Fuel pressure gauge misbehaving.

[charlesual]

Question for those smarter then I.

Mechanical fuel pump recently changed. No change when I turn on electric fuel pump.

 

file:///var/mobile/Library/SMS/Attachments/bb/11/9182CA9D-466D-41C1-9EC2-3B77654DE289/IMG_2275.MOV

 

 

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[charlesual]

Well Damn,
Thought I could post a video. Looks like a fail. Anyway, my fuel pressure gauge rapidly fluctuates between 25-30psi.


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[charlesual]

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[carusoam]

Just now, charlesual said:

Well Damn,
Thought I could post a video. Looks like a fail. Anyway, my fuel pressure gauge rapidly fluctuates between 25-30psi.


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Post video in YouTube... copy link, share link here...

-a-

[charlesual]

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[charlesual]

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[carusoam]

Charles you are now looking at a pair of things to get that straight...

1) Make sure the snubber was installed at the back of the gauge... a small hole in the line...

2) Make sure the installation procedure included an appropriate air bubble in the line... to pass in and out of the snubber’s hole...

An ordinary instrument person will know about these things...

The old pressure gauge may have the snubber still attached to it...

PP thoughts only, not an instrument guy...

Best regards,

-a-

[75_M20F]

I have been dealing with the same issue. In your case it appears the fuel line that comes off the fuel servo and runs to your sending unit or the back of the fuel pressure gauge has had all the air bled out of it and is full of fuel. That line needs to be purged of all the fuel. Disconnect one end from the servo and the other end off the gauge or sending unit, blow it out with compressed air. Then reattach the line. That will probably take care of that needle flying around. (The air in the line acts as a shock absorber to lesson the pulses from the mechanical pump). Otherwise I would say the mechanical fuel pump has issues.

 

 

Edited February 4, 2021 by 75_M20F

[carusoam]

2 hours ago, 75_M20F said:

I have been dealing with the same issue. In your case it appears the fuel line that comes off the fuel servo and runs to your sending unit or the back of the fuel pressure gauge has had all the air bled out of it and is full of fuel. That line needs to be purged of all the fuel. Disconnect one end from the servo and the other end off the gauge or sending unit, blow it out with compressed air. Then reattach the line. That will probably take care of that needle flying around. (The air in the line acts as a shock absorber to lesson the pulses from the mechanical pump). Otherwise I would say the mechanical fuel pump has issues.

 

 

A combination of air and the hardware part, the snubber... dampens the vibrations caused by the fuel pump...

Which are probably occurring at 2X 2700 rpm... about 90 per second... so fast, the needle won’t be able to show that frequency of vibrations...

 

If the snubber is full of fuel, it becomes ineffective...

Check the parts manual for details on the snubber... some get mounted directly to the back of the gauge...

If using a high end JPI... one gets added externally...

PP thoughts only...

-a-

[75_M20F]

In my 75 F, there is no snubber on the fuel pressure gauge. The only snubber is on the back of the fuel servo where the line attaches to go the pressure gauge.

I still have some minor fuel pressure needle rapid movement issues after clearing the line out, but it is much better then before (when it was similar to the video posted above before clearing out the line with air) My mechanic is puzzled as to what is causing it. We have discussed replacing the fuel pump and he suggested the issue could be the fuel servo. However, the fuel servo was overhauled 3 years ago, so I would not think it was the culprit.

 

[75_M20F]

13 hours ago, charlesual said:

 

Question for those smarter then I.

Mechanical fuel pump recently changed. No change when I turn on electric fuel pump.

 

file:///var/mobile/Library/SMS/Attachments/bb/11/9182CA9D-466D-41C1-9EC2-3B77654DE289/IMG_2275.MOV

 

 

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What was the reason for changing the mechanical pump? And how soon after changing the pump did the fuel pressure issues show up?

[RobertGary1]

It’s not the fuel pump. If it was it would go away when the electric pump was on. I agree with just clearing the line. 

[carusoam]

interesting detail regarding the location of the snubber in the F’s fuel injection system...

It will change how easy it is to keep air in the snubber...

 

The mechanical pump is the source of pressure oscillation....

Each revolution of the engine operates the pump’s plunger...

Turning on the electric pump probably won’t remove the oscillation from the system...  it adds pressure closer to the beginning of the fuel system... where the mechanical pump adds pressure and noise closer to the end...

Our fuel handling isn’t affected by the oscillation in pressure... just the interpretation of the instrument is a challenge...

It would be interesting if the electric pump had an affect on the oscillation...

We might learn something about the cause of the oscillation...

PP thoughts only, not a mechanic...

Best regards,

-a-

 

Let’s send an invite to @M20Doc to see what he knows on the subject... (fuelP snubbers and pressure oscillations)