Help Troubleshooting Fluctuating Fuel Pressure

[Ragsf15e]

IO-360, A1A:

I need some help troubleshooting my fuel pressure.  My IA is busy with other stuff and not interested in chasing this.  I don’t mind getting my hands dirty, but I only know enough to get myself in trouble…

Fuel flow fluctuates pretty much all the time but more noticeably in full power than in idle.  In idle it’s not really that big a deal, say 28-30psi.  In full power, it bounces 22-30 randomly but pretty constant.  I have a jpi-930.  Jpi says it’s probably not the sender as that’s not how they die but it could be emi from something else or it’s real.  It didn’t use to fluctuate like this.  I searched my flight logs and found where it started last year, in cruise in the middle of a flight.  Before that it was very steady.

Several things I know/have done- this year at annual (2 weeks ago) my IA cleaned/checked the gascolator and the servo fuel screen.  He said they both looked perfect.  The fluctuation happens no matter which tank I select.  Electric boost pump makes no difference.  Mechanical boost pump was replaced 2 years ago because it was leaking oil.  Engine runs good but it’s a 1998 OH with 1200 hours.  I have a Surefly and a standard mag, both P leads seem reasonably shielded.

How can I check the sender without buying a new one?

Should I go through all the fuel fittings and “put a wrench” on them to make sure they’re tight?  I can’t see any leaks and it was just inspected at annual?

How do I find out what is causing this and how to narrow it down?

Id prefer to be out in front of an issue instead of waiting for the engine to stumble on takeoff or worse…

 

[47U]

Is it both fuel pressure and fuel flow that are fluctuating?  Certainly check that all the fittings are tight and hard-lines inspected for corrosion and damage.    

How old are your fuel hoses?  The fuel system in my C is nothing like your F, but I had fluctuating fuel pressure and fuel flow.  In troubleshooting, I discovered the -6 hose on the inlet side of the engine driven fuel pump had failed internally.  It was sucking air, but had no evidence of external leakage.  

The way I found it, I placed the tank selector in “OFF”, then disconnected the hose coming off the engine driven fuel pump and put a couple inches of vacuum on the system.  It leaked down immediately and wouldn’t hold the vacuum.  Working backwards through the system, I discovered the inlet hose to the fuel pump had failed internally.  

I’m not sure if this is a sound troubleshooting technique… someone will chime in, I’m sure.  

Be sure to use a hand-actuated vacuum pump from the local HF or auto parts store.  You don’t want to run fuel fumes through an electric vacuum pump, just say’n.    

[Marauder]

IO-360, A1A:
I need some help troubleshooting my fuel pressure.  My IA is busy with other stuff and not interested in chasing this.  I don’t mind getting my hands dirty, but I only know enough to get myself in trouble…
Fuel flow fluctuates pretty much all the time but more noticeably in full power than in idle.  In idle it’s not really that big a deal, say 28-30psi.  In full power, it bounces 22-30 randomly but pretty constant.  I have a jpi-930.  Jpi says it’s probably not the sender as that’s not how they die but it could be emi from something else or it’s real.  It didn’t use to fluctuate like this.  I searched my flight logs and found where it started last year, in cruise in the middle of a flight.  Before that it was very steady.
Several things I know/have done- this year at annual (2 weeks ago) my IA cleaned/checked the gascolator and the servo fuel screen.  He said they both looked perfect.  The fluctuation happens no matter which tank I select.  Electric boost pump makes no difference.  Mechanical boost pump was replaced 2 years ago because it was leaking oil.  Engine runs good but it’s a 1998 OH with 1200 hours.  I have a Surefly and a standard mag, both P leads seem reasonably shielded.
How can I check the sender without buying a new one?
Should I go through all the fuel fittings and “put a wrench” on them to make sure they’re tight?  I can’t see any leaks and it was just inspected at annual?
How do I find out what is causing this and how to narrow it down?
Id prefer to be out in front of an issue instead of waiting for the engine to stumble on takeoff or worse…
 

There could be a few things involve. If there are no obvious leaks, here are the things I’d consider:

Some sort of restriction in the sender path.

Loose connections on the sender.

Loose connections on the 930 side.

EMI problem. Check the wiring from the sender to the JPI. I had an interference problem with my JPI that caused a radio problem. I don’t think their units are very robust against EMI.

Infant mortality of that mechanical pump.

