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Tank Switch Cough....

Bluelabel

Well Known Member
Wondering if anyone else has experienced this...
It's not consistent and I can't duplicate it.
Once in a while, when I switch tanks, it's almost as if the engine is without fuel for about a half a second. It comes right back, but at 160KTS, it will sure get your attention. Always scares the **** out of my wife and I. She won't watch when I switch tanks:eek:
On the 23 hours of flying to and from Oshkosh, it probably happened 3 times.
It does seem to always be when at altitude (=>10k)
I haven't documented the exact situations when it happens, because, to be honest, we are usually looking for clean underwear for a few minutes after it happens.
I thought I had the issue solved last year when I added an item to my runup checklist. "1800 rpm, toggle fuel tanks".
I always switch tanks very fast and smooth.
Andair duplex valve is installed per plans. I've checked for leaks. Nothing I can find.
Rich or lean has no effect.
I went over the issue with the Andair guys and went through inards of a valve at Oshkosh. Can't figure out what would cause it.
Only thing I can think is there is some situation causing an air bubble in the system. With our fuel system, we do have about 40 gallons/hour moving around the system at any time.

Anyone else had this issue?
 
No, but my SOP is to turn on my boost pump before I switch tanks and let it run for about a minute after.
 
Echoing Auburntsts .. in the Cherokee I'm flying now if I don't enable the boost pump before switching and leave on a few seconds after switching it will cough/complain.
 
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Maybe I should have mentioned. With an electronic fuel injection system, there is no mechanical pump. I have pump 1 and pump 2. Pump 2 is back up and automatically kicks on if the fuel pressure drops below 22psi. My normal PSI in flight is around 28-30
I do have the ability to manually turn on both pumps at the same time. I may make that part of the procedure.
It's still puzzling as it's wildly inconsistent.
 
Shut the fuel off for 1 minute each flight for 10 flights, after that you n the wife will never notice that cough again.:D
 
Mine does this also, just switch tanks real fast, you are closing and then opening the ports during this movement and our fuel system requires constant fuel pressure to the injectors.
 
Is there a possibility that the electric pump could suck air in the process of switching tanks that could cause the cough? I know it shouldn't be able to, but a loose line or fitting could cause something like this, especially when pulling so much fuel through. Also, something that comes to mind is that you have a small amount of fuel from the unused tank that is sitting in the tunnel for a long period of time before being used, which could be warming up slowly to the point that you switch and the suction drops the fuel pressure in that line below the vapor point and it gives you a very temporary vapor lock which the pump is able to overcome quickly but not immediately.
 
I wonder if while flying, and the non used tank is low, and you encounter turbulence, could the pickup in the tank become momentarily un ported and allow some fuel to empty out of the pick up tube and allow a small air pocket to remain in there until that tank is selected and then you have your momentary air pocket getting circulated through the fuel system?
 
Is it always the same tank?
That's what I need to determine. Like I said I'm always a little freaked out. I'm going to start documenting it.

I wonder if while flying, and the non used tank is low, and you encounter turbulence, could the pickup in the tank become momentarily un ported and allow some fuel to empty out of the pick up tube and allow a small air pocket to remain in there until that tank is selected and then you have your momentary air pocket getting circulated through the fuel system?

It happens even when both are full.
But glad to hear it not just me. ;)
Is there a possibility that the electric pump could suck air in the process of switching tanks that could cause the cough? I know it shouldn't be able to, but a loose line or fitting could cause something like this, especially when pulling so much fuel through. Also, something that comes to mind is that you have a small amount of fuel from the unused tank that is sitting in the tunnel for a long period of time before being used, which could be warming up slowly to the point that you switch and the suction drops the fuel pressure in that line below the vapor point and it gives you a very temporary vapor lock which the pump is able to overcome quickly but not immediately.
Good info. Maybe on the ground I'll let each side run a little longer. Generally I climb out and if I'm going high (which usually do) i switch tanks after I level off.
 
Mine does this also, just switch tanks real fast, you are closing and then opening the ports during this movement and our fuel system requires constant fuel pressure to the injectors.

Bret,
Does your do it randomly?
 
pressure

I thought I read that the fuel pressure was to be set at 35 psi. Not flying yet but I think I have the same system...
 
28psi with electronic injection? Is that what Ross recommends? Typical for auto style inj is 42-60psi.
It's not Ross's system thank god. This is the PSI that Robert at EFII recommends.(I'm running slightly above actually) And with over 400 planes flying, I'll trust that setting. But that's not the point of this thread.

as I said, it's super inconsistent. In 150 hours of flight it's maybe happened 8 times, but it's still no fun.
 
