Fuel pressure fluctuating

[Dialed In]

Good afternoon guys, I have been having a fuel pressure fluctuation for a bit now and was looking for some insite. A little background on my plane it is a 65 E with the a1a engine and 2180 hrs on the rebuild and fuel system.  A few months ago I noticed a fluctuation in the fuel pressure that happens at all altitudes, any fuel level, any power level, on either tank. Seems about every twenty minutes or so the pressure would drop from 25 psi to 18 and fluctuate then return to 25 and be steady.  The fuel pressure would leak down after shut down to zero in about fifteen minutes and I did have a bit of a fuel drip out of one of the tubes coming from the bottom of the cowling although from what I’ve read that is normal?  The engine does not stumble or lose power when this happens there are no other indications other then the gauge. Since the mechanical fuel pump had not been changed since the rebuild I had a service centre change it and was very pleased with my experience there however this problem still exists.  There are no other leaks that have been found elsewhere in the fuel system so I was thinking of having the fuel selector gone through what do you think?   
 

-Chase

[chriscalandro]

Does the boost pump make a difference?  When was the last time you checked the fuel screen?

[Dialed In]

The boost pump brings it back up to normal pressure.  I have not ever checked the fuel screen however the last annual was very extensive it was not a service center but done by an IA with Mooney experience however I don’t know that they specifically checked that screen.  I am mechanically inclined and am going up tomorrow to change the oil how difficult is it to check the screen and is it considered preventive maintenance?  

[chriscalandro]

Remove the panel On the bottom near the gear door, it should be near the boost pump. 
 

Seems like an odd failure for the mechanical pump. 

[David Lloyd]

If you get fuel dripping from the fuel pump drain, the fuel side diaphragm is leaking.  That also would cause erratic or low pressure.  Oil dripping from the fuel pump drain, the oil side diaphragm is leaking. Either one, replace the pump.  Fuel dripping from the manifold drain unless excessive is normal.

[M20F-1968]

What are you using for fuel gauges, the original gauges or electronic?  

I have a JPI 930 and the fuel pressure fluctuated arbitrarily for some time.  The problem was solved by installing a snubber just before the fuel sensor.

The manufacturer is Chemiquip and the snubber has a porosity 40 microns which is designed for light oils.

John Breda

[skykrawler]

First...highly recommend downloading and printing a maintenance manual.  Learn more about the fuel system on your airplane - could save your life.

One of the hoses out the cowl flap is from the sniffle valve on the bottom of the oil pan that drains fuel from the intake plenum when it accumulates from flooding or leak down after shutdown.   The other is from the mechanical pump.   So knowing which one the fuel is coming from is important.

Since you are changing the oil check the wire attachment to the fuel pressure transducer.  A poor connection there (high vibration environment under the cowl) causes change in resistance and fluctuating indications.  From the movie it seems to be at the high end before it drops.

Before you disassemble your airplane..... look in the logbook and/or call the IA that did your annual and ASK which screens they checked.  There is the selector screen (gascolator) and the fuel servo input screen.  Of course garbage could have been introduced since the annual.   The boost pump draws and pushes fuel through the same screens as the mechanical pump.

[Dialed In]

Thanks for the suggestions guys, I do have a service manual circa 1980 that I think is the last rendition for my plane but IMO it is lacking in the fuel system area.  I went through my airframe log book again and they did service the fuel strainer after the dukes pump at annual however nothing was said about the gascolator. I went out and ran the plane up then changed the oil, while under there I pulled the panel and removed the fuel strainer after the dukes pump and it was clean as a whistle.  I installed a new oring and put it back together, the anodized fitting on the exit of the strainer had been relatively loose not hand tight but close so I hoped this was the issue.  Once checking for fuel leaks, running up the engine and checking for oil leaks, then putting the cowlings back on I went for a flight in the pattern and thought I had it licked because it did not act up once in the 30 minutes I was up there.  I landed checked everything over again then went up and flew for an hour and fuel issue came back.  It happened twice while I was up there pretty much identical to what the video shows with the new pump installed.  After flight I get an area on the floor about the size of a quarter where the fuel drips out of the sniffle valve nothing else leaks externally.  I'm going to have my A&P look at it when he can I'm still thinking of having him go through the gascolator but is there anything else easy I could check before that?  I always assumed the original fuel pressure gauge I have was mechanical but I did not look under the dash today to make sure and I can't find anything about it in the manual is it electric?  If so where is the traducer located so I could check the wire?  Is there a chance it could be in the fuel servo even though there is no noticeable change in engine performance?  A JPI 900 is on the list but I have been getting some of these other issues sorted while trying to save up for it BTW sorry for the book.

