure. It wouldn’t stay there for more a second and then it recover, so wasn’t able to check if boost pump would help. After about 2 hours it stop and never re-occur again.

Any ideas?

Yes, I drained the both tanks during preflight and found no water as expected since there was no rain.

[mike_elliott]

I had a fluctuating fuel pressure that dropped with the lowering of DA. New mechanical pump was the fix, probably pinhole in the diaphragm. IO360 like yours 

[N201MKTurbo]

At cruise, if the pressure actually drops that low the engine will stumble.

The later models with the electric gauges, have been known to do that sort of thing.

[Z W]

I had a fuel pressure gauge that would occasionally drop to 0. Sure got your attention, but the engine never faltered. New sensor, new fittings, checked the connections, no more problems.

[ArtVandelay]

At cruise, if the pressure actually drops that low the engine will stumble.
The later models with the electric gauges, have been known to do that sort of thing.

It did stumble a little, as I said the low pressure was just for a second.
I have a JPI, so this wasn’t a gauge issue.


You can see it gets worse and then gets better. The next several flights at the same altitude and essentially the same DA were fine. The pressure always fluctuates a little but never this bad, this is the next flight on the following day:

[Bartman]

I'm no expert, but if this were my plane, I would replace that mechanical fuel pump before the next flight.

[jcolgan]

how long has it been since the fuel system was last opened? Like the gasculator being cleaned and inspected during annual.

Could it possibly be some air that was entrained somewhere in the system shaking loose and going through the pump.?

[tony]

the next time this happens, switch tanks, see if it does the same thing on both tanks.  Then try the boost pump if its on a single tank.  Was it altitude dependent?  It could be a clogged vent.

[ArtVandelay]

the next time this happens, switch tanks, see if it does the same thing on both tanks.  Then try the boost pump if its on a single tank.  Was it altitude dependent?  It could be a clogged vent.

I did, no difference between tanks.

Not altitude dependent, it stopped doing it on its own at level altitude after about 30 minutes, I would a clogged vent to just keep getting worse. I will have to check the vents anyway.

[201Steve]

Mechanical gauge or electric transducer?

[ArtVandelay]

how long has it been since the fuel system was last opened? Like the gasculator being cleaned and inspected during annual.
Could it possibly be some air that was entrained somewhere in the system shaking loose and going through the pump.?

The engine was overhauled in 2019, and now has 500+ hours on it.
The old fuel pump ran at a lower pressure, and fluctuated by only 2 psi.
The new fuel pump runs at a higher pressure, even exceeding the 30 psi limit during the break in and always has had more fluctuations, 4-5 psi.

The fact that it stops seems to imply something like air/debris/something else that was in the fuel.

[ArtVandelay]

Mechanical gauge or electric transducer?

JPI.

[tony]

2 hours ago, ArtVandelay said:

I did, no difference between tanks.

Not altitude dependent, it stopped doing it on its own at level altitude after about 30 minutes, I would a clogged vent to just keep getting worse. I will have to check the vents anyway.

Have you ever taken the gascolator apart and checked the screen?  Its a pain because you'll need to quickly cap the gas lines.

[PT20J]

I see this on my M20J with a G3X/EIS. Usually it looks like the second plot varying a lot but not going below about 20 psi. Once or twice, I've had it look like your first plot dipping down below 14 psi. Sometimes it will be varying and then decide to get calm for a while and then start varying again. My factory rebuilt was installed in 2018 and has about 540 hours on it. Switching tanks, turning on the boost pump, changing altitude has no effect. @kortopates says he's seen this a lot with M20Js with Garmin EIS but not so much with JPI which he believes to have a better smoothing algorithm.

I've checked the system for leaks, had the boost pump overhauled (it was beginning to make noise), replaced the o-rings in the gascolator. None had any effect. If I aim a heat gun at the fuel pump (engine off) it doesn't take much to cause the fuel pressure to rise. Pilots on vansairforce agree that this is common on IO-360s and that the engine driven pump heating the fuel is the source of vapor. I tried filling the hose to the pressure transducer with fuel but after a flight it is dry again, so I know the fuel is vaporizing in the lines.

[tony]

2 hours ago, ArtVandelay said:

The engine was overhauled in 2019, and now has 500+ hours on it.
The old fuel pump ran at a lower pressure, and fluctuated by only 2 psi.
The new fuel pump runs at a higher pressure, even exceeding the 30 psi limit during the break in and always has had more fluctuations, 4-5 psi.

The fact that it stops seems to imply something like air/debris/something else that was in the fuel.

