jaylw314

Low fuel pressure

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The last couple flights, I've been noticing my fuel pressure is intermittently dropping below 15 psi, the lower limit.  It kind of freaks me out because I don't know how long it's been going on, since the FP-5L is out of my normal field of view, and the problem has been intermittent, but the last few flights, it seems go drop to about 14.0 psi for  few seconds before recovering to 16 psi every 30 seconds or so, and this happens on some flights but not others.  When it happens, wiggling the fuel selector valve or changing tanks did not seem to make a difference.

I am ashamed to admit that I cannot recall if the fuel pressure of 16 psi is typical of my "normal" FP, since I'm not sure for how long this has been going on.  Nor is it recorded, since I only have an FP-5L for fuel pressure.

I have noticed no perceptible change in fuel flow or fuel consumption.  Running at full throttle and full rich, fuel flow goes all the way about to 16.9 gph at sea level, which is typical for my motor, and cruise fuel flows are the same as they have always been.  I have not noticed any perceptible changes in power or sound when the fuel pressure drops, either.  On my most recent flight, I was getting 150 KTAS at 8.5 gph, which, if anything, seems a little better than what I normally get.

When I turn on the electric pump, the fuel pressure goes up to 17 psi and seems to stay constant.  That means I've not been seeing low FP on takeoff, the only time I'm making a point of looking at the FP-5L.

Based on the above, I'm suspecting it's something in the gascolator fuel filter.  Since the fuel flow and consumption is normal, it's not something clogging the fuel servo or divider.  If it was a blockage before the electric pump, that would not fix the problem.  Hence, somewhere between the electric fuel pump and the fuel pressure sensor.  Of course, if the normal pressure should be much higher than 16 psi, that might suggested a blockage before the electric fuel pump.  I'm not sure what the odds are that it could be a problem with the mechanical fuel pump or how to diagnose that.

I've relayed all of the above to my mechanic and am waiting to hear back, but any thoughts in the meantime or other things to check?

 

 

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The mechanical pump has two sides of the diagram so one can go bad and you can still fly. Might be going bad.

The collator isn't really a filter. Stuff just falls to the bottom. Typically a few bits. If you had enough to fill it you'd probably have far worse issues. The collator is pulled and cleaned at annual.

Not bad to check the finger filter on the servo (should be done at annual) and the secret filter that is usually before the firewall just past the electric pump (its designed to catch debris if the electric pump sheds).  The secret filter isn't mentioned in the maint manual and the AD that required it forgot to include mandatory inspection but its a good idea.

-Robert

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I read about the "secret filter", but I had assumed it was before the electric pump, thanks

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Seems low.  Do a leak down test on the mechanical fuel pump

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9 minutes ago, Yetti said:

Seems low.  Do a leak down test on the mechanical fuel pump

What is a leak down test?

If the FP looks low, could it simply be a FP sensor problem?  I'd expect the electric fuel pump to produce a FP similar to the mechanical fuel pump, even with a partial blockage before the fuel servo.

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11 minutes ago, jaylw314 said:

What is a leak down test?

If the FP looks low, could it simply be a FP sensor problem?  I'd expect the electric fuel pump to produce a FP similar to the mechanical fuel pump, even with a partial blockage before the fuel servo.

Before cranking, run fuel pump to peak pressure. Move Throttle closed, Mixture to ICO, turn pump off. The FP needle shouldn't move for a minute or two, then only bleed down slowly. If it starts moving soon, or moves fast enough to see the needle move, your mechanical pump / diaphragm may be failing.

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16 minutes ago, jaylw314 said:

What is a leak down test?

If the FP looks low, could it simply be a FP sensor problem?  I'd expect the electric fuel pump to produce a FP similar to the mechanical fuel pump, even with a partial blockage before the fuel servo.

What gauge?  If it is the Mooney stock gauge there is no sensor.   It is a mechanical gauge with a line coming off the fuel servo to the back of the gauge.

 

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Your pressure seems low. What is it with the electric on? It should be about 25 PSI.