I can’t tell from the installation manual but I suspect that the sensor probably has an acceptable output range that can be measured. Maybe ask JPI what they recommend for testing these sensors.


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[Ragsf15e]

Thanks guys, that is definitely helpful. I would really like a way to check the sender (without buying a new one), so I’ll talk to jpi about that.  I feel like it’s hard to narrow down the problem which could be:

1.  Wiring for the JPI or EMI from somewhere else.

2.  Sender.

3. Mechanical.

I feel like I need to narrow that down to 1 area before really attacking it.

[Old Chub]

This may be the same on the IO as the O mooneys but my airplane (O-360) has a snubber in the fuel pressure line to help eliminate the pulses from mechanical pump.   In the past my snubber had gotten crud in it and my pressure would be fine then drop to near zero in flight (something to really catch your attention).   You may try cleaning the snubber and see if that is the source of your issue as its an easy one to at least eliminate from the troubleshooting.

 

Warren 

[Ragsf15e]

1 hour ago, Old Chub said:

This may be the same on the IO as the O mooneys but my airplane (O-360) has a snubber in the fuel pressure line to help eliminate the pulses from mechanical pump.   In the past my snubber had gotten crud in it and my pressure would be fine then drop to near zero in flight (something to really catch your attention).   You may try cleaning the snubber and see if that is the source of your issue as its an easy one to at least eliminate from the troubleshooting.

 

Warren 

That is something Ive thought about.  Currently no snubber on that line, but I do have them on MP and Oil Pressure.  I would put one on there, but it started in the middle of a flight, so I might just be covering up a real problem.  I’m definitely not against having a snubber, just don’t want to hide something else…

[jaylw314]

A failing sender can give you fluctuating readings.  When mine went, I had FP that read about 20, and would slowly fluctuate between 15-20 every minute or so.  It wasn't a bad connection, because turning on the fuel pump would increase that to 18-23 psi.  Replacing it fixed the problem.  Unfortunately, that doesn't really narrow down the list of culprits

Edited February 3, 2022 by jaylw314

[Guest]

43 minutes ago, Ragsf15e said:

That is something Ive thought about.  Currently no snubber on that line, but I do have them on MP and Oil Pressure.  I would put one on there, but it started in the middle of a flight, so I might just be covering up a real problem.  I’m definitely not against having a snubber, just don’t want to hide something else…

The Mooney fuel pressure was taken from a special fitting on the aft side of the fuel servo finger screen body.  The fitting is a #4 ORB to a #3 flared fitting with a built in restrictor.  The hose led to the fuel pressure gauge in the cockpit.  
 

Clarence

 

[Ragsf15e]

31 minutes ago, M20Doc said:

The Mooney fuel pressure was taken from a special fitting on the aft side of the fuel servo finger screen body.  The fitting is a #4 ORB to a #3 flared fitting with a built in restrictor.  The hose led to the fuel pressure gauge in the cockpit.  
 

Clarence

 

That seems like exactly where the connection is for my JPI Sender.  There’s a line off the back of the servo routed under 1/3 cylinders to the sender on the firewall.  Sounds like it might already have a built in restrictor?

How do you go about troubleshooting these kinds of things?

I definitely appreciate all your guys inputs.  I’m just trying to build a plan before attacking it with tools…. Also I think I have as much chance of messing with something that isn’t broken while searching blindly for the problem.

[PT20J]

Does it bounce around rapidly or sort of wander up and down?

Is the JPI new and has always done this, or has it suddenly started but was fine before?

Does it do it with the engine off and the boost pump on, or only with the engine running?

Is the fuel flow steady?

I'm having a similar issue with my G3X EIS. The fuel pressure is steady with just the boost pump on and the engine off. However, when the engine is running at cruise power the pressure varies randomly +/- 2 (sometime 5) psi. It's not a really rapid variation, but more of a random wandering. I never noticed this with the factory gauge. The engine only has 300 hours on it with a new fuel pump and servo from the factory. In flight the boost pump only increases the pressure by perhaps 1 psi, but it has no apparent effect on the wandering. The engine runs fine and the fuel flow is steady. I'm leaning to thinking it's a transducer except that it's strange that it works fine when the engine isn't running.

The way to find out for sure is to get an analog pressure gauge and plumb it in place of the transducer. Your A&P may have one you can borrow and some fittings to hook it up.

