Air bubble of doom in gascolator!

[hilltop170]

Unless I don't understand how the fuel flows into the carb, vacuum does not pull fuel into the bowl. Head pressure from the wing tanks pushes fuel thru the fuel lines, valve, gascolator, and in this case a fuel flow turbine meter as well. There is no vacuum anywhere between the tanks and carb bowl, there is always positive pressure as long as there is fuel in the tanks and the flow path is open. If there is a blockage somewhere, when the fuel in the bowl is exhausted, the engine will just quit, it cannot suck fuel into the bowl.

I would disconnect the fuel line at the carb and run gas into a bucket and measure the flow rate. Any blockage should show up by doing that. The only other wild idea to create vapor in the gascolator I can think of is if an exhaust leak leak was impinging directly onto the gascolator, but even then the normal fuel flow should pass it right on thru to the carb. I'm still baffled by the description of the issue. I could see it if there was a fuel pump sucking fuel thru the system and a small leak occurred somewhere allowing air in, but with head pressure only, I don't get it. I thought 48 170s had an engine driven fuel pump.

I have the same fuel flow computer and have never seen any issue with it restricting flow. Can't see where that would cause any problem.

[bat443]

I'm going to start off on another track, please bare with me, and as a group please do not turn this into an avgas, auto fuel debate. I think you may have a fuel vapor problem. What was the density altitude in cruise when this has been happening? The fuel temperature? Even though the outside air temperature was fine at cruising altitude had the fuel in the tanks been raised to a higher than normal temperature by the airplane setting out in the sun for several hours. This would make the fuel more prone to forming vapor bubbles which would collect in the fuel strainer. Keep in mind that the fuel vapor pressure of auto fuel is normally lower than that of 100LL and that auto fuel blended for lower elevations has a lower vapor pressure. If any of these conditions seem to apply I would try a cautious test flight in the morning when temperatures are lower and the fuel in the airplane is cooler. If using auto fuel, you might try 100ll. I would also look for any lines which had been dented or had a bend radius reduced during removal or installation as this will cause a venturi effect and a localized drop in pressure in the fuel line which may allow vapor form. I don't know if this will help but you have already address the easy stuff.

Tim

[n2582d]

I'm going to start off on another track, please bare with me ... .

It's getting a little chilly up north for that, don't ya think?

[4BravoWhiskey]

I'm not using auto fuel. This has happened in high density altitude and low density altitude. It has never been a problem in the 1.5 years I have flown this plane, until just recently on my way back from Alaska. I flew almost 100 hours since the fuel flow transducer was installed, all since June. So a lot of flying in a short time, everything from 12,000 ft density altitude to -1500 ft density altitude, with no issues whatsoever, and temps ranging from over 100F to as low as 30F. Then this started. The first time I noticed it was in Utah after the engine roughness event and density altitude was probably about 9,000 ft, temps around 85F. Then upon landing in Texas and the last few hours of flight was at lower MSL, but maybe 6,000 ft density altitude and 80F, although I wasn't recording these parameters. Then after a 45 min flight near the home base, and it was around 2500 ft. density altitude and around 75F outside. I've flown in these conditions many many times with no issues.

I think the proper course of action starting tomorrow when I can finally get back to it is (and I welcome input on this):

1. Disconnect fuel line at carb, measure flow rate from left, right, and both. Should be at least 15 gph (I think). Run at least a few gallons though. Watch for any air bubbles forming in gascolator.

2. If any flow issues detected, check vent system (repeat with fuel caps off, for starters). If not, proceed.

2. Remove fuel lines between gascolator and flow sensor, and flow sensor and carb. Visually inspect.

3. Install new fuel line that bypasses flow sensor. Repeat #1, compare.

4. Replace gascolator gaskets with new. Repeat #1, compare.

5. Ground test with engine running - idle, run up, taxi, etc.

6. Drink a cold beer. Repeat.

I really hope I can replicate the problem on the ground. If not, then I'll eventually have to do some more test flights, close to the field and prior to step #6.

[c170b53]

I think its great to hear from various sources as to what the issue is. I think we are all curious and an issue such as this one, can even really help with an individuals post count
I looked into the install paperwork and EI mentions that depending (I think on length versus rise) on the fuel line install, a bend in the fuel line might be required to position a potential air bubble away from the F.F. transducer. Likely an air bubble forming around the transducer would cause it to err greatly. So I guess in even a simple install there's potential problems.
It's hard to imagine an air bubble to traveling down the lines to the fuel valve, After the fuel valve, the line is parallel to the lower fuselage out to the strainer, then after, the line goes with a slight rise to the carb bowl where you would think, with good fuel flow any air would be taken to the fuel bowl. Wonder if the there's a small restriction is in the fuel valve and if the fuel line dips after the strainer before it rises to the carb.
Then Again I like the simplicity of a vapour bubble.

[GAHorn]

I believe that the problem is from excessive length of the line from the gascolator to the carburetor. It will droop too much, there is not enough flow during normal operation to push the air through but with the line disconnected at the carb there is enough flow to push the air through the low spot and out.

There should never be "droop" in the fuel line from gascolator to carburetor.
The purpose of the gascolator is to allow water/sediment to settle-out such that the carb only gets clean fuel, but if there is a "droop" in the line....then a low spot may allow water to settle in that area and disrupt fuel-flow. From gascolator to carburetor the fuel line should only rise.

[4BravoWhiskey]

No droopy hoses here. Or on the airplane. Pretty straight shot with just enough flex.

I checked flow today. Disconnected line at carb. Kept it as close to normal position as possible.
left only: 33.0 gph
right only: 33.0 gph
both tanks: 41.8 gph

I think there is no venting issue and no flow restriction with those rates. I checked the fuel hoses and they look totally smooth and clear. Collected about 8 gal and didn't see any new air accumulation in gascolator.

Repeated with tail raised. flow rates increased by about 2 gph. that was the only change.

???!

[GAHorn]

We need to get together in-person to sort this out. (However you probably don't want to wait until December.... My previous employer has found they can't live without me and made me an offer I can't refuse to come back to DFW for two months to train my replacements. HA! If only they knew....!

[4BravoWhiskey]

Yes that would be great! Fly over to 50R tomorrow. I'll counter your former employer's pathetic offer with lunch at Black's BBQ and a bag of potato chips. I know they can't beat that!

[hilltop170]

If there will be bar-b-que and airplanes involved, I would like to be a part of that!

[4BravoWhiskey]

Come on out Richard! It's a beautiful day, especially if you have a working airplane.
It was great to meet you at your Alaskan home base, 'bout time we got together in Texas!

Jack

[hilltop170]

Jack-
I'm in Dallas today and tomorrow for my 45th high school reunion. Any time after that will work.

[FredMa]

Always having to use exscessive prime/accelerator pump not working, engine cutting out, air in gascalator that does not discipate through to the carb as it should with normal fuel head pressure from the tank: sounds like a malfunctioning carb/float issue to me.

[4BravoWhiskey]

It does sound that way, but I don't know what's normal for these engines in terms of starting. A shot or two of primer does the trick for me, usually.

And the only time the engine ever started cutting out was the one time over Utah. It normally runs just fine. It will develop an air bubble during normal flight with no signs of any engine roughness or anything.

I'm going to try and post a picture of the fuel lines from gascolator to carb.

[counsellj]

Throw a Go-Pro under the cowl to film the gascolator in flight and see what it is doing.