Bad Fuel Pressure Sensor? Or bad main fuel pump?

[Saville]

Originally Posted by lr172

It is a sandwich of rubber slices. Someone posted a teardown of a pump. I encourage you to look at his pictures. LOTS of small rubber parts. failure can happen at any time. It is not just a wear issue. I just had a pump fail with only 400 hours on it.

Larry

Yes...anything can fail after 20 minutes use.

[Saville]

Originally Posted by Steve Iacoviello

Sam, the sensor is bad as evidenced by the "blank" reading. With a good sensor installed one can determine if the fuel pump needs changing.

Exactly. Also I can determine of the fuel pump is bad by hooking up a separate pressure gauge to the manifold block and read that at the same time I'm reading the EFIS.

Or another way to look at it is this:

With a good sensor/EFIS I can know that replacing the fuel pump fixed the problem.

With a bad sensor I'll still get a bad reading. Did replacing the fuel pump fix it?

People are assuming the EFIS/sensor is absolutely perfect and that the fuel pump is the spawn of the devil. I wonder why they assume the former in the face of what I've found.

If the fuel pump is bad (and I never said it wasn't), then I have two problems don't I? Because that reading on the EFIS is all wrong even if there were zero PSI in the line.

And so what if I replaced the fuel pump - I would still get an erroneous reading wouldn't I? So how do I know the new pump is working?

Since I can cause the fault to occur on the ground, I think a little dilly dallying and testing to pin down the precise cause is not completely unwarranted.

Yes I know a lot of you already "know" where the problem lies. And you all may very well be right. Do not take my resistance to diving in right now and changing the pump to mean I think the pump is fine. It could very well be the culprit - I've never said otherwise.

[Saville]

Ok time for a little update:

Last weekend, I was testing out the fuel pressure sender per Van's instructions when I noticed some fuel dripping from the left hand fuel tank air vent inlet.

(See The Case of the Dripping Fuel tank vent" in Ongoing Maintenance)

So here is my latest working theory:

When I start the engine the fuel pressure reads fine. By the time I get to the run-up area the EFIS shows "---".

I talked to TruTrak and they verified that this means either No Contact (i.e. broken wire) or out of range.

After noticing the dripping I opened the fuel cap for that tank and heard the suction sound - the tank wasn't getting any air from the vent. The exit end of the tank vent is above the level of the fuel in the tank. I blew air into the tank vent inlet and felt both air and gas coming out of the exit. Then I put my ear right on the wing over the point where the air vent enters the tank and blew some air. I could plainly hear bubbles.

So the interior connection to the fuel vent is loose - though not completely disconnected. I did see some fuel dripping from the vent once before but I thought that was because I filled the tank to the absolute brim prior to a cross country I was going to do the next day. I figured fuel sloshed in the tank when I pushed the plane back into the hangar and merely dripped out. Now I know differently.

My latest working theory is to why the fuel pressure slowly falls after starting is that the tank is not being vented. The open connection at the tank wall is always "underwater", so no air gets in. Over time, it becomes harder and harder for the Main Fuel Pump to pull gas from the tank. The pressure in the lines goes down over time. Turning on the Aux pump provides added force and so the pressure goes back up.

Luckily the tank didn't implode.

Why did the fuel pressure reading come back on the last flight as I was descending to the airport? My theory is that I was going pretty fast and the ram air forced some air into the tank and equalized the pressure: the connection isn't completely loose - I could still blow air into the tank if i blew hard enough. So my initial thought that this is heat related loses some credibility.

Anyway that's the working theory for now. As I have to fix the vent connection anyway, we will see if that was actually the problem.

So this weekend I drain the tank, remove the large access plate, and see if I can get a wrench onto the connection.

[istrumit]

Originally Posted by Saville

Ok time for a little update:
The open connection at the tank wall is always "underwater", so no air gets in. O

Maybe I am not understanding (wouldn't be my first time), but, if you are dripping from the vent port, then there has to be an open path from that port to the tank...in which case, fuel being pumped out of the tank will be replaced by air through the vent, regardless if the vent is above or below the fuel level.

If the end of the line, in the tank, is below the fuel level, then, in the opposite scenario, positive pressure in the tank (e.g. a low pressure system moving through the area) will cause fuel to exit via the vent line, rather than air venting through that line.

In either case, the tank pressure differential vs Atm should be negligible and would not affect pump effectiveness.

I occasionally get a pressure drop from the mechanical pump after a long climb. I attribute that to either a failing pump or hot fuel close to the vapor point boiling at the pump inlet. Aux pump cures this condition when it occurs, but I will be inspecting and replacing the pump soon.

