Help Troubleshooting Fluctuating Fuel Pressure

[carusoam]

Great video!

Nothing steady about that…

Probably doesn’t tell enough about what you want to know…

Got anything to compare to?

 

My M20C’s FP would rise steadily and stopped near the red line…. And hold there until the pressure bread off slowly….

My M20R doesn’t have an FP gauge…. FF only….

 

Best regards,

-a-

[Ragsf15e]

3 minutes ago, PT20J said:

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

Probably because you didn’t want to flood it!

Im sure my mental model of this is not exactly right, but IF it’s a mechanical issue (ie air bubbles), I keep thinking that they must be coming in prior to the pump because the line after the pump is pressurized and it shouldn’t let air in.  If anything, it would let fuel out.  Before the pump though, I think anything is possible.

After that video, I put it in ico and it held pressure for a long time.  I wiggled the transducer and it’s wires with zero fluctuations.

[carusoam]

Air bubbles have been known to be drawn through leaky seals at the fuel selector valve…

Leaky seals after fuel pressure has been built, show themselves as blue stains externally….

Air bubbles in the line, probably won’t show much of a pressure change, unless or until they are squeezing through a restriction….

Air viscosity is much lower than fuel viscosity so I will flow through a restriction very quickly, and not create the same pressure drop that fuel will…

next steps… setting up a second FP instrument to see what it says, whike the engine goes through preflight, start up, and running…. (?)

This should separate out a gage issue vs. a real issue….

 

PP thoughts only, used to be a viscosity and flow kinda guy… 

-a-

[Guest]

2 hours 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 tyrap the gauge and hose through one of the Camloc receptacles with the face of the gauge aimed toward the pilot’s seat.

Clarence

[PT20J]

14 minutes ago, M20Doc said:

We tyrap the gauge and hose through one of the Camloc receptacles with the face of the gauge aimed toward the pilot’s seat.

Clarence

Clarence, when you've done this, what issues have you found that cause such fluctuations?

[Guest]

3 minutes ago, PT20J said:

Clarence, when you've done this, what issues have you found that cause such fluctuations?

Usually the fitting with the snubber is missing, lost during the overhaul.

Clarence

[Ragsf15e]

1 hour ago, M20Doc said:

Usually the fitting with the snubber is missing, lost during the overhaul.

Clarence

So, it’s an actual fitting between the servo and the line going to the pressure gage?  Like I could take a photo of that connection and we should see it?

[PT20J]

17 minutes ago, Ragsf15e said:

So, it’s an actual fitting between the servo and the line going to the pressure gage?  Like I could take a photo of that connection and we should see it?

The fitting goes on the servo where the line to the pressure transducer attaches. LASAR has pictures on it's website if you have the part number. https://lasar.com/hardwarefittings The M20J IPC list this one which LASAR describes as for all 200 hp models.

 

 

[Marauder]

So, it’s an actual fitting between the servo and the line going to the pressure gage?  Like I could take a photo of that connection and we should see it?

What is troubling me is that your pressure goes above 30 psi. I’ve never seen my pressure ever get anywhere near the max let alone exceed it.

Looking at the installation manual, the resistive sender looks like something you can measure the resistance on. Did JPI ever provide you any troubleshooting guidance? I would think they know what the resistance should read across the working pressure range.



It looks like the transducer for your injected engine is sold through Aircraft Spruce.

https://www.aircraftspruce.com/catalog/inpages/jpifuelpressuresensor.php


Sent from my iPhone using Tapatalk Pro

[Ragsf15e]

8 minutes ago, Marauder said:

What is troubling me is that your pressure goes above 30 psi. I’ve never seen my pressure ever get anywhere near the max let alone exceed it.

Looking at the installation manual, the resistive sender looks like something you can measure the resistance on. Did JPI ever provide you any troubleshooting guidance? I would think they know what the resistance should read across the working pressure range.



It looks like the transducer for your injected engine is sold through Aircraft Spruce.

https://www.aircraftspruce.com/catalog/inpages/jpifuelpressuresensor.php


Sent from my iPhone using Tapatalk Pro

I don’t think the 30psi is an issue. It was mid 20s before I replaced the mech fuel pump.  The new pumps are notoriously strong and sit right at the 30psi limit. There’s a few threads on that.

I agree that sender shows up in the manual, but I think I have a different one with a clip on connector and 3 wires which is almost exactly the same as the oil pressure transducer.  Tough to tell in these pictures with an adel clamp around it.  Maybe it is the one listed?  But the wire connections are definitely through 1 clip.

[carusoam]

Hmmmm…

Something to consider with pressure gauges….

Some Mooney pressure gauges get a vent hole…. In their case…

Unknowing people have covered that hole and got strange pressure readings because of it…

The one I recall was an OilP sensor for the Bravo…. That was made of unobtanium…

Something to consider if swapping out the fuelP sensor…

The vent hole, when there is one, keeps the pressure reading from changing with altitude….

