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Aux Fuel Tanks Not Feeding

Hello all, brand new to the community. Approximately 1 month ago I purchased my first plane, a 1957 V-tail Bonanza H-35. Plane has 20 gal mains, 10 gal aux, and 20 gal tips. She's always been hangared but the owner got sick and stopped flying so she's been sitting for about 15 years with full tanks (except tips). I bought it with a fresh completed annual and inspection passed. Anyway, I've taken her up 5 times both with my instructor and without. Each outing, we have switched to the aux tanks. Every time we switch, there's no fuel pressure and engine dies. When switching back to mains, the engine comes back to life. Once the switch was made to aux, we only gave them about 10-15 seconds to feed to the engine in the hopes that it would start, except this last time. I was solo on a 1.5 hour flight cruising @ 8,500. Decided to try aux again knowing that it would fail, but with the intent of giving it more time to try feeding the engine. I figured that since it had been sitting for so long, there could be air in the lines that would need purging. I made the switch to aux and as predicted there was no fuel pressure and the engine died. I tried using the electric fuel pump, nothing, I tried using the wobble pump (yes it's still installed), nothing. I waited for an ETERNAL 45 seconds before switching back to mains. Engine didn't start right up, but once I used the wobble pump to get a little pressure, all was good again.

Aux tanks are full, they sump fine, and there are no leaks that I can tell. Is there something that I'm missing? What could be the issue? Thank you in advance for your support.

Model: Bonanza H35
Posted 9/11/2023 - 5 months ago
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The first thing I would suggest is to verify that the vent system to the AUX tanks is clean and clear. One vent tube on the right lower wing surface feeds both AUX tanks. The second thing would be to replace the fuel cap inner and outer seals on both tank caps. If the caps are not sealing completely the low pressure area on top of the wing in flight will suck air out of the tanks and not allow the fuel to feed the engine.
I have attached a diagram for the vent system, Item #41 is the vent tube on right wing.

Bob Ripley
ABS Technical Advisor

Posted 9/11/2023 - 5 months ago
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15 years is a long time. Assuming all fuel that was in the airplane when you bought it was drained and disposed of properly? As Bob says, all vents need to be confirmed clear and fuel cap seals double checked. Rubber dries out and generally needs to be replaced every 5 years or so. Your aux tanks feed together thru a check valve for each tank. These prevent back flow. If the airplane sat for that long, you can expect the fuel to deteriorate. Old fuel residue can glue the check valves closed. Recommend pulling check valves and anti-siphon valves and making sure they are clear and function as required. Do not neglect the fine fuel screen in the carburetor inlet. It could easily be blocked with debris stirred up by the first flights after many years.

Now that the airplane has been run a few times, you need to be especially vigilant. I would do a 5 hour, 10 hour and 25 hour oil change and cut the oil filters open for inspection each time. Any corrosion on the cylinder walls, camshaft or cam followers will make it self evident in the oil filter.

Posted 9/11/2023 - 5 months ago
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Thank you Bob & Steve. I failed to mention that the engine was replaced just before it was stowed away for so long. A brand new Continental IO-520 was installed and there is no longer a carburetor. Additionally, before it was "put away" the owner pickled the engine. We scoped out each cylinder and they are pristine. I will still do the oil changes as suggested, but I'm willing to bet that the filters will be clear of debris.

The "gluing" closed of the valves makes sense. Since the tanks have bladders, I'm sure that old fuel was stored with the plane for who knows how long. I do know that all fuel was drained and new gas put back in when the annual was completed in July of this year.

Posted 9/11/2023 - 5 months ago
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Brian, I have to say I am amazed that someone actually followed Continental’s excellent long term storage procedure. That should be a great airplane!

Posted 9/11/2023 - 5 months ago
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Brian, you mention that you have tip tanks. I asume they are the old style Osborns as you mentioned 20 gal tips.
The fuel selector probaly needs to be selected to AUX and the tips turned off to burn off you wings auxs. This is done with the right tip selector. If both tips are selected in the on position you will be feeding from the tips.
Hope this makes sense.

