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Mechanical Fuel Pump Going Out


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My last two flights have seen some irregularities on my fuel pressure read out. The problem occurs during climb out after I have switched off the electric boost pump and no where else during the flight (I.e. I Can't recreate the issue on the ground during normal run up). Here is what happens:

-Fuel pressure slowly drops once the electric pump is turned off (from 22psi all the way down to the bottom of the Green Arc ~15psi)

-Once the electric pump is turned back on the pressure comes back up to about 24psi

-I tried switching tanks and no luck, the issue is persistent on both sides. 

-Once I reach cruise altitude and pull the prop back/lean the fuel, the fuel pressure remains steady (22psi) without the electric pump on and I have no further issues

My assumption is that the mechanical fuel pump is going out and can't handle the pressure requirements when I am at full throttle and prop but I would like to see what the Mooney verse had to say about this.

 

BTW: IO-360 in a 1976 M20F

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1) Falls out of the green arc...

2) Is this a new behavior?

 

Time to see the mechanic to find out what is going on...

You might look at the fuel drain on the pump itself... if it has lost a seal, it may be leaking externally... check for blue stains at or near the nipple...

PP thoughts only, not a mechanic...

 

Best regards,

-a-

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4 minutes ago, carusoam said:

1) Falls out of the green arc...

2) Is this a new behavior?

 

Time to see the mechanic to find out what is going on...

You might look at the fuel drain on the pump itself... if it has lost a seal, it may be leaking externally... check for blue stains at or near the nipple...

PP thoughts only, not a mechanic...

 

Best regards,

-a-

They can leak either fuel or oil out of the small drain that comes out just in front of the nose gear.  Mine fluctuated a bit during climb after shutting off the electric fuel pump but didn’t drop out of the green.  It leaked a lot of oil and made a mess.  Replacement fixed both issues.  If you replace it, the new one will likely run right at the top of the green, maybe slightly over and then decrease slightly as you break it in.  I used overhauled from a well known shop in OK who’s name escapes me...

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6 minutes ago, carusoam said:

1) Falls out of the green arc...

2) Is this a new behavior?

 

Time to see the mechanic to find out what is going on...

You might look at the fuel drain on the pump itself... if it has lost a seal, it may be leaking externally... check for blue stains at or near the nipple...

PP thoughts only, not a mechanic...

 

Best regards,

-a-

Yep it is a new behavior. Happened on my last two flights. First one thought it was an anomaly since the problem went away at cruise. This one the same thing happened. 

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2 minutes ago, Ragsf15e said:

They can leak either fuel or oil out of the small drain that comes out just in front of the nose gear.  Mine fluctuated a bit during climb after shutting off the electric fuel pump but didn’t drop out of the green.  It leaked a lot of oil and made a mess.  Replacement fixed both issues.  If you replace it, the new one will likely run right at the top of the green, maybe slightly over and then decrease slightly as you break it in.  I used overhauled from a well known shop in OK who’s name escapes me...

You replaced the mechanical fuel pump? 

Did it make a mess in flight? 

Haven't seen or smelled any fuel, maybe that's because my last couple flights have been 2-3 hours

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3 minutes ago, Prestonalton said:

You replaced the mechanical fuel pump? 

Did it make a mess in flight? 

Haven't seen or smelled any fuel, maybe that's because my last couple flights have been 2-3 hours

Yeah the mechanical pump has a drain coming out of the bottom of it.  Depending on which seal in it is leaking, it can leak oil or fuel.  I never noticed it in flight other than some erratic ff but it made a mess all over the nose gear truss.  The drain comes out right next to the sniffle valve drain on my 68F.

As mentioned, I think you’ll need to check out connections through your fuel system to make sure you isolate the problem before throwing parts at it.

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Often various things can change the reading...

This happens more for carbureted engines where the max FP is about 6psi...  so a change in attitude changes the head pressure a touch...

But, once it falls out of the green... that’s an indication to go find out why... it may last for hours doing this... or just a few more Minutes...
 

Our fuel pumps use dual diaphragms... sounds like one may be going on hiatus...  

Air leaking into the system may cause pressure fluctuations...  Wherever air leaks into the system while it is running.... fuel typically leaks out after shut-down...

Be on the look out for blue stains, starting back at the selector valve... and working toward the engine...

Expect it to be the mechanical pump, but the electrical pump can cover many difficulties at the same time...

 

When the electric pump is turned on and the fuelP rises... does it stay there for a while, or leak down quickly?

PP thoughts only, not a mechanic...

Best regards,

-a-

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1 minute ago, carusoam said:

Often various things can change the reading...

This happens more for carbureted engines where the max FP is about 6psi...  so a change in attitude changes the head pressure a touch...

But, once it falls out of the green... that’s an indication to go find out why... it may last for hours doing this... or just a few more Minutes...
 

Our fuel pumps use dual diaphragms... sounds like one may be going on hiatus...  

Air leaking into the system may cause pressure fluctuations...  Wherever air leaks into the system while it is running.... fuel typically leaks out after shut-down...

Be on the look out for blue stains, starting back at the selector valve... and working toward the engine...

Expect it to be the mechanical pump, but the electrical pump can cover many difficulties at the same time...

 

When the electric pump is turned on and the fuelP rises... doe si stay there for a while, or leak down quickly?

PP thoughts only, not a mechanic...