Skip

[Guest]

1 hour ago, Ragsf15e said:

That seems like exactly where the connection is for my JPI Sender.  There’s a line off the back of the servo routed under 1/3 cylinders to the sender on the firewall.  Sounds like it might already have a built in restrictor?

How do you go about troubleshooting these kinds of things?

I definitely appreciate all your guys inputs.  I’m just trying to build a plan before attacking it with tools…. Also I think I have as much chance of messing with something that isn’t broken while searching blindly for the problem.

I would add a T to the line and a mechanical pressure gauge to confirm that the pressure fluctuates, or if it’s a fault with the JPI.

Clarence

[Ragsf15e]

57 minutes ago, PT20J said:

Does it bounce around rapidly or sort of wander up and down?

Is the JPI new and has always done this, or has it suddenly started but was fine before?

Does it do it with the engine off and the boost pump on, or only with the engine running?

Is the fuel flow steady?

I'm having a similar issue with my G3X EIS. The fuel pressure is steady with just the boost pump on and the engine off. However, when the engine is running at cruise power the pressure varies randomly +/- 2 (sometime 5) psi. It's not a really rapid variation, but more of a random wandering. I never noticed this with the factory gauge. The engine only has 300 hours on it with a new fuel pump and servo from the factory. In flight the boost pump only increases the pressure by perhaps 1 psi, but it has no apparent effect on the wandering. The engine runs fine and the fuel flow is steady. I'm leaning to thinking it's a transducer except that it's strange that it works fine when the engine isn't running.

The way to find out for sure is to get an analog pressure gauge and plumb it in place of the transducer. Your A&P may have one you can borrow and some fittings to hook it up.

Skip

What you describe sounds similar to mine, but mine is more than 2-3psi.  More like 8psi.  It hasn’t always done this.  I’ll have to look closer with the pump on, engine off to see.

Flow does fluctuate very little, like 0.1-0.2gph but I just figured that’s how they are since it’s tiny.

 I guess I’m going to have to find a mechanical transducer and test the pressure as I think I really need to eliminate the sender.

Ill try to post a plot from my edm data.  Maybe ours look similar? Is yours 1” recording?

[PT20J]

Here's a plot of data I pulled from my last flight. The G3X samples every second. This is the first time I've plotted it. It's interesting that as time went on, the peak variation decreased but the frequency increased. Fuel Flow was constant to within 0.1 gph around 9.0.

 

 

 

 

 

 

EDIT: @Ragsf15e, I added an expanded view of the first 156 seconds of the data to give a better idea of the frequency of variation

[carusoam]

Expect real FF oscillations to come with real FP oscillations….

FF oscillations come with air leaks into the system… bubbles seen when doing the baby food jar test….

 

For quirky subtle problem solving… sharing a video of what you are seeing will really be helpful…

a Video is really good at adding a timeframe to the whole thing….

 

If a fuel line liner is going bad… it may be blocking the FP sensor’s line….  It could get worse and block other things down stream…

If you see black bits on any screens downstream… this is probably a sign the inner layer of the fuel line getting old….

 

PP stuff I heard about on MS…

-a-

[Ragsf15e]

16 hours ago, PT20J said:

Here's a plot of data I pulled from my last flight. The G3X samples every second. This is the first time I've plotted it. It's interesting that as time went on, the peak variation decreased but the frequency increased. Fuel Flow was constant to within 0.1 gph around 9.0.

 

 

 

 

 

 

EDIT: @Ragsf15e, I added an expanded view of the first 156 seconds of the data to give a better idea of the frequency of variation

Here's mine on a similar time scale.  It's right after start, but is relatively constant through the flight.  It does gain in amplitude during takeoff, but settles back to this in the climb.  Fuel Flow is about like this, but just barely more fluctuation when it's higher.

 

[A64Pilot]

20 hours ago, M20Doc said:

I would add a T to the line and a mechanical pressure gauge to confirm that the pressure fluctuates, or if it’s a fault with the JPI.

Clarence

Do this, any time you have any kind of indicating problem, first verify the gauge is good before going crazy and tearing things apart and or spending money.

A cheap automotive fuel pressure gauge will suffice as you don’t have to fly it I assume?

[Ragsf15e]

1 hour ago, A64Pilot said:

Do this, any time you have any kind of indicating problem, first verify the gauge is good before going crazy and tearing things apart and or spending money.

A cheap automotive fuel pressure gauge will suffice as you don’t have to fly it I assume?