I thought I read that the fuel pressure was to be set at 35 psi. Not flying yet but I think I have the same system...

35 static... once flying, MAP will change the pressure.
I may bump it up a bit and see if it changes, or just run both pumps when switching.
 
If you electric fuel pump is on all the time, that leaves only an air leak (or fuel leak depending on how you look at it) or some issue with the added complexity of the duplex fuel selector valve.

Look for some blue stains.

Carl
 
With a fuel flow of 40 gal/hr I think this is unavoidable, unless you can get a "make before break" valve (e.g. Going from left to right there is always fuel flowing).
 
If I may---think about this--you're running on tank 1. Tank 2 doesnt necessarily have fuel all the way to the selector valve. (hopefully it does) So when you switch tanks, the pump has to pull full from tank 2 through the lines, through the valve, through the pre filter, to the pump, then under pressure of 40+- psi ( or whatever its set at) to the outlet hoses, to the injector rails and then the injectors. Even with GREAT suction of the pump, there 'may' be a slight lag in fuel delivery. Like filling up your garden hose with 35 psi of water pressure.

There 'may' be a situation where vacuum in the tank(s) are drawing fuel back from the selector valve to the tanks, creating this dead space that has to be filled.

Tom
 
Fuel pressure

John,
I believe you are running your fuel pressure too low. I run my system at 50PSI for an IO-390. The standard pressure from Ross SDSEFI is 40PSI. I understand that you are using EFii, but physics is physics and automotive injectors typically need more pressure. Your static pressure and inflight pressure should be the same. Do you have your fuel pressure regulator hooked up to your Manifold Pressure?
 
Pressure

35 static... once flying, MAP will change the pressure.
I may bump it up a bit and see if it changes, or just run both pumps when switching.

Hello John,
Your fuel pressure static and in flight should be constant.
 
Have you pulled your EMS data and looked at your fuel pressure coinciding with the known event?
 
I'm a neophyte to aviation systems but these type of problems are solved with header tanks or swirl pots in other applications. Some added weight and plumbing complication, maybe six pounds including the tank/pot and plumbing.
 
My RV-10 has Bendix mechanical fuel injection, so this may be useless information.

I had a similar issue that was resolved by one (or both) of two changes. I moved my fuel flow transducer from the tunnel where it was on the suction side of the system to the pressure side of the system between the servo and spider. I did this because having it on the suction side invites vapor formation at the transducer, especially when heat soaked. While performing this work, I did find evidence of a slight leak at the outlet of the electric boost pump. I could never smell fuel--I just found a little bit of staining at the fittings. I had no more issues after those two corrections.

Best,
Guy
 
Thanks for all the input... even though I just got done my condition inspection and had the tunnel open, maybe there is a chance of a leak that will invite air yet not leak any fuel. Time to put a wrench on each fitting again.
I will also see if running both pumps at the same time helps.(the problem is the the situation happens so infrequently).
I can pull my Garmin data logs. I'll should be able to at least see the conditions of flight.
 
+1 for Jonjay and Jesse.

I've never experienced a hiccup in the 800+ hours of switching. I have a stock setup engine with an andair fuel valve.
I don't use the electric pump in the process.
 
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Here is my theory:
I assume your valve is above the tank fuel level. The fuel line is also in suction upstream the electric pump. I think you have a small leak in the line, probably at the valve connection. When sucking fuel, there is no fuel leak, just air suction, so small that the engine is ok with it. When the fuel line is inactive, the small leak is causing air to be sucked in causing venting and leveling with tank level. Now you have a air pocket in the line and next time you switch over, the engine stumbles.
 
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i've had this happen twice in my 8 years with n668g
both were on hot days running lean of peak and changing tanks without using the boost pump. i've never had it if happen when using the boost pump.
Alan
n668g
 
What does Robert have say about it?
He must have an explanation, it's his system.

We talked and he doesn't have an answer as I need to have more info about the facts when it happens... I flew last week, it was hot climbing out of Bakersfield, Ca. Hit both pumps before switching tanks and didn't have an issue.
EFIS data also doesn't show anything. For some reason, the one thing not shown on my EFIS data when loaded to Savvy is Fuel pressure. I went back and looked at a flight coming out of Arizona, where I know when it happened, and could not find any data irregularities.

