Fuel pressure fluctuations / rough engine

[TShort]

On my return leg home from CO, KAEJ->KUMP, I developed fuel pressure fluctuations that intermittently caused slight engine roughness.
This after solving the oil pressure reading problem on the outbound leg (https://vansairforce.net/threads/low-oil-pressure-reading.229707/).

I have a stock lycoming IO-540-D4A5 from Van's. Just under 500 hours since first flight in 2017, but the engine was delivered well before that (I am not the builder, will have to check records but I think the engine was built/delivered around 2009-2010).

Took off on the R tank. I have the G3X set to remind me every 10 gallons. Typically I turn on the boost pump, switch tanks, then boost off. I switched to the left tank using this procedure, and when the boost pump was turned off FP dropped from 24psi to 14-15psi. I turned the boost back on, pressure came back up. There was a brief period of roughness after a slight delay. Turned the boost pump off, pressure then fluctuated a lot for quite some time (I can post the data), occasionally down into the teens, and then gradually stabilized at 23.1-23.2 with intermittent small drops.

FP did the same with subsequent tank switches, and continued to fluctuate throughout the flight though it was less than after the initial episode.

Back home I pulled the cowl. Externally the fuel pump looks fine. There is no fuel or blue staining from the drain tube or on the belly of the airplane.

This one is not a sensor issue; the engine ran rough, and the FP normalized with boost pump on.

I am looking for suggestions for troubleshooting and what to look for as a cause. Is this a symptom of a failing mech fuel pump?

Thanks in advance.

T.

 

[Avanza]

Normal fuel pressure is 20-25 psi.
Fuel pressure is important alltough the engine should run without faltering on 10 psi. I would inspect all fuel filters including the one on servo.
Does the fuel line have a fire sleeve?
When you turn the fuel pump ON
the fuel pressure increases and air bubbles will implode. When the pump is turned OFF the pressure will be lower and the margin to air bubbles in the fuel line is reduced. All interference with the
fuel flow may cause disruption in fuel pressure and flow. This is especially true
if there are a lot of 90 deg fittings and
no air cooling to the fuel pump.

Good luck

 

[Freemasm]

Will add to the above.

It may not be two phase fuel but rather air in the fuel. Even with no blue staining, I'd still check your lines/connections down stream of the boost pump as a first suspect. While probably not a contributor, I'd also check the left lines from tank to boost pump. The post BP section rarely sees positive pressure so lack of staining wouldn't necessarily be a surprise. Additionally, and you can get "one way" leaks where air gets in under a slight vacuum e.g. boost pump off .

Easy additional checks. Best of luck.

 

[BoydBirchler]

On my return leg home from CO, KAEJ->KUMP, I developed fuel pressure fluctuations that intermittently caused slight engine roughness.
This after solving the oil pressure reading problem on the outbound leg (https://vansairforce.net/threads/low-oil-pressure-reading.229707/).

I have a stock lycoming IO-540-D4A5 from Van's. Just under 500 hours since first flight in 2017, but the engine was delivered well before that (I am not the builder, will have to check records but I think the engine was built/delivered around 2009-2010).

Took off on the R tank. I have the G3X set to remind me every 10 gallons. Typically I turn on the boost pump, switch tanks, then boost off. I switched to the left tank using this procedure, and when the boost pump was turned off FP dropped from 24psi to 14-15psi. I turned the boost back on, pressure came back up. There was a brief period of roughness after a slight delay. Turned the boost pump off, pressure then fluctuated a lot for quite some time (I can post the data), occasionally down into the teens, and then gradually stabilized at 23.1-23.2 with intermittent small drops.

FP did the same with subsequent tank switches, and continued to fluctuate throughout the flight though it was less than after the initial episode.

Back home I pulled the cowl. Externally the fuel pump looks fine. There is no fuel or blue staining from the drain tube or on the belly of the airplane.

This one is not a sensor issue; the engine ran rough, and the FP normalized with boost pump on.

I am looking for suggestions for troubleshooting and what to look for as a cause. Is this a symptom of a failing mech fuel pump?

Thanks in advance.

T.

Make sure the tank breather is not partially blocked (bug next has happened to me on more than one occasion ).

 

[Walt]

Normal fuel pressure is 20-25 psi.
Fuel pressure is important alltough the engine should run without faltering on 10 psi. I would inspect all fuel filters including the one on servo.

Lycoming fuel pressure specs are 14-45 psi, however anything below 18 may cause issues at full power.
It might idle at 10psi but that's about it.

 

[TShort]

Lycoming fuel pressure specs are 14-45 psi, however anything below 18 may cause issues at full power.
It might idle at 10psi but that's about it.

I was at 13,500', WOT 2450 RPM LOP and I think 55% power roughly.
When it dropped to 14-15psi it didn't shake like a wet dog or shut down, but there was a noticeable change.

 

[srothert]

On my return leg home from CO, KAEJ->KUMP, I developed fuel pressure fluctuations that intermittently caused slight engine roughness.
This after solving the oil pressure reading problem on the outbound leg (https://vansairforce.net/threads/low-oil-pressure-reading.229707/).

