Jump to content

another fuel pressure question


Recommended Posts

18 hours ago, marcusku said:

I still haven't gotten to the bottom of this problem.  I've replaced the gascolator o rings and the fuel selector o rings, inspected the fuel lines going to both pumps.

To recap, I occasionally get fuel pressure fluctuations from 25 psi down to 22 psi.  Also, sometimes fuel pressure is slow to build after start up, other times it snaps right up.  I have a two gauges and they seem to agree.  Am I chasing a problem that doesn't exist or is this not normal?  I've also observed that a slight reduction in throttle during the decent will make the pressure come back up.

Are the two gauges completely independent including the transducer? The slow rising pressure on start up sounds like an instrumentation problem since it doesn’t always happen. Also, the fuel flow at idle is low and should result in high pressure.

The pump is designed to produce sufficient flow as required by the engine and regulated by the servo. The pressure is not regulated - it’s whatever it needs to be. But it shouldn’t fluctuate if the fuel flow doesn’t change. 
Here’s a good video on fuel pumps:

Also, from the Tempestplus website FAQ:

Q: What is causing my fuel pressure to fall below the “green arc”?
A: The most common reasons for low fuel flow/pressure are the following:
1. A worn or broken O-ring on the inlet side fuel fitting can cause an inlet suction leak. An inlet air leak will result in low Fuel Pump output pressure.
2. A weak operating spring in the Fuel Pump diaphragm assembly;
3. A worn fuel pressure gauge;
4. A clogged fuel vent or fuel cap;
5. A constriction in a fuel line either externally (a kinked hose), or internally (due to a collapsed hose lining).
6. Wrong fuel pump for the installation.

Q: What is causing my fuel pressure to fluctuate?
A: Fluctuating fuel pressure can be the result of the following:
1. A worn fuel pressure gauge;
2. A worn or broken O-ring on the inlet side fuel fitting;
3. A defective fuel selector valve (boost pump may labor);
4. An obstruction in the induction system (air intake);
5. A clogged or dirty fuel strainer;
6. Foreign material inside the fuel body (such as fuel cell sealant, thread sealant, or a drill shaving).

Skip

  • Thanks 1
Link to comment
Share on other sites

Yes the two gauges use separate transducers.

I haven't checked that fitting.  Would I be looking for a leak or blockage with the hole?

I've also noticed that fuel flow seems to be higher when it's fluctuating--about 10.6 gal/hr instead of 10.0 gal/hr when the pressure is reading higher and not fluctuating (same power settings). 

Link to comment
Share on other sites

3 hours ago, marcusku said:

If I choose to send in the fuel servo to get looked at, what's good place to do so?

I assume this is a specialty thing that most shops wouldn't do?

Many reputable engine shops are authorized repair stations for the servos.   Just ask around your area.   Mine went to Aircraft Engine Specialists in Chandler, AZ, but they're local to me.   

Link to comment
Share on other sites

  I sent my servo to Aircraft Accessories of Oklahoma in Tulsa, (918) 835-9924. They said it had internal leakage. It was not cheap to overhaul (surprise!).

  Unfortunately, it didn't fix the problem. The gauge still read 5-8 psi. I spent about $5k running this problem down, including replacing the mech pump, which is very expensive because the mags and vacuum pump have to be removed.

So, what was the actual problem? The FP sending unit, which costs $50 at Aircraft Spruce, was bad and would give low or fluctuating readings.  Now it reads 24 psi except when at idle.

  What did I learn from this? Start by working through things that are easiest and cheapest to do, like replacing the sending unit. Also, some A&Ps are not all that knowledgeable when it comes to the IO-360's injection system and will do normal troubleshooting, e.g. checking for clogged lines, bad fuel pump, etc. That will cost a you over a grand right there.

  If the servo is leaking you will see blue stains in the inlet, which should not be there since the venturi is for measuring air volume not dispensing fuel. If you have questions call Aircraft Accessories, they know way more than your average A&P about servos.

 

 

 

 

 

  • Like 1
Link to comment
Share on other sites

There are two clues that point to an indication problem:

1. The engine runs just fine from idle to full power, and 

2. The pressure is sometimes slow to build after start when the fuel flow is low and thus the demand on the pump is minimal so the pressure should be the greatest.