When it drops below 15 or so the engine usually starts to stumble. If it shows low with the electric on, I would suspect a sensor problem.

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1 hour ago, Yetti said:

What gauge?  If it is the Mooney stock gauge there is no sensor.   It is a mechanical gauge with a line coming off the fuel servo to the back of the gauge.

 

I have an FP-5L, primary for fuel pressure, so there is a fuel pressure transducer (PT-30GA) installed at the same place on the fuel line as the factory.

49 minutes ago, N201MKTurbo said:

Your pressure seems low. What is it with the electric on? It should be about 25 PSI.

When it drops below 15 or so the engine usually starts to stumble. If it shows low with the electric on, I would suspect a sensor problem.

That's what gets me, when I turn on the electric fuel pump, it only went up to 17 psi while the motor was running.  I hadn't thought to try it with the motor off yet, but I can't imagine it would make a difference--I suspect even at max FF, the electric pump should be able to keep up, right?

Likewise, the fact that my fuel usage (based on refueling) and power are unchanged means I'm not simply losing a bunch of fuel overboard somewhere.

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4 hours ago, Hank said:

Before cranking, run fuel pump to peak pressure. Move Throttle closed, Mixture to ICO, turn pump off. The FP needle shouldn't move for a minute or two, then only bleed down slowly. If it starts moving soon, or moves fast enough to see the needle move, your mechanical pump / diaphragm may be failing.

Hmm.  I ran by the hangar on the way back from work.  When running the electric fuel pump with mixture to ICO, I only got a FP of 16.0 psi.  However, once I turned the fuel pump off, it remained at 16.0 psi, and only decreased to about 15.5 after a minute.  That suggests the electric pump's check valve is fine.

I'm confused.  If the normal pressure from both the engine and electric boost pump is about 25 psi, why would I get the same low FP on both of them alone?  I'm wondering more if this is a sensor issue now...

Are people getting about 25 psi typically for FP?

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3 minutes ago, jaylw314 said:

Hmm.  I ran by the hangar on the way back from work.  When running the electric fuel pump with mixture to ICO, I only got a FP of 16.0 psi.  However, once I turned the fuel pump off, it remained at 16.0 psi, and only decreased to about 15.5 after a minute.  That suggests the electric pump's check valve is fine.

I'm confused.  If the normal pressure from both the engine and electric boost pump is about 25 psi, why would I get the same low FP on both of them alone?  I'm wondering more if this is a sensor issue now...

Are people getting about 25 psi typically for FP?

You need to check your sensor.

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M20Js have an electric gauge and transducer. It could be a measurement issue, since both pumps have about the same pressure and the engine runs fine. The way a good A&P would troubleshoot it would be to connect a known accurate gauge in the line and see what it reads. 

Skip

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1 hour ago, PT20J said:

M20Js have an electric gauge and transducer. It could be a measurement issue, since both pumps have about the same pressure and the engine runs fine. The way a good A&P would troubleshoot it would be to connect a known accurate gauge in the line and see what it reads. 

Skip

Yes, I suggested that to my IA, he said he'd go look some stuff up and get back to me.

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Got a JPI?

having the second transducer and indicator would be great right about.... now!  :)

Downloading data through Savvy would be helpful to share the data with your MS friends...

Having a FF indicator would tell you that the FF hasn’t changed either...

The Engine doesn’t stumble...

Did you try to lean it at all?

 

So using PP logic...

If the fuel pressure was going low... the FF would try to follow it...

Lower FF than normal would tend to cause a stumble, especially when trying to lean the normal way...

With low fuelP, getting max FF for T/O will be challenging...

The electric fuel pump would supply the extra FF needed... it would have to show the FP in the green arc if it was actually supplying the FF...

FF and FP are directly related, but not very linear...

Any fuel leaked would be draining external... lots of blue everywhere if the a couple of psi were draining out...

Some Mooneys have a fuel line that returns to the tank... to have that fail... that wouldn't show low P and deliver full FF to the engine... low power and stumbling combined with really ill feelings....

It sounds like a bad fuel pressure sensor is high on the suspect list...

