Low fuel pressure problem, HELP!

[DoctorJay69]

My son-in-law and I are at our wits end.  Our '69 M20E, IO-360-A1A, has developed a fuel pressure problem that 2 AP's have not been able to solve.  All filters have been checked and cleaned out as both had contaminants in them  A new fuel pump was just installed today, but still will not read in the green without the boost pump on.

Any realistic suggestions by those who have experienced the problem themselves would be greatly appreciated.

Thanks to everyone in advance.

[slowflyin]

I'd check the FP with a different gauge.

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[Guest]

Installing a test gauge to verify pump output pressure and panel gauge accuracy should help.

Clarence

[Marauder]

My son-in-law and I are at our wits end.  Our '69 M20E, IO-360-A1A, has developed a fuel pressure problem that 2 AP's have not been able to solve.  All filters have been checked and cleaned out as both had contaminants in them  A new fuel pump was just installed today, but still will not read in the green without the boost pump on.

Any realistic suggestions by those who have experienced the problem themselves would be greatly appreciated.

Thanks to everyone in advance.

Did they clean out the line going to the gauge? If I remember correctly the orifice of the gauge inlet is not that large.

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[carusoam]

Original FP gauges often have a snubber in line on the back of the gauge.  The snubber is, essentially, a tiny hole that mechanically smooths out the pressure being read by the gauge. A small obstruction may make a large reading error.

 

The other thing we have learned this week...

The M20E and M20C both share the same maintenance manual.  Somebody reported getting an IO360 fuel pump for an O360 engine.  The pressure difference is very large.  One uses 3psi and the other use 23psi (I'm guessing on these particular numbers...)

Fuzzy memories of a PP, not a mechanic.

Best regards,

-a-

[RLCarter]

I'm on the other end of the spectrum, Just replaced the mechanical fuel pump and now have 30 psi fuel pressure at idle with the boost pump off, goes to 31 or so with it on. To bad we cant average them which you put us both in the green

[Bob_Belville]

8 hours ago, RLCarter said:

I'm on the other end of the spectrum, Just replaced the mechanical fuel pump and now have 30 psi fuel pressure at idle with the boost pump off, goes to 31 or so with it on. To bad we cant average them which you put us both in the green

As with the low pressure problem, I'd be suspicious of the gauge and check it first. Does fuel flow, if you have it, confirm the high pressure? FF @ takeoff should be 18 gph +/-. Done engine run normal @ slow idle? 

[Guest]

20 hours ago, carusoam said:

Original FP gauges often have a snubber in line on the back of the gauge.  The snubber is, essentially, a tiny hole that mechanically smooths out the pressure being read by the gauge. A small obstruction may make a large reading error.

 

The other thing we have learned this week...

The M20E and M20C both share the same maintenance manual.  Somebody reported getting an IO360 fuel pump for an O360 engine.  The pressure difference is very large.  One uses 3psi and the other use 23psi (I'm guessing on these particular numbers...)

Fuzzy memories of a PP, not a mechanic.

Best regards,

-a-

The snubber fitting is usually at the injector or carb.

Clarence

[Brian Scranton]

Same thing happened to me. Discovered after looking over and over again that I had a leak in the fuel pressure line. Any fuel smell or blue stains?

[DrBill]

22 hours ago, RLCarter said:

I'm on the other end of the spectrum, Just replaced the mechanical fuel pump and now have 30 psi fuel pressure at idle with the boost pump off, goes to 31 or so with it on. To bad we cant average them which you put us both in the green

I'm 20 hrs after major OH and the mech pressure went to 30+.  Has to be bad mech pump so I got another under warranty but not changed yet.  Elec pressure is ok <25 w/o engine running so gauge is ok.

Pump failure mode is lever binds on shaft causing diaphragm to pump both directions with lever not just down as designed.

I'll post when I's fixed.

Bill

 

[bradp]

For educational purposes the takeoff for the pressure gauge is downstream of the boot pump and upstream of the engine driven pump? 

Thanks

brad  

[Guest]

In fuel injected engines the fuel pressure is taken from a fitting on the finger screen portion of the servo.  The fitting is between the  servo and the oil sump, the hose goes to the pressure gauge in older models or to a pressure transducer in newer models.

On carbureted models the pressure pick up is at the carburetor.

Clarence

[JoshMan]

Hey, guys.  Is there any way the servo could be involved?

[carusoam]

Welcome aboard Josh.

Looking at the challenge from an engineering point of view...

1) flow volume is developed by the pump.

2) pressure builds wherever restriction to flow occurs.

3) Since the fuel servo is down stream, for it to cause a low pressure situation, it would need to allow more fuel to pass through it than normal...

4) how much resistance to flow does the servo cause?

5) testing flow through the fuel injection system is done with identical glass jars and a timing device.  This test is probably written up around here as the glass jar test(?)...

6) the glass jar test is usually looking for a blocked fuel injector.  But it would show low volume being produced by the pump and/or high volume being allowed by the fuel servo...

7) a leak downstream would be a serious low pressure challenge.  Somebody recently posted a picture of a leaking fuel hose that got melted by an exhaust pipe.

8) Look for how wear can cause the low pressure situation.  Wear is a common problem on mechanical devices.  Also, Look for broken parts or incorrect parts.  These things happen...

9) know what the right pressure for your plane is and check that your instrument is reading it properly.  Instruments get old over the decades of use...

Keep in mind, I am only a PP, not a mechanic.

Best regards,

-a-

[buddy]

Try checking your fuel tank vents to make sure they are clear. This happened to me one time in my 201 at altitude on a really cold day my fuel vents froze up and the only way to get the fuel pressure back up was to turn on the boost pump. Once I got down to a lower altitude, below freezing the fuel pressure was normal without the boost pump on.