Agree, Im just having a hard time imagining the cowling off, engine running, mechanical gage in the engine compartment?  Probably gonna be bouncing around just due to the engine running.  

[jaylw314]

23 minutes ago, Ragsf15e said:

Agree, Im just having a hard time imagining the cowling off, engine running, mechanical gage in the engine compartment?  Probably gonna be bouncing around just due to the engine running.  

The electric fuel pump should suffice, at least for starters, right? 

Edited February 4, 2022 by jaylw314

[Ragsf15e]

12 minutes ago, jaylw314 said:

The electric fuel pump should suffice, at least for starters, right? 

Yeah, I gotta check and see if it fluctuates with just the electric pump.  I’m not sure if it does.  Also, if I run that pump for a minute or two won’t i flood the heck out of it?  See this is why I need help, I’m only smart enough to be dangerous!

[carusoam]

The mechanical fuel pump may have some rpm related bumps implied in its output… ranges with rpm that can change each time rpm gets changed…

Testing this on the ground should be interesting….  Turn the pump on, watch the pressure build, turn it off… see what the FP gauge says, for how long before it leaks down…

If it leaks down quickly, expect that fuel is going somewhere…

Typically, pulling the mixture back to ICO… keeps the fuel from going forwards….

In the case of the Ovation, the fuel system is plumbed to allow fuel to return to tank from which it came… by pulling the mixture back.

The throttle position has a lot to do with how much fuel is going to be delivered….

Priming the engine with full throttle or pulled back, delivers a different rate of fuel…

 

FP is a complex relationship between volume of the pump output, and how much restriction is in the line…

When the engine is running there is going to be a lot of variables… especially if a tube is deteriorating…

When the engine is not running… things will be pretty stable… even with the tube fully degraded…

 

PP thoughts about FP measurements…

-a-

[201Mooniac]

1 hour ago, Ragsf15e said:

Agree, Im just having a hard time imagining the cowling off, engine running, mechanical gage in the engine compartment?  Probably gonna be bouncing around just due to the engine running.  

We did this once on a friends plane and plumbed in the gauge with a hose long enough so I could hand it to him with his arm out the pilot side vent window and he could hold it out while running the engine.

[A64Pilot]

1 hour ago, Ragsf15e said:

Agree, Im just having a hard time imagining the cowling off, engine running, mechanical gage in the engine compartment?  Probably gonna be bouncing around just due to the engine running.  

Run the little plastic line out of the oil door and through the storm window and of course have the gauge there, you can use copper line if plastic concerns you, but for a few minutes I think the plastic is fine.

 Back in the day all pressure gauges were “wet” type, meaning the hoses ran into the cockpit and to the gauge, it was common to have small restrictor’s in line to prevent excessive fluid lose in the event of a busted line or gauge, but your doing a temp install and aren’t flying so you don’t need a restrictor etc

Edited February 5, 2022 by A64Pilot

[PT20J]

55 minutes ago, Ragsf15e said:

Yeah, I gotta check and see if it fluctuates with just the electric pump.  I’m not sure if it does.  Also, if I run that pump for a minute or two won’t i flood the heck out of it?  See this is why I need help, I’m only smart enough to be dangerous!

You can run the electric pump without flooding it as long as the mixture is in ICO. Try that first, because it's easy. In my case the problem only occurs when the engine is running -- it's stead on the boost pump.

[Ragsf15e]

21 minutes ago, PT20J said:

You can run the electric pump without flooding it as long as the mixture is in ICO. Try that first, because it's easy. In my case the problem only occurs when the engine is running -- it's stead on the boost pump.

So mine is steady with boost pump on, mixture ico.  However, I think it was bouncing when I put the mixture forward and let it run fuel  (but it was really quick because I didn’t want to flood it too bad).  Video of pressure as BP comes on with mixture in middle position.

https://photos.app.goo.gl/A9c5jKgQDLdTccHw8

 

[PT20J]

6 minutes ago, Ragsf15e said:

So mine is steady with boost pump on, mixture ico.  However, I think it was bouncing when I put the mixture forward and let it run fuel  (but it was really quick because I didn’t want to flood it too bad).  Video of pressure as BP comes on with mixture in middle position.

https://photos.app.goo.gl/A9c5jKgQDLdTccHw8

 

Interesting -- I didn't think to try advancing the mixture with the boost pump on.