[Yetti]

Leak down test seems reasonable, but I would say your mechanical pump is going south.   If you have the MP and Fuel pressure then it is mechanical.  

[carusoam]

Fuel dripping out of the right place is OK... sniffle valve...

Fuel dripping out the wrong places is not OK... fuel pump vents...

Chase, you did a nice job describing what you know!

Original Fuel Pressure gauge is mechanical... the fuel gets delivered to the instrument in the panel...

 

Expect that the FP is telling something important... failed mechanical instruments are typically on/off...

Might be time for a fuel system OH(?).

 

See if @jetdriven is around... Byron knows IO360fuel systems pretty well...

@M20Doc is also a pretty good resource for Mooney fuel systems...

PP thoughts only not a mechanic...

Best regards,

-a-

[chriscalandro]

Sounds like the next step is the mechanical pump. 
was hoping maybe there was something in the screen causing it. Seems a bit odd it’s intermittent, but I don’t know the inner workings of the unit. 

[Dialed In]

Mechanical pump was replaced at the service center last week that is why it is now reading 30 psi they told me is was a sb by lycoming and that is the pump that should be on there. I know I’ve read on here about the new pumps being higher pressure but they assured me it was the way it should be. Anyway the video is after the replacement on the way home, before the replacement it was 25 psi steady dropping to around 18. 

[N201MKTurbo]

Blow out your hose between the servo and the firewall. Make sure it is not full of fuel. Take out your gauge and point the ports down and see if any fuel drips out. If it does, set your compressor to 25 PSI and pressurize the gauge with the ports pointing down. Then remove the pressure and see if any fuel is sprayed out. Do this until no fuel comes out. Then let it set ports down for a day or so.

I had this problem and found that the bellows in the gauge was full of fuel. It finally fixed the problem.

[chriscalandro]

A lot of times new things are the most broken things. I’d follow the compressor suggestion then move on to the mechanical pump. Even if it’s new. 

[Guest]

Your fuel pressure gauge is a mechanical one, not electric.  The fuel pressure pick up is on the fuel injection servo finger screen body between the servo and the oil sump.

When the fuel pressure gauge develops a heart beat it is usually the pressure line filling with fuel.  The gauge shows the pump pulses, blowing the fuel out as Rich says will cure this.  

Another thing to check is the electric fuel pump outlet filter.  Remove the left fairing by the gear door to expose the electric fuel pump.  On the outlet side you likely have an Airmaze filter, many people don’t know it’s there and never clean it.  
It’s required by AD 80-13-03 and SBM20-222B.  https://cdn2.hubspot.net/hubfs/4147179/technical_documents/service_bulletins/sbm20-222b-1.pdf

Clarence

 

[Dialed In]

I appreciate the insite guys, I will check that when I get back next week and let you know. 
-Chase

[romeotango]

If you do check the filter on the fuel selector valve, be sure to note the following:

https://cdn2.hubspot.net/hubfs/4147179/technical_documents/service_bulletins/sbm20-200-1.pdf

 

 

[carusoam]

+1 on verify gauge accuracy...

+1 on draining the mechanical gauge first...  As mechanical gauges misbehave when no longer set-up properly...

there is probably a small calibrated hole called a snubber in the line near the gauge that is supposed to smooth out the readings.....

it needs to be dirt and liquid free...

if the liquid gets all the way to the gauge, this buffer starts to mis-behave...

