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ROTAX 912ULS2 Fuel Flow Anomalies (not CT)


FastEddieB

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Guys,

 

Karen and I just did a quick round-trip to S FL in our Sky Arrow.

 

The indications I'm getting re: fuel pressure are starting to become more problematical. I think the numbers have always been low, but they seem even worse now.

 

On the way home, I used my iPhone to capture some of the behavior:

 

 

 

 

 

Summary:

 

1) Prior to the first start of the day, turning on the boost pump barely moves the needle, esp. if the plane has been sitting a while

 

2) After takeoff, the gauge shows the bottom of the green, and turning off the boost pump causes it to drop to about zero before climbing back up to the green :o

 

3) In normal cruise, I'm right about at the bottom of the green

 

4) I just started noticing that after landing, the fuel pressure shows very low, even with the engine running above idle. It may or may not have been doing this a while - its not something I generally look at while taxiing

 

I'm trying to figure out whether I have a bad boost pump, a bad engine-driven pump or maybe even a bad gauge and/or sending unit - or maybe a blockage in the line leading to the sending unit. Or possibly some combination of the above or something else entirely. The engine seems to be getting enough fuel regardless of pressure indications.

 

I have a ROTAX mechanic nearby in Dahlonega, GA (Tim Dawson, cc'd this), and will want this looked at, but I'm curious as to the indications you guys get on your CT's or other ROTAX 912-equipped planes. This will help give Tim and I an idea of what's normal.

 

Thanks in advance,

 

Fast Eddie B. (Ed Benson - Mineral Bluff, GA)

706-492-3118

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Hi Eddie,

 

You have a couple of things to rule out, bad mechanical and/or electric pump, bad cockpit gauge, bad fuel pressure sending unit or poor fuel flow.

The easiest way is to make a quick diagnostic mechanical gauge from parts at the hardware store. Get some 1/16" (smallest diameter) copper tubing and some compression fittings, a tee to put it in line with the fuel line, plus a pressure gauge that will read 10psi and below. No large diameter tubing as we only want to measure pressure and not flow. Plus if you had a leak the stream out a small tube is much less. The gauge only needs to be a ballpark and does not need to be some high dollar gauge reading to the tenth of a psi. Any standard gauge for this check will work. Put it in line and do your different rpm run ups and boost pump runs on the ground, but up to 4500-5000 rpm, just like in flight. Check the mechanical gauge against the electronic gauge. This will give you answers to all your questions in one sitting. It will tell you if the mechanical and electric fuel pumps are putting out the correct pressure. It will tell you if the gauge in the cockpit is wrong and depending on what the readings are it could tell you if you have a fuel flow issue to the carbs. If you do a check any other way it will be guess work and hit and miss. The set up from the hardware store will run $20-$30 and you will be able to use it again and by just swapping the gauge on the end to a 100psi or lower can be used to check oil pressure.

Normal or average fuel pressure for most is between 3.1 - 4.0psi. Mine is 3.1 and has been for years and some see over 4.0 psi. When the fuel pressure takes a dive in cruise rpm and you suffer no loss of power then it may be just a fuel pressure sending unit or gauge. If you are at 5000+ rpm when the pressure drops that far down and the engine doesn't drop rpm then I usually start to suspect the sending unit first. The diagnostic gauge setup is well worth doing and keeping on hand.

 

Keep us informed.

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Thanks, Roger.

 

I've had two Sky Arrow owners respond, and both said (in effect) that the stock gauge and sender were cr*p. Both had had problems similar to mine.

 

I have a Vacuum/Fuel Pressure gauge like you mentioned.

 

Here's the setup on my Sky Arrow:

 

5472042440_027527823c_z.jpg

 

Looks straightforward.

 

Looking through the Aircraft Spruce catalog showed a variety of fuel pressure gauge/sender combos, including digital, if I want to replace my setup with something more durable. I recall seeing the brand "UMA", just like the one in the photo.

 

Let me see if I can get gauge hooked up - I should be able to - I have a whole box of hoses/fittings and should be able to find something that works.

 

Will report findings as soon as I get to the airport and check.

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Hi Eddie,

 

Now that I see where your pressure sender is I'm leaning more on the sender issue side. You can just pull a hose off in the fuel line system on top and put a little 4" jumper hose back in its place and put the tee fitting I mention in line between the two hoses.