+1 with sharing your thoughts with MS and your mechanic..!

Pp thoughts only, not a mechanic...

Best regards,

-a-

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10 hours ago, carusoam said:

Got a JPI?

having the second transducer and indicator would be great right about.... now!  :)

Downloading data through Savvy would be helpful to share the data with your MS friends...

Having a FF indicator would tell you that the FF hasn’t changed either...

The Engine doesn’t stumble...

Did you try to lean it at all?

 

So using PP logic...

If the fuel pressure was going low... the FF would try to follow it...

Lower FF than normal would tend to cause a stumble, especially when trying to lean the normal way...

With low fuelP, getting max FF for T/O will be challenging...

The electric fuel pump would supply the extra FF needed... it would have to show the FP in the green arc if it was actually supplying the FF...

FF and FP are directly related, but not very linear...

Any fuel leaked would be draining external... lots of blue everywhere if the a couple of psi were draining out...

Some Mooneys have a fuel line that returns to the tank... to have that fail... that wouldn't show low P and deliver full FF to the engine... low power and stumbling combined with really ill feelings....

It sounds like a bad fuel pressure sensor is high on the suspect list...

+1 with sharing your thoughts with MS and your mechanic..!

Pp thoughts only, not a mechanic...

Best regards,

-a-

Sorry, the JPI isn't hooked up to FP, just FF, so there's no relevant data other than completely normal looking fuel flows in climb and cruise.

I calculated my fuel used based on indicated FF, and compared that to the amount of gas I have been refueling, and they match up, nor is there any blue stuff on the plane or in the hangar, so the evidence supports the conclusion I am not losing fuel overboard.

Lycoming IO-360 doesn't have a fuel return line, so I'm not losing FP there

FF is dependent on fuel pressure in the divider, and that is regulated to a lower pressure than the line FP.  I would expect low FP only to cause stumbling on high fuel flows first, since the FP should be capable of providing more than the max fuel necessary at full power/full rich?

I would expect low FP would be more likely to cause problems when richening or increasing throttle, not leaning?

 

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Does jpi have a fuel pressure sensor? If it does I’d be interested in it but I haven’t seen it listed back when I was looking. 

-Robert 

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17 minutes ago, RobertGary1 said:

Does jpi have a fuel pressure sensor? If it does I’d be interested in it but I haven’t seen it listed back when I was looking. 

-Robert 

IIRC the primary 900 and 930 do, but not the 730/830.  Don't quote me on that though...

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The engine monitors are capable of having FP... but, not all sensors have to be purchased or connected to them... depends on who has extra AMUs at the time of purchase and install...

 

To clear up one point I made earlier... if the engine isn’t being supplied with enough fuel, because of low pressure.... it may be running pretty lean already... without the pilot knowing it.

when the pilot tries to lean as expected... things suddenly become more noticeable...

 

0) What causes pressure to form in a fuel line?  (Number zero for Jay :))

1) To generate pressure, We first have to generate fuel flow... the fuel pumps do this pretty well.

2) flow can exist without pressure... restriction to flow causes pressure to rise...

3) It is important to know where the pressure is being measured... there may be a pressure regulator in the system...

4) Expect the FP sensor to be early in the system... mostly to monitor the Fuel Pump...

5) Expect that there is some form of pressure regulation near the end... could be as simple as using a small calibrated internal hole in the line... or a pressure sensitive valve...

6) a pressure regulator near the end, allows for a wide range of pressure to be used. Thus allowing worn pumps, or a leak in the line to not kill the engine for a while...

7) having two independent pumps... shows that FF is being delivered as expected... or both pumps are failing the same way?

8) the leak down test... shows that fuel isn’t leaking out of the system...

9) if the fuel pressure gauge IS working correctly... FF is normal, but pressure is low...expect a leak is occurring somewhere...

10) fuel leaking externally... would show a big mess... and danger of fire.

11) fuel leaking internally... if that is possible, would send fuel somewhere where it can’t be used, in a hidden way... 

 

12) fuel pressure gauge... we have two types....

