Air bubble of doom in gascolator!

[hilltop170]

Are you sure the problem you had in Utah was because of the vapor bubble in the gascolator?

[4BravoWhiskey]

BL - yeah, sorry. I wish I could have double checked sooner, but just didn't have a chance to do it. Everything else in here is accurate, to the best of my knowledge.

Richard - No, I can't be sure of that. My mechanic told me at the time there should not be an air bubble in there (I called him from Blanding), so it seemed to be the culprit. He also questioned me a lot about carb ice. When I noticed the problem I pulled carb heat and it reduced rpm (as expected whether there is carb ice or not), but did not improve things. I don't know how long I held it on, but I think plenty long enough to know. Probably 20 sec. Maybe that's not long enough, but I have a carb temp gauge and it was in the green. Not that those are foolproof, but it was also an extremely dry day in the desert, and even though you can still get carb ice when it doesn't seem likely... it doesn''t seem likely. But who knows, maybe that was it.

No matter what, I still don't understand air in the gascolator!? Would like to know how it gets there, and whether it's more common than people think.

[krines]

Just read through the entire thread. What a mystery. The only thing I could come up with and I mean no offense by this but how are your piloting skills. Do you find yourself having trouble controlling the airplane. Brief periods of inverted flight could result air getting into the fuel lines. Just kidding. Eagerly awaiting the solution to this one.

[canav8]

Jack. I posted earlier. Did you replace the gaskets with known new? Do you have a chip in the glass? Did you cap the primer and check for air? Inquiring minds?

[4BravoWhiskey]

I did replace the gaskets with new ones (from Cessna). The glass was fine, no chips along the edges.

I did not get a chance to plug the primer outlet and run it that way. I have what I need to do it, other than time. Hopefully this weekend!

[Mayday]

Did you ever figure out what was causing the air in the gascolator? I'm asking because I'm having some issues with slight power loss at full throttle and was suspecting a fuel starvation issue. I noticed I also have a good bit of air in the gascolator, I would say between 1/2-3/4 inch. I am having this problem on a 59 175 with a GO-300 but the fuel system and gascolator seem to be identical. My GO-300 turns approx 2700rpm static, and when held there for much more than 10-12 seconds it will "shudder" slightly and loose 1-200rpm for a second or two before returning to 2700rpm for another 1-2 seconds and just kind of repeats this.

I don't mean to thread jack but I just read through this whole thing and was really hoping to see a conclusion and was left feeling a little unsatisfied

[4BravoWhiskey]

Sorry for the delay in responding, but the answer is maybe. I have been waiting to confirm before posting, and I haven't been able to do much flying for the past several months. But since you asked, here's an update. I replaced the two hoses that were between the gascolator and carb (via the fuel flow sensor in between) with a single, new hose that bypassed the fuel flow sensor (so, like it was before the fuel flow sensor was installed). I flew the plane about 15 hours that way and never saw an air bubble. So my mechanic made two brand new short hoses and put them back where the previous ones were, taking the single hose back out. Ground tests showed no air bubble (not that those ever did), but then I only flew about an hour before my generator crapped out and I haven't flown it since.

So the current hypothesis is that one of the two fuel hoses was the source. But I need more time to know for sure. My mechanic found an article in a Cessna publication discussing how rubber fuel lines can do this... let air into the system while not leaking fuel out. So hopefully it is solved but I don't know for sure yet.

And even though this point has been debated quite a lot, I do not think that you should have air in the gascolator. Does yours usually go away after sitting overnight, like mine did?

[Mayday]

And even though this point has been debated quite a lot, I do not think that you should have air in the gascolator. Does yours usually go away after sitting overnight, like mine did?

Thanks for the update, I know there seems to be a bit of debate on the issue. I was getting the air bubble as well. When I would crack the rubber line at the carb or even the primer fitting on top of the gascolator, I couldn't get all the air out. There would be at least a small bubble (maybe 1/8-1/4in) at the top of the gascolator yet. After I would run the engine or let it sit overnight the bubble would be big, like half the gascolator was air. The air bubble would grow while sitting, even overnight and there were still no signs of leaking.

One by one I tried eliminating the sources, first I capped the primer fitting, purged the air and let it sit, the air bubble came back.