If you find it is the sender which looks like a good bet then I would remote mount that sender off the engine. Many that have done it that way have had sender failures due to vibration and heat. You can't reuse yours it's probably dead. Turn the sender hose around and mount it back on the firewall. That is what FD found out when they started having failures. They now mount the fuel pressure senders on the firewall.

 

Second little item I might mention,if I may, is that all those fire sleeve hoses should have band clamps on all the ends and the ends technically should be dipped in a product called "End Dip".

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I'll get the part number off my sender and probably try that first (assuming it IS just an indication problem).

 

Not much of a firewall on my Sky Arrow, but sure to be somewhere to mount the sender away from the engine.

 

Second little item I might mention,if I may, is that all those fire sleeve hoses should have band clamps on all the ends and the ends technically should be dipped in a product called "End Dip".

 

Maybe when you replace all my freakin' hoses in 2012 we can take care of that! :P

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Went out the the airport today, and determined I have an indication problem, not a real fuel pressure problem.

 

Hooked up the pressure gauge via a "T". Turned on the boost pump and saw this (needle barely off the stop):

 

5480490814_3806fbdcc9.jpg

 

But the pressure gauge showed a full 3 psi:

 

5480491256_35c8df1d70_z.jpg

 

I did not try it with the engine running.

 

It seems like it has to be the gauge or the sender - and I'll probably start with the sender:

 

5480490342_386aa0c53a.jpg

 

Is it a common part? It has both UMA and VDO labels When I do a Google search for the VDO part, I come up with this:

 

5479946729_162edb4bbf_z.jpg

 

Looks the same, and the price is right, but says 0-30 psi while my gauge is calibrated 0-8psi. But the selection of the Rochester gauge may take that into account.

 

Thoughts?

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Hi Eddie,

 

That's the right one.

So the heat and vibration doesn't kill it again remote mount it some where low and off the engine. I was pretty sure it wasn't really a fuel flow issue from what you said. Number one issue to always look at is the sender first and the best way to diagnose it is with a tee in line with a mechanical gauge. Usually I keep a new spare in the shop and try it first since it's fast and easy and it's usually the problem.

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Hi Eddie,

 

That's the right one.

 

I'll order it online today. The price is right, so if that IS the problem (and I assume it is, though it could also be the gauge, which another Sky Arrow owner says is equally flimsy), I'll order a second as a backup.

 

In my high mounted pusher configuration, relocating to something "non-engine" may not be so easy. I would certainly need a much longer fuel line and fire sleeve, and would probably need to fabricate an extension for the wiring.

 

1) Can you help me out on the hose and fire sleeve? Part numbers and a good source?

 

2) In lieu of all that, how about using some shock mounts to at least help with the vibration - something like this:

 

j6984-64.jpg

 

or even something from these guys: http://www.banaire.com/shockmounts.html.

 

Thanks - I hope to get to this later this week when the part arrives.

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Hi Eddie,

 

Our (CT) fuel senders used to be mounted right on top of the fuel cross over tube between the carbs if you had a Dynon. Vibration used to kill them. All FD did was to do exactly as you described. Just get a longer piece of hose and fire sleeve. The fitting on top where the hose attaches is not metric. It is 1/8" NPT thread. Just get a male 1/8" NPT-barb end for one end of the hose and the other get a female with a barbed end. You can use an Adel clamp to hold the sender where ever you relocate it to. To help insulate it from the heat use a fire sleeve wrap, a piece cut up the middle and wrapped around it, then use wire ties to hold the wrap in place. You may have enough wire on your connecting wires already if you just unwind the two wires and stretch them out. If not adding a little extra wire is easy enough to do. By the way the oil pressure sender threads are 1/8" NPT if you ever needed to use you mechanical test gauge setup on it or want to relocate that sender which some do and again because of vibration. Fire sleeve can be had through Aircraft Spruce. You can also call Lockwood as they have a little kit for this they made up originally for the CT, but should be fine for what you are doing. It has everything you need. The have the sender too. You could use your vibration isolator with an Adel clamp on an engine tube, but if you can get it off the engine and down low out of the heat all the better.

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Thanks, Roger - I'll call Lockwood in the morning.

 

On a vaguely related note, we talked about float bowl gaskets before. Mine have been reused at least twice that I know of.