Posted 9/11/2023 - 5 months ago
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Carl makes a great point. You should have a POH supplement that calls out exact procedures for selecting fuel tanks. This should include switch and valve positions, as well as the order in which the tanks should be used. I have seen some “creative” tip tank plumbing configurations…

Posted 9/12/2023 - 5 months ago
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What happens when you access the drain site to drain the sumps.

With the aux tanks there should be a common drain for the mains and an aux drain aft of the main tank drain.

When you turn the valve for the aux tanks, does fuel come out?

Posted 9/12/2023 - 5 months ago
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Thank you all. Let me answer these in sequence.

Carl - The tip tanks are 20 gal Brittains and each time we have tried to use the aux tanks, the tip selectors were in the off position. Also, they do not have bladders in them so they are dry as they should be. I have not tried using those yet as I'm not sure where they feed and if they feed into the aux tanks, they won't help anyway.

Steve - Unfortunately, although the previous owner was super great at maintenance (he was the planes mechanic), he was not very good with POH updates. I have scoured the POH and there is NOTHING in there about fueling procedures. I do have a page in the tips installation manual which I will put in the POH once it is determined where the tips feed.

Tom - There are individual sumps for each main and each aux. As far as the drain pipes, there is a panel above which 2 pipes with valves are positioned. These would be the drains that you mention, one for main and one for aux. I have actually not tried draining from either of them yet as every time I'm with the plane I don't have a collecting receptacle. That is next on my list tho. Probably safe to assume that the mains will drain fine. If one drains and one does not, clearly I'll know which drain is which and I'll label them. If they both drain, it will be a bit more to figure out which is which. However, since all fuel was replaced in the plane during the annual in July, I can only assume that both drains will flow freely.

Posted 9/12/2023 - 5 months ago
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You should be checking the fuel for water prior to each flight.

The lowest fuel point in the plane is at the access door on the left under the wing.

It is easily opened. The point in front is the bottom of the main fuel pump. It is opened by turning the bottom valve about 25 degrees. You do not need a tool to check it. All you need is a pan to collect the fluid. I drain about 4 ounces from this site.

The drain immediately aft of the main fuel pump is a valve which is vertical. You turn this one about 180 degrees and allow about four ounces of fluid to drain.

If there is water coming out of either drain, you will be able to tell the difference because the fuel will float on top of the water.

IF no fuel comes out of the one behind the main fuel drain, this might explain why you have a problem. No fuel is getting from the aux tank to the fuel pump.

If this is the case, you should have a mechanic remove your seat, the floor boards in front of the seat and then the cover of the fuel selector, etc. Then you should have good access the the fuel pump, the fuel cell selector, and its connections.

Then you can figure out what is wrong. All of the fuel has to go through the fuel selector.

Let us know what you find.

OH. When everything is opened up use your cell phone and photograph the whole thing. When I had to have my fuel selector overhauled, I labeled every pipe with a Letter (such as A, B, C, etc) and then photoed the whole thing. It is a night mare. of tubes there.

One other thing, consider having your electric fuel pump also overhauled as long as you have that area opened up!!!!!

Another thing. Have fuel line caps available to prevent drainage of the fuel into the cabin. All of the fuel cells are higher than the fuel selector.

Tom

Posted 9/12/2023 - 5 months ago
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Brian, I may suggest that you select the wing aux tanks on the ground and run the engine and see if it will run.
Then check to see what the fuel flow is and compare it to each main tank while on the ground.
If it runs good on the ground with the Aux selected with fuel flow matching the main tanks, it would be a vent issue I believe while air is traveling over the aux vent.
I have been here on my J35 with 20 gal tip tanks and the right tip would not feed in the air but ran great on the ground.
I found that the vent was slightly bent and causing a vacuum .

Keep at it we will find out why!

Posted 9/12/2023 - 5 months ago
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Aux Fuel Tanks Not Feeding