  • Mechanical...the fuel line delivering pressurized fuel in a tiny line to the back of the instrument panel.
  • electronic...  sensor delivering a voltage signal to the gauge...

 

13) There are so many things designed to overcome certain problems that may occur over time... It is possible to keep flying as a failure continues to worse over time...

So what happens if flying with a bad sensor? Not much...

What if it’s not a bad sensor? Something else is failing...

Makes you want to identify at a minimum... what is failing... before relying on things to work...

 

14) Mechanical pressure gauges can get cleaned or OH’d... or replaced...

15) electronic gauges can be tested, and have connections cleaned...

 

In brief... check that the gauge is not working before ignoring it... :)

Ordinary PP thoughts only,  being shared, not an instrument tech or mechanic...

Best regards,

-a-

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3 hours ago, carusoam said:

2) flow can exist without pressure... restriction to flow causes pressure to rise...

@carusoam Nitpicking, but you can have pressure without flow, but you can't have flow without pressure :) 

In the IO-360, the FF sensor is actually a pressure gauge in the fuel divider in the Bendix RSA fuel injection, and is directly proportional to the actual fuel flow out the injectors.  So if I'm seeing normal FF on the gauge, that means I also have normal (and sufficient) pressure in the fuel divider. 

I assume there would be no drop in FF at max power until the line fuel pressure drops below about 11 psi, since the minimum FP (14 psi) is supposed to be enough to provide 125% of the fuel flow at max power, AFAIK...

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1 hour ago, jaylw314 said:

In the IO-360, the FF sensor is actually a pressure gauge in the fuel divider in the Bendix RSA fuel injection, and is directly proportional to the actual fuel flow out the injectors.  So if I'm seeing normal FF on the gauge, that means I also have normal (and sufficient) pressure in the fuel divider. 

There is a pressure port on the flow divider to drive a pressure gauge calibrated in fuel flow on the RSA injection systems. Some Pipers and Cessna use it -- probably others. Mooney installations I've seen (including the factory installations) use the more accurate Floscan transducers in the fuel line between the fuel pump and the servo. Are you sure yours is a pressure takeoff?

Skip

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1 hour ago, PT20J said:

There is a pressure port on the flow divider to drive a pressure gauge calibrated in fuel flow on the RSA injection systems. Some Pipers and Cessna use it -- probably others. Mooney installations I've seen (including the factory installations) use the more accurate Floscan transducers in the fuel line between the fuel pump and the servo. Are you sure yours is a pressure takeoff?

Skip

I hadn't realized that, I assumed the Floscan was just a pressure gauge as well.  I hadn't looked at the installation closely enough, so I assume it is installed the way you are describing.  Thanks, and sorry @carusoam :) 

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2 hours ago, jaylw314 said:

@carusoam Nitpicking, but you can have pressure without flow, but you can't have flow without pressure :) 

In the IO-360, the FF sensor is actually a pressure gauge in the fuel divider in the Bendix RSA fuel injection, and is directly proportional to the actual fuel flow out the injectors.  So if I'm seeing normal FF on the gauge, that means I also have normal (and sufficient) pressure in the fuel divider. 

I assume there would be no drop in FF at max power until the line fuel pressure drops below about 11 psi, since the minimum FP (14 psi) is supposed to be enough to provide 125% of the fuel flow at max power, AFAIK...

Jay,

You missed my first line... i’ll Put a 0 next to it...

0) What causes pressure to form in a fuel line?  (Number zero for Jay :))

it is not nit picking... it is an important detail... kind of like stating initial conditions... yet keeping it interesting... sort of.

Studying flow and restrictions and pressure is kind of like studying lift, drag, and gravity... not nearly as fun as using these things for flight...

Best regards,

-a-

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26 minutes ago, jaylw314 said:

I hadn't realized that, I assumed the Floscan was just a pressure gauge as well.  I hadn't looked at the installation closely enough, so I assume it is installed the way you are describing.  Thanks, and sorry @carusoam :) 

Look for a bulge in a firesleeve on the fuel line - like a python ate a mouse :) The transducer should be inside. 

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