 

I was shocked to find they're $16 each in the Lockwood catalog. Google came up with some "nitrile" gaskets for the Bing 64 for $9 each:

 

http://compare.ebay.com/like/320617413157?ltyp=AllFixedPriceItemTypes&var=sbar&rvr_id=213937361363&crlp=1_263602_304662&UA=M*S%3F&GUID=08af7c0c11f0a040b145c045fffdc8f7&itemid=320617413157&ff4=263602_304662

 

Would those be worthwhile? Seems like rubber would be more durable than cork if that's a suitable replacement.

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Hi Eddie,

 

Interesting. I have never seen Nitrile rubber Bing 64 carb bowl gaskets before. I don't know of a reason they won't work. Nitrile is okay around ethanol. The newer gaskets aren't cork, but a fiber gasket. I get mine from CPS. They are around $15. I like the fiber gaskets better, they just seem to be more solid. Most of the carb bowl leaks I seem to get in is from the cork gaskets. If you don't use the rubber gaskets then ask for the fiber ones. That's all CPS carries. If you do get the rubber ones let us know how they work.

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The gaskets I have are probably fiber - the "cork" thing was just a mind f*rt taking me back.

 

I don't plan on dropping the bowls any time soon, but I may order a pair of those nitrile ones to see how they compare.

 

BTW, $15 for a gasket is kinda outrageous. I'll have to check to see if the ones from a BMW dealer are the same and how much they cost.

 

ETA...just to show I'm not totally crazy, the BMW ones list as cork and about $3:

 

http://www.maxbmwmotorcycles.com/fiche/DiagramsMain.aspx?vid=51689&rnd=03192010

 

Click on "fuel preparation" and scroll down to the exploded diagram. I'm pretty sure the gaskets are the same, since I installed the independent float option on my R100GS and I think the Bings on the ROTAX evolved from these.

 

BMW MOA Rally, Missoula, MT many year ago, installing the newfangled float bowl and floats:

 

5482570017_64b1574931_z.jpg

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1) I ordered a pair of the "nitrile" float bowl gaskets for $18 + .50 shipping. I'll let you know how they look when they arrive.

 

2) I ordered the VDO sender from Lockwood. The "relocation kit" was $75 and seemed to have a lot of extraneous parts, so I didn't order that. Really all I should need is the longer fuel line and fire sleeve - I already have all the little fittings and mounts in place.

 

3) If I just want to order the fuel line and fire sleeve from Aircraft Spruce or Wicks, would you happen to know the exact dimensions of each, or even the part numbers? I figure if I get 6' or so I'll be able to run the hose wherever I want to locate the sender away from the engine.

 

4) The Aircraft Spruce catalog says the sender unit has tapered threads and no sealant should be used - for proper grounding. Can you really get away with no sealer of any kind on the threads?

 

Again, thanks so much for all the help.

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Hi Eddie,

 

The Rotax manual's say to use a little Loctite 243 which is only a medium strength thread locker and not a sealer. Yes you can get away without any thread sealant. You may want to try it this way first. Absolutely do not use Teflon tape on this. The ground for the sender is with the contact to the engine body. I have used Loctite 567 thread sealant with success and no affect on the grounding. The 567is a very good product for sealing all hydraulic pressure fittings. It is recommended by Matco for all brake fittings and works very well to seal fuel threads, too. Any time I have a trouble spot that oozes liquid I use a little of the 567 and I have never had a leak. It doesn't dry hard so disassembly is easy.

You can buy the 1/4" ID fuel hose locally, nothing magical about that hose and the firesleeve is number 10 in size. Get however long a piece you need. Use clamps on both ends. Technically the fire sleeve should have "End Dip" on the ends of the fire sleeve hose which prevents them from wicking up oil or fuel.

 

http://www.aircrafts...es/stratofs.php

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Thanks again, Roger.

 

I guess we could just dialog via email, but I suppose others here might eventually have similar issues and be interested.

 

My plan is...

 

1) Buy some fuel line locally (I assumed it was something "special"*)

 

2) Wait for the sender to arrive, install it in the current location and be sure that's the problem.

 

3) If it is, find a location off the engine for the sender, mount it and run a line from the "T" back to it**.

 

4) Then go ahead and order the proper length of firesleeve and install it

 

5) At some point in the future, secure the ends of the firesleeve properly (ALL the firesleeve!)