I can say, that I think it only happens when hot and high. I have started using both tanks for equal time on the ground to prevent any hot fuel from sitting in one side.
I'll see if it ever happens again, and note all the facts.
 
fuel return

Hey John,

I've had a similar issue with my -6 , fingers crossed we got it figured out last week when doing the condition inspection. Somewhat different symptoms, but it may give you some insight.

I've had an intermittent issue when switching to the right tank - air was entering the fuel lines and the AFP injection system would react with a rise in rpm and fuel flow, followed almost immediately by cutting out/stumble when the air made it to the injectors. I did the tighten all the b-nuts, check the vents and look for fuel staining thing with no relief. I thought for sure it was a leak at the Andair valve letting air in on the right tank line (fuel selector is the high point in the system before the pump). But again, no fuel smell or stain to help diagnose.

At shut down before starting the compression checks this year, I had an a-ha moment when I heard the vapor pushing fuel past the injectors. The fuel return line is in the plenum also, and subject to the same heat/vaporization - could it be pushing fuel all the way through to the line and putting vapor in the system?

The only difference between the right and left side fuel systems in my plane is the fuel return from the injection system (AFP with purge valve installed). The fuel return is T'd into the supply side line on the right. The T fitting is below the mid-tank level and fuel level in the tank has never made a difference in the occurrence of the issue. I didn't consider that air could enter the fuel flow there since the fuel supply line is lower, and fuel level higher, than the T. I also didn't consider the volume of vapor that could be produced in the return line back to the supply line.

Only one way to find out. We capped the T in the supply line and plumbed the fuel return into the vent line. Checked on the ground with the electric pump only, didn't push any fuel out the vent, but it did go right into the tank as expected. Run up checks all good with normal operation.

Five flights later, with both cold and hot starts, I haven't had an issue when switching to the right tank. Like I said, fingers crossed, but I think we've got it figured out.

Long story short, if you have a fuel return line from the injection system, check how it's plumbed.
 
Kind of resolved....
I did not find an issue as far as something I could fix.
No leaking in the fuel system. Put a wrench on all the fittings and gave them a little snug.
My new OP is to hit "Both Pumps" when switching tanks.
Hasn't happened since July.

Of course it hasn't been as hot.
I really think it has something to do with the heat. I'm certainly not able to duplicate it.
 
Fuel pressure

I agree, that is a lot of engine, extra fuel won't hurt in high temps. Keep us up to date. Are your fuel pumps on the cold side of the firewall?
 
They are... even thought the tunnel in the 10 notoriously has heat issues.

I do have gold heat reflective panels on the belly right above the exhaust (which did make significant difference the the temp of the floors when flying). Plus I have the tunnel well insulated as well as the firewall.... so, I've done what I can to control those temps.
 
They are... even thought the tunnel in the 10 notoriously has heat issues.

I do have gold heat reflective panels on the belly right above the exhaust (which did make significant difference the the temp of the floors when flying). Plus I have the tunnel well insulated as well as the firewall.... so, I've done what I can to control those temps.

In addition to insulating the tunnel, I fire-sleeved all the aluminum fuel lines in there and had a split fire sleeve that fit around my Andair fuel pump, with the the filter and Red Cube transducer. Never a hiccup switching tanks, even without the boost pump ON running mogas. I used the red silicone Stratoflex stuff.
 
My vote would still be that air is getting into the unused line at the fuel selector valve. Depends a lot on the design of the valve, and I am not familiar with the Andair, so I can't help with details.
But I think a lack of fuel in the line you are switching to is the only hypothesis that fits the observations.
 
Cones

My vote would still be that air is getting into the unused line at the fuel selector valve. Depends a lot on the design of the valve, and I am not familiar with the Andair, so I can't help with details.
But I think a lack of fuel in the line you are switching to is the only hypothesis that fits the observations.

John,
Have you tried conical seals on the valve line fittings? This would address the possibility of air in the lines.
 
I'm thinking vapour lock. Where in the system is your fuel flow transducer?

Bevan.

Well, the good thing about the EFII system is I don?t need one of those. The system sends a fuel flow signal to my Garmin. Once you get the Kfactor number dialed in, it?s super accurate.

John,
Have you tried conical seals on the valve line fittings? This would address the possibility of air in the lines.

I have not, I didn?t even know that existed, but I?ll do some research on them. THX
 
Switching fuel tanks.

Never ever experienced that ( 270 hrs on RV10), Have fuel injection and never turn on fuel pump while switching tanks in either my RV 8 or RV10. Is all your system in the tunnel or part on top the engine? D Pfeiffer
 
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