Next I went Fter the drain valve. I the cable running from the drain valve on the bottom of the gascolator to the panel. I pulled the cable out and took off the bracket/lever assembly to reveal your standard brass drain valve. After leaving it sit overnight, I came back to find no air in the gascolator..... Turns out there must have been too much pressure put on that lever with the cable to the cockpit, it wasn't leaking fuel that I could see or it was leaking so slow it was evaporating as fast as it was leaking.

I was getting an engine stumble at full power that I was associating with the air bubble, turns out they were two different issues as the stumble remained after I got rid of the air bubble. After another week of pulling my hair out and changing parts I found my full power stumble was from weak valve springs. I changed them and now it runs like a top.

I talked to a few experienced GA mechanics over the past few weeks and they all have said there should be no air at all in the gascolator, if there is something is leaking, now that I have my problem fixed and flew the plane, even the tiny air bubble I can't get out by purging is gone. No air whatsoever.

[blueldr]

In viEw of the fact that the only thing to cause the filling of the gascolator is the head pressure from the tanks, and the fall distance being maybe only about three feet, I would guess the pressure to be only a few ounces. If your gascolator is getting air in it, I would guess that something is restricting the free flow from the tanks to the gascolator. I can't immagine that the fuel consumption of the engine is causing a vacuum to form between the carburetor and thesource of the fuel.
Are you sure that you're getting a full, free flow of fuel from the tanks to the gascolator?
Are your tanks properly vented to provide a full flow for the period of time it takes for fuel starvation to start?
It wouldn't take much of a negative pressure in the tanks to restrict the flow requiring such a small amount of head pressure.

If you've solved all of this problem, Just disregard this comment.

[johneeb]

In viEw of the fact that the only thing to cause the filling of the gascolator is the head pressure from the tanks, and the fall distance being maybe only about three feet, I would guess the pressure to be only a few ounces. If your gascolator is getting air in it, I would guess that something is restricting the free flow from the tanks to the gascolator. I can't immagine that the fuel consumption of the engine is causing a vacuum to form between the carburetor and thesource of the fuel.
Are you sure that you're getting a full, free flow of fuel from the tanks to the gascolator?
Are your tanks properly vented to provide a full flow for the period of time it takes for fuel starvation to start?
It wouldn't take much of a negative pressure in the tanks to restrict the flow requiring such a small amount of head pressure.

If you've solved all of this problem, Just disregard this comment.

Dick,
How about this airplane is parked outside, still has the original top of the cabin fuel tank vent and the prevailing wind at its tie-down spot is from back toward the front over the vent causing a vacuum in the fuel tanks.

[blueldr]

Geez, John. You're asking the wrong guy. Try that "Cray Super Computer" of yours<

[4BravoWhiskey]

Next I went Fter the drain valve. I the cable running from the drain valve on the bottom of the gascolator to the panel. I pulled the cable out and took off the bracket/lever assembly to reveal your standard brass drain valve. After leaving it sit overnight, I came back to find no air in the gascolator..... Turns out there must have been too much pressure put on that lever with the cable to the cockpit, it wasn't leaking fuel that I could see or it was leaking so slow it was evaporating as fast as it was leaking.

I'm not following you here, maybe your plane has something different than 170s (at least those that I've seen). A cable running to the drain valve on the gascolator, running to the panel??

[Mayday]

I'm not following you here, maybe your plane has something different than 170s (at least those that I've seen). A cable running to the drain valve on the gascolator, running to the panel??

Yeah! I've got a 59 175, my 63 172 had the same fuel strainer cable thing. Not sure when they started putting them on but figured it was the earlier 100 series with them. I think the idea was to be able to purge out the low point (gascolator) right before starting the engine to ensure clean fuel only made it to the carb. I'm guessing the EPA really loves those strainer cables, about 2 seconds of pulling that cable lets probably a cup or more of fuel to drain onto the ground.

[MoonlightVFR]

Seven months I have been following this subject "Air bubble of doom"

Over 4,000 views this forum,

And then , and then I learn the subject was not about a C170 at all!

[blueldr]

It isn't a C-170 at all ??? This has been one helluva waste of time. Everyone thinking and picturing the fuel system of a C-170 and trying to figure out "why".
Are we really sure this problem craft isn't a helicopter? They have a lot of problems on helicopters that never seem to occur on a C-170.