PP thoughts only,

-a-

[PT20J]

49 minutes ago, carusoam said:

Hmmmm…

Something to consider with pressure gauges….

Some Mooney pressure gauges get a vent hole…. In their case…

Unknowing people have covered that hole and got strange pressure readings because of it…

The one I recall was an OilP sensor for the Bravo…. That was made of unobtanium…

Something to consider if swapping out the fuelP sensor…

The vent hole, when there is one, keeps the pressure reading from changing with altitude….

PP thoughts only,

-a-

Transducers can measure absolute pressure (like manifold pressure) or gauge pressure (pressure above ambient like oil and fuel pressure). The latter need a vent to the atmosphere. It would be worth checking that the mounting hardware doesn’t obscure the vent.

Skip

 

 

[Ragsf15e]

1 hour ago, Marauder said:

What is troubling me is that your pressure goes above 30 psi. I’ve never seen my pressure ever get anywhere near the max let alone exceed it.

Looking at the installation manual, the resistive sender looks like something you can measure the resistance on. Did JPI ever provide you any troubleshooting guidance? I would think they know what the resistance should read across the working pressure range.



It looks like the transducer for your injected engine is sold through Aircraft Spruce.

https://www.aircraftspruce.com/catalog/inpages/jpifuelpressuresensor.php


Sent from my iPhone using Tapatalk Pro

I knew my sender looked different… the page you added is from a 2008 manual. I have a newer sender.  It’s $423 at Chief!  Let’s hope it’s a loose fitting!

Below is an updated manual with the sender I have:

 

[Ragsf15e]

24 minutes ago, PT20J said:

Transducers can measure absolute pressure (like manifold pressure) or gauge pressure (pressure above ambient like oil and fuel pressure). The latter need a vent to the atmosphere. It would be worth checking that the mounting hardware doesn’t obscure the vent.

Skip

 

 

So you guys are saying there’s a tiny “breather” hole in this somewhere?

(picture from Chief)

[PT20J]

There has to be a vent somewhere. The OEM Kulite transducer Mooney used on my plane had an obvious small hole in the case. I don’t know where it is on your transducer or the Kavlico transducer Garmin uses for the G3X since I didn’t install it. 

[carusoam]

Some times the vent hole is indicated by a line on the body of the transducer….  Just to help avoid accidentally covering the vent hole…

Unfortunately, I don’t know if the FuelP sensor here uses one….

PP thoughts as usual…

-a-

[Ragsf15e]

On 2/4/2022 at 7:39 PM, PT20J said:

The fitting goes on the servo where the line to the pressure transducer attaches. LASAR has pictures on it's website if you have the part number. https://lasar.com/hardwarefittings The M20J IPC list this one which LASAR describes as for all 200 hp models.

 

 

Tough to say if it’s that exact part with the restrictor or just an elbow, but there’s something like that at the back of the servo.  Sorry, pipes were too hot to get a good picture!

 

 

[PT20J]

On 2/4/2022 at 4:58 PM, 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

 

I tried this today and mine is steady with only the boost pump whether the mixture is at ICO or full rich. (And, as a bonus, I have now proved that the sniffle valve is unrestricted )

Skip

[Guest]

7 minutes ago, Ragsf15e said:

Tough to say if it’s that exact part with the restrictor or just an elbow, but there’s something like that at the back of the servo.  Sorry, pipes were too hot to get a good picture!

 

 

From your first picture, the fitting is the one between the servo and the oil sump.  The Mooney fitting is a #4 or 7/16-20 thread which screws into the servo body and the hose is -3 flared fitting and is supposed to be restricted.

Clarence

[Ragsf15e]

UNEXPECTED TESTING OUTCOME:

I’ve been flying the airplane while reading about how to troubleshoot the pressure issue.  I’ve talked to the folks at Tempest (fuel pump) and JPI (edm sensors).  Well today I got together with my mechanic for step 1 and there was an unexpected outcome.  As everyone said, we had to verify the gage, so we T’d in a line with a nice electric pressure gage and about 6 feet of hose so I could hold it in the cockpit while doing a runup. I thought either both gages would fluctuate (problem is real) as mine always does, or mine would fluctuate and the new one wouldn’t (my gage/sender is bad).  Nope, option 3 happened, both were rock solid and in agreement.  Theory is that the long hose dampened the fluctuations.

So, we reassembled with a snubber on the sender and I went flying.  Of course, right when I starter the engine, it was fluctuating just like before.

 I did learn that my gage is accurate and it’s not the sender or an electrical issue. Unfortunately I think it’s pulling air in somewhere and it seems to be hard to find.  Videos to follow…

[carusoam]

Check for the procedure on how to leave some headspace in front of the sensor…

If you have a snubber…. It should have air passing in and out of the hole…

I would invite @Bob_Belville to stop by and discuss snubbers….