Some examples of a similar challenge in even more modern gauges...

https://mooneyspace.com/search/?q=JPI pressure oscillation&updated_after=any&sortby=relevancy&search_and_or=and

So have your mechanic make sure your pressure gauge is working properly first, or T in a reliable gauge...

PP thoughts only, not a mechanic...

Best regards,

-a-

[Dialed In]

I got the Mooney into the shop three weeks ago and my A&P let me tackle it with his oversight.  I did what N201MKTurbo said and removed the line between the servo and the fire wall, I blew it out and sure enough there was a decent amount of fuel in it.  I then removed the line from the gauge to the fire wall and blew through it and there was fuel all the way up there.  I finished up with the gauge and let it dry.  Once it was back together everything works as it should I wanted to put some hours on it before replying to make sure it was repaired which it is, thanks a bunch guys.

-Chase

[PT20J]

Proof again that when the engine's running fine, but a gauge reads funny, check the gauge first.

[carusoam]

Great follow-up,  Chase!

Go MS!

Best regards,

-a-

[Ragsf15e]

On 2/23/2020 at 8:10 PM, N201MKTurbo said:

Blow out your hose between the servo and the firewall. Make sure it is not full of fuel. Take out your gauge and point the ports down and see if any fuel drips out. If it does, set your compressor to 25 PSI and pressurize the gauge with the ports pointing down. Then remove the pressure and see if any fuel is sprayed out. Do this until no fuel comes out. Then let it set ports down for a day or so.

I had this problem and found that the bellows in the gauge was full of fuel. It finally fixed the problem.

Ok, I gotta ask because I thought this was supposed to be the other way around, and I’m having fuel and oil pressure fluctuations in my jpi…

Are you saying there’s supposed to be air in the line leading to the fuel pressure (and oil pressure) transducer?  I have the jpi electrical (and super/overly sensitive) transducers and I’m getting jumpy indications.  The oil pressure line has no air in it at all.  Not sure about the fuel line.

How are they supposed to be?

[carusoam]

If the sensor has a snubber in front of it...

Expect air passing in and out of the snubber’s in-line hole to filter out the noise in the line...

If you don’t have a snubber... you are getting no filter for mechanical noise in the line...

@Bob_Belville had some discussions with this and JPI back in the day...

JPI started out with a snubber isn’t needed... Bob was an engineer, and knows most pressure gauges for non-steady flows have plenty of noise oscillations...   snubbers filter out the noise...

Best regards,

-a-

[Ragsf15e]

12 hours ago, carusoam said:

If the sensor has a snubber in front of it...

Expect air passing in and out of the snubber’s in-line hole to filter out the noise in the line...

If you don’t have a snubber... you are getting no filter for mechanical noise in the line...

@Bob_Belville had some discussions with this and JPI back in the day...

JPI started out with a snubber isn’t needed... Bob was an engineer, and knows most pressure gauges for non-steady flows have plenty of noise oscillations...   snubbers filter out the noise...

Best regards,

-a-

Yes, I’ve been reading through his old posts.  I think there’s also some differences in the mechanical gage discussed in this thread vs the electronic transducer in my jpi.

[Ragsf15e]

On 2/23/2020 at 4:29 AM, M20F-1968 said:

What are you using for fuel gauges, the original gauges or electronic?  

I have a JPI 930 and the fuel pressure fluctuated arbitrarily for some time.  The problem was solved by installing a snubber just before the fuel sensor.

The manufacturer is Chemiquip and the snubber has a porosity 40 microns which is designed for light oils.

John Breda

Can you point me to the exact snubber you used?  I am looking for a fuel pressure one as well since my jpi 930 has some erratic indications.  I see one for air and one for light oil and water.  Not one for fuel?  Jpi had steered me toward Omega snubbers but said Chemiquip also had them.