[Saville]

Originally Posted by istrumit

Maybe I am not understanding (wouldn't be my first time), but, if you are dripping from the vent port, then there has to be an open path from that port to the tank...in which case, fuel being pumped out of the tank will be replaced by air through the vent, regardless if the vent is above or below the fuel level.

If the end of the line, in the tank, is below the fuel level, then, in the opposite scenario, positive pressure in the tank (e.g. a low pressure system moving through the area) will cause fuel to exit via the vent line, rather than air venting through that line.

In either case, the tank pressure differential vs Atm should be negligible and would not affect pump effectiveness.

I occasionally get a pressure drop from the mechanical pump after a long climb. I attribute that to either a failing pump or hot fuel close to the vapor point boiling at the pump inlet. Aux pump cures this condition when it occurs, but I will be inspecting and replacing the pump soon.

Yes there is an open path from the port to the tank and that path is inside the tank where that vent line goes through the tank. The vent line is connected to a fitting on the inboard tank wall. The vent line comes in 2 parts...intake (under the fuselage) to the inboard tank wall exterior. And then another tube from the inboard tank wall fitting INSIDE the tank, outboard to the end of the tank.

What is loose is the connection on the INSIDE of the tank closest to the fuselage. The dihedral moves the gas to that wall. The outboard end of the vent line is near the gas cap and is NOT submerged.

Since the effective intake of the vent line is now "underwater" at all times, there can be no equalization of tank pressure.

As the fuel pump pressurizes the system, gas is drawn from the tank but the vent cannot replace the lost volume with air because the effective exit of the air vent is underwater.

If the air vent lines were connected solidly, then there would be a path from atmospheric air to the air pocket inside the tanks. But the connection is faulty so the atmospheric air cannot get bast the volume of gas into which it is submerged. Gas is incompressible.

This is a test being done on the ground.

[lr172]

Originally Posted by Saville

But the connection is faulty so the atmospheric air cannot get bast the volume of gas into which it is submerged.

This is incorrect. As that tank develops a negative pressure (i.e. vacuum) it will pull air from the external, higher pressure source even if that source is submerged in an imcompressable liquid. The liquid doesn't need to compress in order to let the air in.

[Carl Froehlich]

Originally Posted by Saville

Ok time for a little update:

Last weekend, I was testing out the fuel pressure sender per Van's instructions when I noticed some fuel dripping from the left hand fuel tank air vent inlet.

(See The Case of the Dripping Fuel tank vent" in Ongoing Maintenance)

So here is my latest working theory:

When I start the engine the fuel pressure reads fine. By the time I get to the run-up area the EFIS shows "---".

I talked to TruTrak and they verified that this means either No Contact (i.e. broken wire) or out of range.

After noticing the dripping I opened the fuel cap for that tank and heard the suction sound - the tank wasn't getting any air from the vent. The exit end of the tank vent is above the level of the fuel in the tank. I blew air into the tank vent inlet and felt both air and gas coming out of the exit. Then I put my ear right on the wing over the point where the air vent enters the tank and blew some air. I could plainly hear bubbles.

So the interior connection to the fuel vent is loose - though not completely disconnected. I did see some fuel dripping from the vent once before but I thought that was because I filled the tank to the absolute brim prior to a cross country I was going to do the next day. I figured fuel sloshed in the tank when I pushed the plane back into the hangar and merely dripped out. Now I know differently.

My latest working theory is to why the fuel pressure slowly falls after starting is that the tank is not being vented. The open connection at the tank wall is always "underwater", so no air gets in. Over time, it becomes harder and harder for the Main Fuel Pump to pull gas from the tank. The pressure in the lines goes down over time. Turning on the Aux pump provides added force and so the pressure goes back up.

Luckily the tank didn't implode.

Why did the fuel pressure reading come back on the last flight as I was descending to the airport? My theory is that I was going pretty fast and the ram air forced some air into the tank and equalized the pressure: the connection isn't completely loose - I could still blow air into the tank if i blew hard enough. So my initial thought that this is heat related loses some credibility.

Anyway that's the working theory for now. As I have to fix the vent connection anyway, we will see if that was actually the problem.

So this weekend I drain the tank, remove the large access plate, and see if I can get a wrench onto the connection.