Best regards,

-a-

Pressure remains constant when the electric pump is on, but I know the electric pump is not rated for continuous operation so I don't want to leave it on throughout climb. Maybe I will go check for stains tomorrow evening...currently away from home in the Miami area so I'm limited on tools :/

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Preston,

If you were to run the electric pump to cover for the Fuel pressure not being there...

That would draw some red flags...  :)

Using the back-up system to fly would be bad... 

Especially if it isn’t actually rated for continuous use...  That would kill the back-up system before the primary system has a chance to croak...   :)

Check for leaks, borrow a screw driver, or buy one at the nearest convenience store...
 

We have a few mechanics around here... wait for their responses...

Best regards,

-a-

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https://skyvector.com/airport/X51/Miami-Homestead-General-Aviation-Airport
 

If you have time... you may get some recommendations...

If you want to get a more direct answer...

Consider starting a thread in the Florida pilots area... do a search first in case there is a collection of mechanics cited there already...

Best regards,

-a- 

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I know of a guy whose engine driven fuel pump was not putting out the rated fuel flow / pressure. He didn’t want to spend the money to get this fixed. He also, reportedly, had developed the bad habit of not using his checklist. 

One day, while taking his new girlfriend for her first flight In his airplane, he neglected to use his checklist and failed to remember to turn on his electric boost pump. On climbout, his engine was burning more fuel than his engine drive fuel pump was supplying to his carburetor, which caused his engine to quit. I was up in my airplane at the time when he made his “mayday” call. Let’s just say that it got his attention.

Fortunately, he had enough altitude to return to the runway for a downwind landing.

He told his IA to fix his fuel pump.

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What's your FP without the engine running?  The electric pump is supposed to be able to get it up to 30 psi in the IO-360.  If it only gets up to 24, there's at least the possibility your FP transducer has gone bad, which happened to me a couple years ago and scared the bejesus out of me.  That was only a $200 repair.

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6 hours ago, jaylw314 said:

What's your FP without the engine running?  The electric pump is supposed to be able to get it up to 30 psi in the IO-360.  If it only gets up to 24, there's at least the possibility your FP transducer has gone bad, which happened to me a couple years ago and scared the bejesus out of me.  That was only a $200 repair.

It definitely doesn't get up to 30. When I prime on a cold start it gets up to about 22-24 psi.

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12 hours ago, Prestonalton said:

It definitely doesn't get up to 30. When I prime on a cold start it gets up to about 22-24 psi.

IIRC, the electric pump has to get at least as high as the max spec fuel pressure from the engine pump (correct me if I'm wrong, someone), so that's already a little wonky.  That's why I thought about the fuel pressure sensor problem I had.

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5 minutes ago, jaylw314 said:

IIRC, the electric pump has to get at least as high as the max spec fuel pressure from the engine pump (correct me if I'm wrong, someone), so that's already a little wonky.  That's why I thought about the fuel pressure sensor problem I had.

Interesting, ever since i've had the plane (~1 year) its read about the same so I haven't noticed anything abnormal until it just started reading a lower pressure. So it could very well be that transducer. Especially since I am not seeing any leaks, fuel stains, or loose connections.

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I had a nearly identical situation where the fuel pressure would drift down to 12–14 psi in the climb but then back to normal with power reduction, leaning or with the electric pump on. I had no visible leaking of oil or fuel from the witness line.

 

Replacing the mechanical fuel pump fixed the issue. M20E

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18 hours ago, Spurious Moppet said:

I had a nearly identical situation where the fuel pressure would drift down to 12–14 psi in the climb but then back to normal with power reduction, leaning or with the electric pump on. I had no visible leaking of oil or fuel from the witness line.

 

Replacing the mechanical fuel pump fixed the issue. M20E

Yeah, I agree.  I’m not telling the OP to run out and throw parts at it, but a new mech fuel pump fixed some erratic fuel pressure issues on takeoff/climb.  Full power, It’s solid at 29psi now with the electric pump on and only drops to about 27 or 28psi with the electric pump off.  

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On 10/15/2020 at 5:12 AM, Prestonalton said:

It definitely doesn't get up to 30. When I prime on a cold start it gets up to about 22-24 psi.

Never mind me, I must have been smoking something. :blink:  My electric fuel pump gets it up to 25-26, not 30, so that's pretty close.  Was working on (faulty) memory, the plane's been in the shop for 2 weeks for annual.

 

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5 hours ago, Ragsf15e said:

Yeah, I agree.  I’m not telling the OP to run out and throw parts at it, but a new mech fuel pump fixed some erratic fuel pressure issues on takeoff/climb.  Full power, It’s solid at 29psi now with the electric pump on and only drops to about 27 or 28psi with the electric pump off.  

Oh I forgot to add, mine failed the leak down test, so I was pretty confident that the pump was the problem when also considering the other symptoms.

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40 minutes ago, Spurious Moppet said:

Oh I forgot to add, mine failed the leak down test, so I was pretty confident that the pump was the problem when also considering the other symptoms.

Yeah, the other symptoms are key.  Mine was pissing oil out the drain.

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So I am out here checking on the fittings and for leaks right now and noticed the l drain in front of the nosewheel tire is actually plugged up. Would this have anything to do with the issue I am having? Thinking about doing a test run-up after I clear the drain. Picture attached

2101B504-30A2-441C-84E6-B65149E15957.jpeg

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