The mechanic fuel pump runs at 30 PSI?  Is that the correct pump?  I think mine runs at 24 and with the boost pump at 27

[tony]

7 minutes ago, PT20J said:

I see this on my M20J with a G3X/EIS. Usually it looks like the second plot varying a lot but not going below about 20 psi. Once or twice, I've had it look like your first plot dipping down below 14 psi. Sometimes it will be varying and then decide to get calm for a while and then start varying again. My factory rebuilt was installed in 2018 and has about 540 hours on it. Switching tanks, turning on the boost pump, changing altitude has no effect. @kortopates says he's seen this a lot with M20Js with Garmin EIS but not so much with JPI which he believes to have a better smoothing algorithm.

I've checked the system for leaks, had the boost pump overhauled (it was beginning to make noise), replaced the o-rings in the gascolator. None had any effect. If I aim a heat gun at the fuel pump (engine off) it doesn't take much to cause the fuel pressure to rise. Pilots on vansairforce agree that this is common on IO-360s and that the engine driven pump heating the fuel is the source of vapor. I tried filling the hose to the pressure transducer with fuel but after a flight it is dry again, so I know the fuel is vaporizing in the lines.

Skip, if your theory is correct have you ever considered putting your fuel line in a fire shield to see if it fixes it?

[PT20J]

1 minute ago, tony said:

The mechanic fuel pump runs at 30 PSI?  Is that the correct pump?  I think mine runs at 24 and with the boost pump at 27

There was a period where Lycoming was shipping pumps that ran high. The servo, according to Precision Airmotive has a range of 20-50 psi, so it runs fine but it exceeded the Mooney redline and caused engine monitors to alarm driving everyone nuts. They must have solved this by late 2018 because my engine came with a fuel pump that puts out about 25 psi.

[PT20J]

2 minutes ago, tony said:

Skip, if your theory is correct have you ever considered putting your fuel line in a fire shield to see if it fixes it?

All the fuel lines forward of the firewall (including the line to the pressure transducer) have molded silicone fire sleeves and the fuel flow transducer is wrapped in fire sleeve material. Aiming the heat gun at them had no effect.

[tony]

17 minutes ago, PT20J said:

There was a period where Lycoming was shipping pumps that ran high. The servo, according to Precision Airmotive has a range of 20-50 psi, so it runs fine but it exceeded the Mooney redline and caused engine monitors to alarm driving everyone nuts. They must have solved this by late 2018 because my engine came with a fuel pump that puts out about 25 psi.

if the mechanic fuel pump runs a higher pressure than the electric boost pump, how can you check if the boost pump is working?

[PT20J]

21 minutes ago, tony said:

if the mechanic fuel pump runs a higher pressure than the electric boost pump, how can you check if the boost pump is working?

It's under my feet - I can hear it when it is running.

[ArtVandelay]

There was a period where Lycoming was shipping pumps that ran high. The servo, according to Precision Airmotive has a range of 20-50 psi, so it runs fine but it exceeded the Mooney redline and caused engine monitors to alarm driving everyone nuts. They must have solved this by late 2018 because my engine came with a fuel pump that puts out about 25 psi.

Mine started high, but after about 20 hours it settled down, peaking around 26 psi. My overhaul was done e/o 2018.

[ArtVandelay]

Have you ever taken the gascolator apart and checked the screen?  Its a pain because you'll need to quickly cap the gas lines.

Yes, it was leaking a few years back, I had it overhauled.

[ArtVandelay]

I see this on my M20J with a G3X/EIS. Usually it looks like the second plot varying a lot but not going below about 20 psi. Once or twice, I've had it look like your first plot dipping down below 14 psi. Sometimes it will be varying and then decide to get calm for a while and then start varying again. My factory rebuilt was installed in 2018 and has about 540 hours on it. Switching tanks, turning on the boost pump, changing altitude has no effect. [mention=7862]kortopates[/mention] says he's seen this a lot with M20Js with Garmin EIS but not so much with JPI which he believes to have a better smoothing algorithm.
I've checked the system for leaks, had the boost pump overhauled (it was beginning to make noise), replaced the o-rings in the gascolator. None had any effect. If I aim a heat gun at the fuel pump (engine off) it doesn't take much to cause the fuel pressure to rise. Pilots on vansairforce agree that this is common on IO-360s and that the engine driven pump heating the fuel is the source of vapor. I tried filling the hose to the pressure transducer with fuel but after a flight it is dry again, so I know the fuel is vaporizing in the lines.

I would guess that the higher you are, the more the fuel will vaporize? I tend to fly above 8000’ when flying long distances.

[EricJ]

4 hours ago, tony said:

if the mechanic fuel pump runs a higher pressure than the electric boost pump, how can you check if the boost pump is working?

When you turn the boost pump on to prime prior to engine start, you can see how much pressure it is making.

My fuel pressure drops a couple psi when I turn the boost pump off during climbout.

[gabez]

for me was the fuel vapor check valve. the vapor couldn't go back into the tanks and the pump eventually couldn't push fuel. High fluctuations same graph as urs.