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Bad Fuel Pressure Sensor? Or bad main fuel pump?

Saville said:
I think you are wrong - or at best do not understand my description:

At no point did I say the liquid needs to compress in order to let air in. Air comes in a working system because the fuel level goes down - the air pocket in the tank grows - the pressure of that air becomes lower than ambient, and so ambient air is sucked in (when testing on the tarmac) to equalize the pressure and keep the gas flowing. When flyign you have ram effect but these are all ground tests.

The incompressibility of the gas is what PREVENTS air from coming into the tank to replace fuel volume when the system is busted.

Difficult thing to discuss with words - I put some crude pictures in my response to Carl below.
Click to expand...

You are treating the tank fuel/air interface as a barrier. It is not. The only things that count are the pressure (or vacuum) in the tank compared to outside pressure. The pressure (or vacuum) in the tank will be the same regardless of measuring below the fuel level or above (ignoring the trivial amount of height difference).

Van's route the vent to the high end of the tank to reduce fuel burping out the vent with full tanks.

From everything you said, I conclude you are drawing a vacuum in your fuel tank. The smoking gun is that you said you suck in air when you removed the fuel cap.

I suspect just good luck prevented a collapsed tank.

Carl
 
Try a long engine run up and such with half tanks and the fuel caps off. That will confirm or deny a clogged vent.

Carl
 
Saville said:
I do not see how ambient air is going to be sucked from the outside into that air pocket, because the gasoline is incompressible, and there is no longer a clear path from ambient to the pocket.

The air pressure in the pocket stays low until I open the gas cap whereupon I hear the sucking sound - the pressure is NOW back at ambient. There was a lower than ambient pressure in the air pocket.
Click to expand...

The fact that you don't see how, doesn't mean it doesn't happen.. I am not going to attempt to describe the physics involved in this post. You can easily crate an experiment to prove it with a water bottle and a straw.

A small negative pressure can be supported until it is strong enough to suck in the air. This is what you experience when you pull the cap. It doesn't prove that you are not equalizeing; just not equalizaing all the way to ambient.

You have proven that the engine can pull in enough fuel in this condition. If you had a plugged vent, you would have seen your fuel tank start to collapse. I have seen pictures of significant oil canning of a fuel tank due to a plugged vent and the pump still draws fuel.

Larry
 
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Progress Report:

This weekend I got the access panel off the left hand fuel tank.

It had the cork ring.

After removing the access plate/sender assembly I reached in and felt for the B-but on the air vent:

I could tighten it with my fingers. I went in with the bore scope to have a look at the flange and this is what I saw"



Flange looks ok from the outside.

I pushed the nut up the vent tube a ways and took this:



Doesn't tell me much but it looks ok. The lighting that comes with the borescope is a little weak - I need to enhance that somehow. I shoved a headlamp into the tank to get this lighting but it isn't great.

Anyhow, that was why I was hearing bubbles coming out of the inside of the tank at the air vent fixture: the B-nut was loose. I don't know if it was always like that, or if it got that way or how it got that way.

I have to figure out how many turns or facets past finger tight I need to make this connection stay put: I REALLY do not want to remove an access plate again if it can be avoided.

Learned a lot though, as always - easy for me to do as I don't know much ;)

I'm also going to have to fashion a new one of these:



As I messed up one of the flange ends. It is the line connecting the tank fuel pickup to the fuselage fuel line plumbing.

Getting it back on without ruining the new one is a real head scratcher for me as well.

Onwards and upwards.
 
Final Report

After fixing the serious problem in the left tank air vent in, I tested the air vent in the right wing tank. Firstly to see if it has the same problem (loose B-nut on the inside of the tank) and secondly there was some evidence this was the cause of my fuel pressure problem.

So when the right hand tank tested fine I knew the vent wasn't the cause of the low fuel pressure.

There was also some ProSeal on the outboard end of the left tank vent and I cleared that away.

So I replaced the fuel pump. I also had one of Van's Fuel Pump shrouds installed.

I noticed that there was a tube running plenum bleed air towards the fuel pump, but that it was only vaguely pointed at the pump. In fact it pointed below the pump. Reading in the Forum I saw several posts where low fuel pressure was caused by vapor lock and the lock was solved by installing the shrouds. So given that the bleed air was doing nothing, I installed the shroud.

Run ups showed that the fuel pressure problem has been solved. I can't really say whether it was the shroud (as some others have experienced) or a bad pump. But if I had to bet I'd bet on a bad pump.

I have the old pump and I will be dissecting it. A mechanic said that in his opinion there could be a failed diaphragm in the pump . We'll see.

Anyway the airplane is airworthy again and I can turn from being a mechanic to a pilot.

I want to thank everyone for all their input.
 
Gregg, long, but well thought out process, AND testing. You did uncover several unknown issues, that could have created an issue down the road.
Test and retest all the systems after the pump install---verify the EFIS pressure reading against the mechanical gauge AGAIN, after the new pump is installed.
Tom
 
TS Flightlines said:
Gregg, long, but well thought out process, AND testing. You did uncover several unknown issues, that could have created an issue down the road.
Test and retest all the systems after the pump install---verify the EFIS pressure reading against the mechanical gauge AGAIN, after the new pump is installed.
Tom
Click to expand...

Thank you Tom, I appreciate all the support.

One thing that I re-learned is:

You have to pay attention to EVERY LITTLE oddity you run across. Like when I saw gas dripping from the left tank air vent intake.

Any time I get a "Hmm that's funny...." thought it needs to be followed up.

Yes the resolution of this took a long time - I hated being grounded. But as you point out, I discovered other problems along the way to fixing the main one. Problems which could have resulted in something worse than being grounded.
 
Thats right Gregg--if you take care of the little things, the big things take care of themselves----well sometimes.
Tom
 
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