I have a stock lycoming IO-540-D4A5 from Van's. Just under 500 hours since first flight in 2017, but the engine was delivered well before that (I am not the builder, will have to check records but I think the engine was built/delivered around 2009-2010).

Took off on the R tank. I have the G3X set to remind me every 10 gallons. Typically I turn on the boost pump, switch tanks, then boost off. I switched to the left tank using this procedure, and when the boost pump was turned off FP dropped from 24psi to 14-15psi. I turned the boost back on, pressure came back up. There was a brief period of roughness after a slight delay. Turned the boost pump off, pressure then fluctuated a lot for quite some time (I can post the data), occasionally down into the teens, and then gradually stabilized at 23.1-23.2 with intermittent small drops.

FP did the same with subsequent tank switches, and continued to fluctuate throughout the flight though it was less than after the initial episode.

Back home I pulled the cowl. Externally the fuel pump looks fine. There is no fuel or blue staining from the drain tube or on the belly of the airplane.

This one is not a sensor issue; the engine ran rough, and the FP normalized with boost pump on.

I am looking for suggestions for troubleshooting and what to look for as a cause. Is this a symptom of a failing mech fuel pump?

Thanks in advance.

T.

Do you happen to have one of those "one way valves", connecting the oil breather to the exhaust system? If so, disconnect the breather hose from the one way valve. Do a runup on the ramp. Observe fuel pressure, and if acceptable, do some flight testing close to your home airport.

I know of an instance where a "one way valve" was added to an existing, flying aircraft with a Lycoming 0-320. Prior to the addition of the "one way valve", the engine would start without an electric fuel pump. It idled normally and would develop full static RPM. After the installation of the "one way valve", the engine would not start or idle, unless the electric fuel pump was on, all the time!!!

 

[TShort]

Do you happen to have one of those "one way valves", connecting the oil breather to the exhaust system? If so, disconnect the breather hose from the one way valve. Do a runup on the ramp. Observe fuel pressure, and if acceptable, do some flight testing close to your home airport.

I know of an instance where a "one way valve" was added to an existing, flying aircraft with a Lycoming 0-320. Prior to the addition of the "one way valve", the engine would start without an electric fuel pump. It idled normally and would develop full static RPM. After the installation of the "one way valve", the engine would not start or idle, unless the electric fuel pump was on, all the time!!!

I do not; the crankcase breather is vented to the lower cowl. I connect my engine dryer to it at shutdown, I am sure there is no valve in it. Appreciate the input!

 

[kiwipete]

As mentioned above it’s worth checking all the fuel lines. I had a slightly loose fitting near the tank which never showed any leaks but I suspect it caused a similar problem with fuel fluctuations until I found it.
Also there’s quite a bit of debate over the fuel flow (red cube or similar) location. If it’s fitted in the tunnel there’s some evidence to show this may cause fluctuations.

regards Peter

 

[airguy]

Do you happen to have one of those "one way valves", connecting the oil breather to the exhaust system? If so, disconnect the breather hose from the one way valve. Do a runup on the ramp. Observe fuel pressure, and if acceptable, do some flight testing close to your home airport.

I know of an instance where a "one way valve" was added to an existing, flying aircraft with a Lycoming 0-320. Prior to the addition of the "one way valve", the engine would start without an electric fuel pump. It idled normally and would develop full static RPM. After the installation of the "one way valve", the engine would not start or idle, unless the electric fuel pump was on, all the time!!!

Since those two systems have nothing in common with each other, that symptom points to the installer disconnecting something to gain access in the FWF area, and not putting it back together correctly. This is how those "Old Wives Tales" get started. Please don't be that guy.

 

[TShort]

As mentioned above it’s worth checking all the fuel lines. I had a slightly loose fitting near the tank which never showed any leaks but I suspect it caused a similar problem with fuel fluctuations until I found it.
Also there’s quite a bit of debate over the fuel flow (red cube or similar) location. If it’s fitted in the tunnel there’s some evidence to show this may cause fluctuations.

regards Peter

Thanks for the info.
I moved the red cube from its original location in the tunnel to the FWF area, between the servo and spider. Did this several years ago due to FP fluctuation issues at the time, no issues since (until now).

In terms of checking all the fuel lines ... if there is a tiny air leak, or a leak only under suction without fuel staining or leakage, does this amount to just putting a wrench on all the fittings and ensuring they are tight? How have you guys tracked down issues like this? There has been no recent maintenance or changes to the airplane, I have flown 40 hours or so since the condition inspection in April.

 

[Freemasm]

Thanks for the info.
I moved the red cube from its original location in the tunnel to the FWF area, between the servo and spider. Did this several years ago due to FP fluctuation issues at the time, no issues since (until now).

In terms of checking all the fuel lines ... if there is a tiny air leak, or a leak only under suction without fuel staining or leakage, does this amount to just putting a wrench on all the fittings and ensuring they are tight? How have you guys tracked down issues like this? There has been no recent maintenance or changes to the airplane, I have flown 40 hours or so since the condition inspection in April.

Basically yes assuming nothing is cracked. Give everything a good-shop-practice sanity check; snug fittings being a first check. Short, straight tube runs can induce stresses. Components supported by tubing vs being firmly, mechanically anchored, cause material fatigue and eventual cracking, filter is free from fowling, etc.