That's why Clarence suggested checking the restrictor because it would be common both your instruments and if it had some debris in it intermittently restricting the small hole, that might be a problem.

Before tearing into the servo, I'd talk to Al Jesmer at Precision Airmotive and get his thoughts.

Skip

Link to comment
Share on other sites

  • 2 months later...

I had the restrictor checked and cleaned but I'm still seeing the fluctuation.   They did say that I had a bit of leakage at a couple of the injectors.  They are GAMI injectors, could injectors be causing anything?  Any other ideas?

Link to comment
Share on other sites

41 minutes ago, marcusku said:

I had the restrictor checked and cleaned but I'm still seeing the fluctuation.   They did say that I had a bit of leakage at a couple of the injectors.  They are GAMI injectors, could injectors be causing anything?  Any other ideas?

Not likely; the servo controls the flow and sends metered fuel to the divider that distributes it to the injectors. If an injector clogs, its fuel just goes to the other three. If an injector leaks, that fuel just doesn’t get to the engine.

Skip

Link to comment
Share on other sites

  The fluctuation sure sounds like my problem, which was a bad sending unit. It's cheaper to replace the sending unit than doing anything else. I promise you that once you start troubleshooting servos and mech pumps you're in for many wasted AMUs.

  You can buy the sending unit from Aircraft Spruce, $50 - $150, depending on paperwork; it's Mitchell either way. Replacing is quick and easy, though you probably should replace the wiring while you're at it. The unit's on the right side next to the fuel servo.

John W

 

Link to comment
Share on other sites

I haven't noticed anything on how the engine runs, full throttle or idle.  I haven't noticed any blue stains around the servo.  EGT in cruise is generally in the low 1400's at 24" & 2400 rpm running around 50 degrees rich of peak.  When it fluctuates the lowest I've seen is 19-20 psi when it normally about 25.  At idle its around 27-28 psi.  It doesn't drop much on runup though does some under full throttle as you'd expect.  When it's fluctuating flow goes up to 10.6 gallons/min when it's normally 10.0.  Sometimes pulling a tiny bit of throttle out will make it stop and the pressure go up.

Link to comment
Share on other sites

Have you cleaned the servo strainer? It's very, very fine. A&Ps often use Hoppe's to dissolve any lead accumulations. Your readings are much better than I saw with the bad FP sender.  It might be the bellows on the servo. If it's been years since OH or has a lot of hours you might consider that route, though you might want to talk with a Bendix tech about your symptoms. There are some o-rings that can cause internal leakage that may not be visible at first glance.

But, at least all your readings are in the green.

Link to comment
Share on other sites

A while ago I added a Mooney split fuel pressure/ manifold pressure gauge to my Comanche. Wouldn’t you know that my fuel pressure wanders too.  Something that I was unaware of until now.  I’ve chosen to send both my fuel servo and fuel pump for overhaul.

Clarence

Link to comment
Share on other sites

On 1/5/2021 at 6:15 PM, M20Doc said:

A while ago I added a Mooney split fuel pressure/ manifold pressure gauge to my Comanche. Wouldn’t you know that my fuel pressure wanders too.  Something that I was unaware of until now.  I’ve chosen to send both my fuel servo and fuel pump for overhaul.

Clarence

Hello Clarence,

Any idea what might be causing this (see video)? My fuel pressure gauge seems to keep time with the engine RPM. The higher the RPM the faster the needle oscillates back and forth. At times the needle is a blur even!  I have cleared out the fuel line from the gauge to the servo like you suggested in another thread by disconnecting both ends and blowing it out with compressed air. The problem is still there. Engine is the IO360 and it a pure mechanical gauge with a hose running from the gauge to the fuel servo.

Thanks! 

Michael

 

 

Link to comment
Share on other sites

1 hour ago, 75_M20F said:

Hello Clarence,

Any idea what might be causing this (see video)? My fuel pressure gauge seems to keep time with the engine RPM. The higher the RPM the faster the needle oscillates back and forth. At times the needle is a blur even!  I have cleared out the fuel line from the gauge to the servo like you suggested in another thread by disconnecting both ends and blowing it out with compressed air. The problem is still there. Engine is the IO360 and it a pure mechanical gauge with a hose running from the gauge to the fuel servo.