 

 

*I was surprised to see that my system just uses barb fittings and clamps. I'm sure they're fine, but not what I'm used to in other planes

 

**Rather than run a long hose from the existing T:

 

5472042440_027527823c_z.jpg

 

...I don't see why I couldn't eliminate the existing "T" with a straight hose run, and then put a "T" pretty close to the carb in the picture:

 

5489631092_35a76175a2_z.jpg

 

...and run a much shorter hose to a location to the right in the photo. From the little I know about pressure dynamics, the pressure anywhere in the system should be the same, right?

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Eddie,

 

Just run a little longer hose from that tee back to the fire wall. Use an Adel clamp to secure it. Your done. Mount the sender at the highest location on that firewall and that will help with allowing the fuel to drain away from the sender and keep the small orifice clean and clear. If it hangs down like the FD relocation 91 oct. over a year's period may cause some fuel psi fluctuations.

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Mission Accomplished!

 

I hope...

 

The VDO sender* arrived Wed. I had a chance to spend some time in the hangar yesterday.

 

Just swapping the senders and turned on the boost pump and, voila':

 

5495202629_5ebec6fc15.jpg

 

Not quite the 3 psi that showed on my impromptu pressure gauge setup, but a lot more than before (barely off the peg, if you recall).

 

I then removed the bracket and mount from the existing location. Removed the "T" from where it was and replaced that hose with a longer one:

 

5495795640_d55a9d57f9_z.jpg

 

I will order some firesleeve to eliminate the gap in the middle and think about properly sealing all the ends when I need a future project.

 

Found what I think is a decent off-engine location for the gauge (not the final wire routing):

 

5495203779_454b08e575_z.jpg

 

Mounted for now with one existing screw - if this works out I'll drill for a second.

 

I then inserted the "T" close to the Bing on that side and this is the final result:

 

5495204159_f06707c640_z.jpg

 

Will need a small piece of firesleeve there as well. I rerouted the wires for the gauge so as to not need any extensions.

 

Checked for leaks and appear to be good to go (did not run the engine yet).

 

Do you see any potential "gotcha's"?

 

Two things I want to verify...

 

1) I assume the aluminum bracket shown here was just to hold the sender and did not serve any other function

 

2) It appears the hole that's left in the "nose" of the case is a blind hole that I don't have to worry about - there's an apparently identical unfilled hole on the other side. Am I correct?

 

5495795046_366ebefea4_z.jpg

 

5495795324_bfc618e80c_z.jpg

 

Anyway, thanks for all the info and encouragement. I hope some of this was relevant to CT owners.

 

*BTW, I think it was Helen Woods who said the VDO ships with a warning to NOT use it for fuel applications. She's right. Any thoughts on bringing this up to the FAA? On the one hand, I hate to make waves. On the other, might there be a real danger when a part knowingly used for an improper application?

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Hi Eddie,

 

Looks like your good to go. It's all relevant. The same dog by a different name. The bracket was just for your sender. The hole is a mounting hole for something else so don't worry about it. I bet the other psi loss is in the gauge itself.

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I bet the other psi loss is in the gauge itself.

 

At some point, for about $152 I could try this:

 

10-02877.jpg

 

From Aircraft Spruce. Says you can calibrate it for a wide variety of senders, including VDO.

 

Probably overkill, unless my gauge fails. After all, what's really important is that there's fuel pressure, not knowing to the nearest tenth exactly what it is.

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I got out to the hangar yesterday and prior to buttoning up the cowling I did a runup to check for leaks.

 

Eureka!

 

web.jpg?ver=12996775560001

 

I don't think I've ever seen fuel pressure indications so high, not even when the plane was new*. Hopefully the little sender will be happier and longer-lived in its new location.

 

Thanks again, guys, for the guidance.

 

*I stand corrected. I found this photo of one of my first flights - note the fuel pressure then:

 

web.jpg?ver=12994991450001

 

Memory is a funny thing!

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Fuel pressure senders should not be mounted on an engine. Even Dynon reiterates this. I glad it worked out for you Eddie. FD CT's have had this problem too early on when the sender was mounted on top of the engine for the Dynon units. Excessive heat isn't good either and should be considered when mounting. I have replaced several oil and fuel pressure senders on 912's.

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