Unfortunately, he can’t stop by, but his writing about it may still live on….  
 

I think JPI told him a snubber wouldn’t work…. Or worse, it wasn’t needed… Which was an invite for the EE inside of Bob to find out how to make it work…

Snubbers have a calibrated hole… it can only be calibrated for one viscosity going through it… air and fuel are much different viscosities…  expect air to be the traditional source for our need…

When buying a snubber… you get to select the hole dimension…

PP thoughts only, not an instrument guru…

Best regards,

-a-

[Ragsf15e]

10 hours ago, carusoam said:

Check for the procedure on how to leave some headspace in front of the sensor…

If you have a snubber…. It should have air passing in and out of the hole…

I would invite @Bob_Belville to stop by and discuss snubbers….

Unfortunately, he can’t stop by, but his writing about it may still live on….  
 

I think JPI told him a snubber wouldn’t work…. Or worse, it wasn’t needed… Which was an invite for the EE inside of Bob to find out how to make it work…

Snubbers have a calibrated hole… it can only be calibrated for one viscosity going through it… air and fuel are much different viscosities…  expect air to be the traditional source for our need…

When buying a snubber… you get to select the hole dimension…

PP thoughts only, not an instrument guru…

Best regards,

-a-

I agree with the viscosity issues on choosing the correct snubber. I’ve been using snubbers from Omega that come in different specs based on fluid type… unfortunately there’s not one specifically for 100ll.  The one I used is for water or light oil.  It was one step up from the one for air.  Unfortunately, the viscosity of 100ll is between the specs for the two choices.  
After seeing the gage steady while testing with a long damper, I’m still convinced the system is sucking air somewhere.

[Ragsf15e]

Here’s video yesterday during testing at ~runup rpm.  It’s worse during takeoff and cruise power/fuel flows.  Fuel pressure is top right on the EDM930.

before testing 

https://photos.app.goo.gl/dpPBA8iMU5JNh4a58

 

During test with another gage T’d in

https://photos.app.goo.gl/2gtD9nfHLXaFinkC8
 

[PT20J]

I'm pretty sure this has to do with air in the line to the transducer. Mine does something similar. In addition, after shutdown with a hot engine and the mixture in ICO, the fuel pressure will rise until it is off scale. Since there is no mechanical pump running, this can only be due to air in the line expanding as the temperature within the cowling rises due to a hot engine and no cooling airflow.

I don't remember seeing the needle wavering on the factory gauge after we installed the new engine, After putting in the G3X with EIS, I noticed this happening. I had a small seep at the new fuel pressure transducer, and when I fixed it I ran the boost pump to bleed all the air out of the line and this didn't help the fluctuations. Thinking back, when we installed the new engine we didn't bleed out the air, so the entire fuel system from the boost pump on was dry. Now I'm thinking that it might be best to keep the line from the servo to the transducer mostly filled with air. This might explain what happened with your test. If the long line to the digital gauge was not full of fuel it may have acted as a snubber. 

I spoke with Alan Jesmer at Precision Airmotive (makers of the RSA injectors) and he pointed out that it is futile to try to keep the air out of the line because when the fuel gets hot it will form bubbles of vapor, and since the line has no flow there is no place for the vapor to go.

Skip

[Ragsf15e]

1 hour ago, PT20J said:

I'm pretty sure this has to do with air in the line to the transducer. Mine does something similar. In addition, after shutdown with a hot engine and the mixture in ICO, the fuel pressure will rise until it is off scale. Since there is no mechanical pump running, this can only be due to air in the line expanding as the temperature within the cowling rises due to a hot engine and no cooling airflow.

I don't remember seeing the needle wavering on the factory gauge after we installed the new engine, After putting in the G3X with EIS, I noticed this happening. I had a small seep at the new fuel pressure transducer, and when I fixed it I ran the boost pump to bleed all the air out of the line and this didn't help the fluctuations. Thinking back, when we installed the new engine we didn't bleed out the air, so the entire fuel system from the boost pump on was dry. Now I'm thinking that it might be best to keep the line from the servo to the transducer mostly filled with air. This might explain what happened with your test. If the long line to the digital gauge was not full of fuel it may have acted as a snubber. 

I spoke with Alan Jesmer at Precision Airmotive (makers of the RSA injectors) and he pointed out that it is futile to try to keep the air out of the line because when the fuel gets hot it will form bubbles of vapor, and since the line has no flow there is no place for the vapor to go.

Skip

Do you think it shows up on our electric senders but not on the old analog ones just because they’re more sensitive?

I must say, it’s annoying to see it jumping around so much during takeoff. It’s hard to get a last quick look for gages steady and green when that one is jumping all over the place.