Thanks! 

Michael

 

 

The fuel pressure fitting on the fuel injection servo is supposed to have a very small restrictor in it.  Check this fitting.  It is on the finger screen body between the servo and the oil sump, a bit of a challenge to get to/out.

Clarence

Link to comment
Share on other sites

2 hours ago, M20Doc said:

The fuel pressure fitting on the fuel injection servo is supposed to have a very small restrictor in it.  Check this fitting.  It is on the finger screen body between the servo and the oil sump, a bit of a challenge to get to/out.

Clarence

Thanks Clarence,

 

Do these restrictors ever go bad and need replacing?

 

Edited by 75_M20F
Link to comment
Share on other sites

28 minutes ago, 75_M20F said:

Thanks Clarence,

 

Do these restrictors ever go bad and need replacing?

 

Sometimes they get lost/misplaced during overhaul.  Then replaced with a non-restricted version.

Clarence

Edited by M20Doc
Link to comment
Share on other sites

17 hours ago, M20Doc said:

Sometimes they get lost/misplaced during overhaul.  Then replaced with a non-restricted version.

Clarence

 

I really appreciate your input Mooney Doctor!

It has been 4 years since the engine overhaul and the fuel pressure has been a okay the entire time up until recently (so I don't think it is a restrictor issue + I cleaned the fuel line out with compressed air).

Now recently the fuel pressure has starting pulsating from startup to shutdown.

Do you think it could be the mechanical fuel pump? The electric fuel pump does not cause any pulsating to the gauge, but once I start the engine the pulsing starts up.

 

Thank you

 

Link to comment
Share on other sites

Diaphragm pump vs vanes...

RPM dependent oscillations... expect the diaphragms that get mechanically pushed in tune with engine rpm to cause the pressure fluctuations...

The snubber is the device that muffles the noise...

Check to see if your snubber has disappeared...(?)

They are calibrated holes like your mechanical MP gauge has... except the MP hole is external... to the outside...  if the calibrated hole in the MP line goes missing... the MP becomes terribly unusable...

PP thoughts only, not a mechanic...

Best regards,

-a-

Link to comment
Share on other sites

4 minutes ago, carusoam said:

Diaphragm pump vs vanes...

RPM dependent oscillations... expect the diaphragms that get mechanically pushed in tune with engine rpm to cause the pressure fluctuations...

The snubber is the device that muffles the noise...

Check to see if your snubber has disappeared...(?)

They are calibrated holes like your mechanical MP gauge has... except the MP hole is external... to the outside...  if the calibrated hole in the MP line goes missing... the MP becomes terribly unusable...

PP thoughts only, not a mechanic...

Best regards,

-a-

Snubber is there, all parts are there. This problem just started up out of the blue and everything had been fine for years. 

Link to comment
Share on other sites

That’s a tough one...

1) gauge appears to work properly...

2) snubber exists...

3) snubber appears to have failed...

4) snubbers are traditionally a calibrated hole drilled in an internal diaphragm/piece of metal...

5) It looks like the hole is no longer calibrated, or air is not the only thing trying to pass through the hole...

6) Got a part number for the snubber?  We can look that up to see what it looks like on the inside...

PP thoughts only...

Best regards,

-a-

Link to comment
Share on other sites

2 minutes ago, carusoam said:

That’s a tough one...

1) gauge appears to work properly...

2) snubber exists...

3) snubber appears to have failed...

4) snubbers are traditionally a calibrated hole drilled in an internal diaphragm/piece of metal...

5) It looks like the hole is no longer calibrated, or air is not the only thing trying to pass through the hole...

6) Got a part number for the snubber?  We can look that up to see what it looks like on the inside...

PP thoughts only...

Best regards,

-a-

RESTRICTOR FITTING F/I SERVO TO F/P HOSE - 610120-000 — LASAR

I think this is the part in question? 

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.