Perhaps a review is in order:
- It makes no difference if the vent line inside the tank is below the fuel level, other than if it is below the fuel level you will syphon vent gas out of the tank when the fuel expands (typically after filling and the sun hits the wing). As long as there is a differential pressure between the inside of the tank (when your fuel pump suck out fuel) you will draw in an equal, by volume, amount of air into the tank via the vent. If the vent is below the fuel level the air will just bubble up to the top of the tank.
- As you heard sucking sound when you removed the gas cap, I would first look to a clogged vent line as your root issue. While you may have been able to blow air into the line that may not mean you do not have a clog.
- I would think a partially blocked vent line would explain why your fuel pressure indication came back on decent - at the lower power you were drawing less fuel so the tank vacuum was not a great.
- I partially clogged line would also explain your "If I blow hard enough into the vent line" issue. If the gas cap is off it takes little to blow air into the tank via the vent line, unless the vent line is clogged.

Carl
RV-10 with 3/8" vent lines that have screens across them to keep out mud dabbers, and a check valve inside the wing so the vent can suck in air if the outside gets clogged with ice.

[Saville]

Originally Posted by Carl Froehlich

Perhaps a review is in order:
- It makes no difference if the vent line inside the tank is below the fuel level, other than if it is below the fuel level you will syphon vent gas out of the tank when the fuel expands (typically after filling and the sun hits the wing). As long as there is a differential pressure between the inside of the tank (when your fuel pump suck out fuel) you will draw in an equal, by volume, amount of air into the tank via the vent. If the vent is below the fuel level the air will just bubble up to the top of the tank.

I don't see how of the vent line is disconnected. If there's no clear bath from the end of the vent line to ambient, how does the pressure get equalized?.

- As you heard sucking sound when you removed the gas cap, I would first look to a clogged vent line as your root issue. While you may have been able to blow air into the line that may not mean you do not have a clog.

I reported that I blew air through the line. Both air and fuel got through. Because BOTH air and fuel got through, I suspect a bad connection. Though is some ways that may behave like a clog. Also don't forget the bubbles..

- I would think a partially blocked vent line would explain why your fuel pressure indication came back on decent - at the lower power you were drawing less fuel so the tank vacuum was not a great.

I tried several different power settings at altitude including idle and the pressure indication did not return. That explanation doesn't hold.

- I partially clogged line would also explain your "If I blow hard enough into the vent line" issue. If the gas cap is off it takes little to blow air into the tank via the vent line, unless the vent line is clogged.

You are forgetting (or may not have read the post) that I could plainly hear bubbles coming out of the vent line at the inboard tank wall. This would not happen if there was only a clog.

Carl
RV-10 with 3/8" vent lines that have screens across them to keep out mud dabbers, and a check valve inside the wing so the vent can suck in air if the outside gets clogged with ice.

I wonder why Van went to the trouble of designing such a complication venting system that requires plumbing through the tank and then the fuselage.

Tough to describe this situation in words so if you will pardon my crude drawings.

The system as it is supposed to be (as I understand it):



I've drawn the tank in grey and tilted it up exaggerating the dihedral of the wing. The left side of the drawing is against the fuselage - the right side is towards the wingtip.

The red lines represent the vent tubing and the green line the fuel line. The blue line in the upper corner of the picture is the level of the fuel in the tank - everything to the left in the tank is fuel. Note that the vent at the upper right end is in the air pocket.

When the system is at equilibrium, fuel pump off, the air pocket is at ambient pressure. This is because there's a direct and clear path to the ambient air via the red vent tube.

Now I turn on the pump, fuel comes out of the tank. The air pocket expands and lowers the pressure in the air pocket. Because the pressure is lower than ambient, the air flows from outside and brings the pressure in the pocket up to ambient.

Fuel pressure stays constant.

This is what I think the situation is in my tank:



The line is open at C - inside the tank.

There is no longer a clear path from the ambient air to the air pocket at the other end of the tank.

At equilibrium, say after I open the fuel cap, the air pocket is at ambient pressure.

Now turn on the fuel pump.

Gas is pumped out of the tank.

The air pocket grows larger and the pressure of the air in that pocket is reduced.

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.

And I know it's not connected because I could hear bubbles coming out of it when I put my ear on the wing skin above "C".

Maybe I do not understand the system, please explain how air from the outside can get into that air pocket if the vent is disconnected. If so I'd appreciate being educated.

Thanks

[Saville]

Originally Posted by lr172

This is incorrect. As that tank develops a negative pressure (i.e. vacuum) it will pull air from the external, higher pressure source even if that source is submerged in an imcompressable liquid. The liquid doesn't need to compress in order to let the air in.

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.

[jpowell13]

I have a Trutrak EFIS and the same sensor manifold as you. I just added a second VDO sensor (using a 45 degree NPT fitting) to my manifold and a 1 1/4" UMA fuel pressure guage as a backup for my EFIS fuel pressure. I like my Trutrak but have backed up all its essential functions in case it goes out on a cross country since they don't make them anymore. John